Mass Converter

Convert mass between metric (kg, g, mg, tonne), avoirdupois (lb, oz, stone, short ton, long ton), troy ounce, grain, and the metric carat with NIST-grade exact factors.

此計算機尚未翻譯成繁體中文（台灣）。

在翻譯完成前，我們先為您顯示英文（美國）版本。

From

Value

Result

0.453592 kg

1 lb= 0.453592 kg

Decimals

All units

Unit	Value
Milligram (mg)	453,592
Gram (g)	453.592
Kilogram (kg)	0.453592
Tonne (metric ton, 1000 kg) (t)	4.53592E-4
Ounce (avoirdupois) (oz)	16
Pound (lb, avoirdupois) (lb)	1
Stone (st, 14 lb) (st)	0.0714286
Short ton (US, 2000 lb) (ton (US))	5E-4
Long ton (UK, 2240 lb) (ton (UK))	4.46429E-4
Troy ounce (precious metals) (oz t)	14.5833
Grain (gr, pharmaceutical) (gr)	7,000
Carat (ct, gemstones — not karat / gold purity) (ct)	2,267.96

Tracking bodyweight? See body fat percentage and lean mass.

最後更新: 2026年5月21日

Mass converter. Kilograms, pounds, ounces, stone, troy ounce, grain and carat with NIST-exact factors.

A mass converter changes a value from one unit of mass to another using exact factors anchored to the SI kilogram. Troy ounce (gold) and avoirdupois ounce (groceries) are kept in separate groups, and metric carat (gem mass) is never confused with karat (gold purity).

What Is a Mass Converter?

A mass converter is a tool that takes a measurement in one unit of mass and returns the equivalent value in any other supported unit. Every conversion in this calculator routes through a single base unit, the kilogram (SI base unit since 1889, redefined in 2019 via the fixed Planck constant), using each unit's defined factor: 1 gram is exactly 1e-3 kg, 1 tonne is exactly 1000 kg, 1 avoirdupois pound is exactly 0.45359237 kg, 1 avoirdupois ounce is exactly 0.028349523125 kg (= lb / 16), 1 stone is exactly 6.35029318 kg (= 14 lb), 1 short ton (US) is exactly 907.18474 kg (= 2000 lb), 1 long ton (UK) is exactly 1016.0469088 kg (= 2240 lb), 1 troy ounce is exactly 31.1034768 g, 1 grain is exactly 64.79891 mg, and 1 metric carat is exactly 200 mg. These are not engineering approximations; they are exact rational factors that drop out of three fixed international agreements: the SI kilogram, the 1959 International Yard and Pound Agreement (1 lb = 0.45359237 kg exact), and the 4th General Conference of the Metric Convention in 1907 (1 carat = 200 mg exact).

The single most-important distinction this converter respects is the troy / avoirdupois split. In daily English the word ounce is overloaded: an ounce of cheese (28.349523125 g, avoirdupois) is a completely different mass from an ounce of gold (31.1034768 g, troy). A troy ounce is about 9.7% heavier than an avoirdupois ounce, and conflating them on a 100-oz precious-metals trade at $2,000/oz silently loses you ~$17,700 (8.85 troy oz × $2,000) — a measurable cost that many online converters ship by default when they collapse both units into a single "oz" picker. The picker in this calculator separates them: "Ounce (avoirdupois)" sits under the everyday group and "Troy ounce (precious metals)" sits under bullion, with "oz t" as the ISO-recommended symbol for the latter. The same goes for the metric carat (ct), kept in its own Gemstone group: a carat measures gemstone mass at exactly 200 mg, while a karat (kt, K) measures gold purity as a fraction of 24 — they share an etymological root (Greek keration, "carob seed") but mean entirely different things. This converter handles the metric carat only; the karat is dimensionless and does not belong in any mass converter.

The second distinction this converter respects is the three-way ambiguity of the word "ton". A short ton (US) is 2000 avoirdupois pounds = 907.18474 kg. A long ton (UK) is 2240 avoirdupois pounds = 1016.0469088 kg. A tonne (metric ton, BIPM-accepted) is exactly 1000 kg = ~2204.62 lb. The three differ by up to 12% — a non-trivial gap on any shipping invoice, crane rating, or international contract — and most converters treat all three as a generic "ton". This calculator labels them "Short ton (US, 2000 lb)", "Long ton (UK, 2240 lb)", and "Tonne (metric ton, 1000 kg)" so a US engineer reading a European spec, or a British engineer reading a US datasheet, never has to guess which one was meant.

What distinguishes a serious mass converter from a marketing widget is honesty about which factors are exact and how many digits matter. Most online converters print 1 kg ≈ 2.2046 lb (4-decimal rounding), 1 lb ≈ 0.4536 kg, 1 oz ≈ 28.35 g — and stop there. That introduces about 0.0000226 lb of drift per kilogram, which compounds invisibly over a barbell at the gym, accumulates measurably on a precious-metals invoice, and adds up to ~226 g (≈ 0.5 lb) over a 10,000 kg shipment. This converter exposes the full 0.45359237 kg exact value, the full 2.20462262184878 lb/kg ratio, and lets you read up to 15 decimal digits (the full IEEE-754 mantissa). Every result is tagged "exact" because every supported pair in the mass set is definitionally exact under NIST SP 811 Appendix B.8, the 1959 IYP Agreement, and the 1907 CIPM carat resolution.

How to Convert Between Mass Units

Every mass conversion is one multiplication and one division through the kilogram. The general formula is:

y = x \cdot \frac{a_{\text{from}}}{a_{\text{to}}}

where $x$ is your input value, $a_\text{from}$ is the source unit's factor to kilograms, and $a_\text{to}$ is the target unit's factor to kilograms. To do it by hand:

1. Look up the source-to-kilogram factor. For pound (avoirdupois), $a_\text{from} = 0.45359237$.

2. Multiply the input by that factor to get kilograms. 180 lb × 0.45359237 = 81.6466266 kg.

3. Look up the target-to-kilogram factor. If the target is the kilogram itself, $a_\text{to} = 1$ and the result is already in kilograms.

4. If the target is a different unit, divide the kilogram value by the target factor. For example, 81.6466266 kg ÷ 0.0311034768 kg/oz t = 2625.61 oz t. The result is real arithmetic, but the framing is noisy — troy ounces are not used for body weight, so this output works only as a sanity check that the formula and factors are wired correctly.

The same procedure works for every supported unit. Going from grams to avoirdupois ounces: 1 g × 1e-3 ÷ 0.028349523125 = 0.03527396 oz (the familiar kitchen 28.35 g ≈ 1 oz factor expressed in reverse). Going from troy ounces to grams: 1 oz t × 0.0311034768 ÷ 1e-3 = 31.1034768 g exact (the canonical gold-trader factor). Going from carat to milligrams: 2.5 ct × 0.0002 ÷ 1e-6 = 500 mg — a 2.5-carat diamond weighs exactly half a gram. Going from grain to milligrams: 5 gr × 6.479891e-5 ÷ 1e-6 = 323.99455 mg ≈ 324 mg — the canonical "5-grain aspirin" dose that older US pharmacy bottles still spell as "5 gr" and that manufacturers round to a clean 325 mg.

To use this calculator, pick the source unit from the "From" dropdown, type a value, then pick the target unit from the "To" dropdown. The result updates on every keystroke. Click the result card to copy it to the clipboard. Use the precision selector to switch between auto (6 significant figures), or a fixed 0, 2, 4, 6, 10 or 15 decimals. Auto-precision switches to scientific notation when the result is larger than 10¹² or smaller than 10⁻³, so converting a long-ton shipment to milligrams still renders readably. The "exact" badge appears next to every mass result, because every unit in this set has a rational, definitionally exact factor to the kilogram.

Mass Conversion Formula

y = x \cdot \frac{a_{\text{from}}}{a_{\text{to}}}

$y$ = The converted value, expressed in the target unit of mass.
$x$ = The input value, expressed in the source unit of mass.
$a_{\text{from}}$ = Factor that converts the source unit to kilograms (e.g. 0.45359237 for pound, 0.0311034768 for troy ounce, 0.0002 for carat, 6.479891e-5 for grain).
$a_{\text{to}}$ = Factor that converts the target unit to kilograms (e.g. 1 for kg, 1e-3 for gram, 1e-6 for milligram, 907.18474 for short ton, 1016.0469088 for long ton, 1000 for tonne).

The formula is a two-step pivot through the kilogram base unit. The factor table this calculator uses is sourced directly from NIST SP 811 Appendix B.8 and anchored to three exact upstream definitions — the SI kilogram, the 1959 International Yard and Pound Agreement (1 lb = 0.45359237 kg exact), and the 4th General Conference of the Metric Convention in 1907 (1 carat = 200 mg exact):

Milligram (mg): 1 × 10⁻⁶ kg (exact, SI prefix)
Gram (g): 1 × 10⁻³ kg (exact, SI prefix)
Kilogram (kg): 1 kg (SI base unit; redefined in 2019 via the fixed Planck constant h = 6.62607015 × 10⁻³⁴ J⋅s exact)
Tonne (t): 1000 kg (exact, BIPM-accepted; also called metric ton)
Ounce avoirdupois (oz): 0.028349523125 kg (exact, = lb / 16; the /16 is a power of 2 and so bit-exact in IEEE-754)
Pound avoirdupois (lb): 0.45359237 kg (exact, IYP 1959; the anchor of every other imperial mass unit)
Stone (st): 6.35029318 kg (exact, = 14 lb; UK body-weight unit)
Short ton (US, ton): 907.18474 kg (exact, = 2000 lb)
Long ton (UK, ton): 1016.0469088 kg (exact, = 2240 lb)
Troy ounce (oz t): 0.0311034768 kg = 31.1034768 g (exact by ISO 80000-4 definition; 480 grains × 64.79891 mg/grain)
Grain (gr): 6.479891 × 10⁻⁵ kg = 64.79891 mg (exact, = 1/7000 lb; pharmaceutical and firearm ammunition)
Carat metric (ct): 2 × 10⁻⁴ kg = 200 mg (exact, CIPM 1907; gemstone mass, bit-exact in IEEE-754 because 200 mg is an integer × 10⁻⁴)

For the kilogram → pound direction the calculator displays the irrational quotient 1/0.45359237 = 2.20462262184877581… lb/kg. Most casual converters print 2.2046 and stop there; this one will print as many digits as the precision selector requests, up to 15. The same applies to the gram → troy-ounce ratio: 1/0.0311034768 = 32.1507466… oz t per kilogram, or 0.0321507466… oz t per gram. Every result is tagged exact because both endpoints are exact rationals — the irrationality is in the printed form, not in the underlying definition.

Worked Mass Conversion Examples

180 lb to kg (the bodyweight workhorse)

Set From = Pound, To = Kilogram, Value = 180. The formula gives 180 × 0.45359237 = 81.6466266 kg exact. At auto-precision the result reads 81.65 kg (6 significant figures). This is the single highest-volume mass query worldwide: a US gym-goer or runner converting their bodyweight to the metric value their doctor, BMI calculator, or European training-plan app expects. The reverse — 81.65 kg back to pounds — gives 81.65 × 2.20462262184878 = 180.0072 lb, which is why a metric bathroom scale that rounds to 0.05 kg can produce "180 vs 181 lb" disagreements between a US and EU display of the same person. For higher precision use 0.45359237 directly; multiplying by the rounded 0.4536 introduces about 0.026 g of drift per pound, invisible for daily use and bothersome only on metrology-grade gear.

1 troy ounce of gold to grams (the precious-metals anchor)

Set From = Troy ounce, To = Gram, Value = 1. The formula gives 1 × 31.1034768 = 31.1034768 g exact. This is the conversion every gold and silver investor does daily: spot prices are quoted per troy ounce (oz t), but bullion bars are often labelled in grams (a 10 g gold bar is 10 / 31.1034768 = 0.3215 troy ounces; a 1 kg bar is 32.1507 troy ounces). The 9.7% gap between the troy ounce (31.1034768 g) and the everyday avoirdupois ounce (28.349523125 g) is what trips up converters that collapse both into a single "oz" picker: 100 oz avoirdupois of gold weighs only 91.146 troy ounces, and on a $2000/oz market that ~8.85-oz-t shortfall is ~$17,700 per 100-bar lot. The arithmetic is unforgiving — a single wrong ounce on a bullion invoice scales linearly with bar count.

14 stone to kg and pounds (UK body-weight reporting)

Set From = Stone, To = Kilogram, Value = 14. The formula gives 14 × 6.35029318 = 88.9041 kg. Switch To = Pound: 14 × 6.35029318 / 0.45359237 = 196 lb exact (the definition: 1 stone = 14 lb). The stone (st) is the dominant UK unit for human body weight: a typical adult will say "I'm 11 stone 4" rather than "71.7 kg" or "158 lb". The mixed stone+pounds format is also common — 11 stone 4 means 11 × 14 + 4 = 158 lb = 71.67 kg. Most American converters omit stone entirely, which is why a UK-trained doctor or athletics coach reading a US weight-tracking app or fitness platform routinely has to do the arithmetic by hand. Including stone in the dropdown is a small differentiator that closes a real gap for British and Irish users (and for anyone reading a British media weight specification — Royal Navy fitness benchmarks, professional boxing weight classes, racehorse jockey limits all quote stones).

5-grain aspirin to milligrams (the legacy pharmaceutical anchor)

Set From = Grain, To = Milligram, Value = 5. The formula gives 5 × 64.79891 = 323.99455 mg ≈ 324 mg. This is the conversion behind the classic US pharmacy label: an old 5-grain aspirin tablet contains exactly 323.99455 mg of acetylsalicylic acid, which manufacturers conventionally round to 325 mg — the standard adult analgesic dose. The low-dose "baby aspirin" 81 mg tablet is one quarter of that 325 mg dose (= 1.25 grains). The grain (gr) is also the standard unit for firearm bullet weights and powder charges: a 9 mm .380 ACP cartridge often uses a 95-grain bullet (= 6.156 g); a .308 Winchester rifle typically fires a 150- to 180-grain bullet (= 9.72–11.66 g). The grain survives in three modern niches — legacy pharmacy labels, US ballistics, and North American water-hardness measurements (grains per gallon) — and the calculator's factor (6.479891 × 10⁻⁵ kg, NIST SP 811 Appendix B.8) is exact to the published specification.

2.5-carat diamond to milligrams and grams (the jeweler's check)

Set From = Carat, To = Milligram, Value = 2.5. The formula gives 2.5 × 200 = 500 mg exact. Switch To = Gram: 2.5 × 0.2 = 0.5 g exact. A 2.5-carat diamond weighs exactly half a gram — a useful sanity check for any retail jewelry spec sheet. The metric carat (ct) was fixed at exactly 200 mg by the International Committee for Weights and Measures (CIPM) in 1907 and adopted unanimously at the 4th General Conference of the Metric Convention (Paris, October 1907); the United States adopted it as the legal standard for diamonds and precious stones on July 1, 1913, and the United Kingdom on April 1, 1914. Because 200 mg = 0.2 g = 2 × 10⁻⁴ kg is a finite-decimal rational, the factor is bit-exact in IEEE-754 doubles, with zero floating-point drift at any precision level. Do not confuse the metric carat (gemstone mass) with the karat (gold purity) — a 14-karat (14K) wedding band describes 14/24 = 58.3% gold content by mass, not a 14-mg ring. This calculator handles the carat (mass) only; the karat is dimensionless and out of scope.

1 short ton vs 1 long ton vs 1 tonne (the three-way disambiguation)

Set Value = 1 and try each of the three "ton" units in turn as the source, with Kilogram as the target.

- Short ton (US, 2000 lb) = 907.18474 kg exact. Standard in US heavy-press, crane and shipping-capacity specifications. About 9.3% lighter than a tonne.

- Long ton (UK, 2240 lb) = 1016.0469088 kg exact. Standard in UK shipping (the "Imperial ton"), legacy naval ratings, and pre-1965 British industrial specs. About 1.6% heavier than a tonne.

- Tonne (metric, 1000 kg) = 1000 kg exact. Standard everywhere outside the US and (legacy) UK; used in international shipping under IMO standards, in EU automotive curb-weight ratings, and in any modern engineering spec that respects SI. About 1.102 short tons or 0.984 long tons.

The practical lesson: any unqualified "ton" on a cross-border shipping invoice, crane data sheet, or industrial press rating is ambiguous by up to 12%. A 10-ton crane bought in the US lifts ~9.07 tonnes; the same nameplate in the UK lifts ~10.16 tonnes; the same nameplate from a German vendor lifts exactly 10 tonnes. This calculator labels every ton variant explicitly so you can never guess which one was meant.

Comparative table: 1 unit in milligrams, grams and kilograms

Unit	mg	g	kg	Exact?
Milligram (mg)	1	0.001	1e-6	yes
Grain (gr)	64.79891	0.06479891	6.479891e-5	yes
Carat (ct)	200	0.2	0.0002	yes
Gram (g)	1000	1	0.001	yes
Ounce avoirdupois (oz)	28349.523125	28.349523125	0.028349523125	yes
Troy ounce (oz t)	31103.4768	31.1034768	0.0311034768	yes
Pound (lb)	453592.37	453.59237	0.45359237	yes
Kilogram (kg)	1000000	1000	1	yes
Stone (st)	6350293.18	6350.29318	6.35029318	yes
Short ton (US)	907184740	907184.74	907.18474	yes
Long ton (UK)	1016046908.8	1016046.9088	1016.0469088	yes
Tonne (t)	1e9	1e6	1000	yes

Five definitional shortcuts worth memorising: 1 lb = 0.45359237 kg exact (IYP 1959); 1 ct = 200 mg exact (CIPM 1907); 1 stone = 14 lb exact; 1 short ton = 2000 lb exact; 1 long ton = 2240 lb exact. Every other row in the table follows mechanically from these five anchors plus the SI prefix powers of ten.

Most common mass conversions

These are the unit pairs people look up most often. Use them as a quick reference, or paste them into the calculator for an exact result to 15 decimals.

- 1 kg to lb: 2.20462262184878 lb (the metric → imperial weight workhorse)

- 1 lb to kg: 0.45359237 kg (IYP 1959, exact)

- 150 lb to kg: 68.0388555 kg (rounded common bodyweight)

- 180 lb to kg: 81.6466266 kg

- 70 kg to lb: 154.3236 lb

- 100 g to oz: 3.5274 oz (avoirdupois — the kitchen ounce)

- 1 oz to g: 28.349523125 g (avoirdupois, kitchen and grocery)

- 1 oz t to g: 31.1034768 g (troy, gold and silver — 9.7% heavier than avdp oz)

- 1 troy ounce to grams of gold: 31.1034768 g (spot-price denominator)

- 1 stone to kg: 6.35029318 kg (UK body weight)

- 14 stone to kg: 88.9041 kg (≈196 lb)

- 1 carat to mg: 200 mg (CIPM 1907, exact)

- 1 carat to grams: 0.2 g (gemstone mass)

- 1 grain to mg: 64.79891 mg (pharmacy, firearms)

- 5 grains to mg: 323.99455 mg ≈ 324 mg (standard aspirin dose)

- 1 short ton to kg: 907.18474 kg (US heavy industry)

- 1 long ton to kg: 1016.0469088 kg (UK shipping)

- 1 tonne to kg: 1000 kg (metric ton; SI-aligned)

- 1 tonne to short tons (US): 1.10231 short tons (≈10% heavier than a US ton)

- 1 tonne to long tons (UK): 0.98421 long tons (≈1.6% lighter than a UK ton)

Mass Conversion Tips

If you are pricing gold, silver, platinum or palladium, the picker label you want is "Troy ounce"; for kitchen scales, grocery labels, US recipes and postage, the label is "Ounce (avoirdupois)". Treat the picker dropdown as the disambiguator — pick first, type second. The (exact) badge fires on both because both factors are NIST-exact (28.349523125 g for the everyday ounce, 31.1034768 g for the bullion one), so once the label is right the arithmetic is right.
Never trust an unqualified "ton". Short ton (US, 2000 lb = 907.18474 kg), long ton (UK, 2240 lb = 1016.0469088 kg) and tonne (metric, 1000 kg = ~2204.62 lb) differ by up to ~12%. If a vendor's spec sheet says "5-ton crane" without disambiguating, ask which one — the same nameplate spans 4.54 tonnes (US 5 short tons) to 5.08 tonnes (UK 5 long tons) to 5.00 tonnes (metric). Cross-border shipping invoices, crane data sheets and industrial press ratings are the three places this bites hardest.
Carat (ct) is gemstone mass; karat (kt, K) is gold purity. A 2-carat diamond weighs 400 mg; an 18-karat gold ring contains 18/24 = 75% gold by mass and has no mass meaning at all. They share an etymological root (Greek keration, carob seed) but mean entirely different things. This converter handles the metric carat only. For gold purity, use the karat fineness fraction directly — there is no mass to convert.
For bodyweight, prefer kg ↔ lb directly over going through stone. The stone is a valid UK unit but adds an extra mental step (1 st = 14 lb = 6.35029318 kg). Use stone when you are reading British media ("He weighs 14 stone 6"), Royal Navy fitness specs, professional boxing weight classes or racehorse jockey limits; for everything else, kg ↔ lb is faster and avoids the divide-and-remainder arithmetic.
If you are reading a legacy US pharmacy bottle that lists doses in grains (gr), 1 grain = 64.79891 mg exactly. The canonical 5-grain aspirin tablet is 323.99455 mg, which manufacturers round to 325 mg. Low-dose "baby aspirin" 81 mg tablets are one quarter of that = 1.25 grains. Modern prescriptions and most jurisdictions have moved fully to mg; the grain survives mainly on older bottles, in US firearm ballistics (bullet and propellant weights), and in North American water-hardness measurements (grains per gallon). The same exact factor — 6.479891 × 10⁻⁵ kg — is the one this calculator uses.
For kitchen and recipe work, the canonical factor is 1 oz (avoirdupois) = 28.349523125 g exact. Most US recipes round to 28.35 g (good enough for any baking ratio) or to 30 g (the casual round). Going the other way: 100 g = 3.5274 oz. If your recipe gives a flour weight in ounces and you want to translate to grams, use this converter rather than the dry-volume cup measurements — flour packs at very different densities depending on whether it's sifted, spooned or scooped, and weight-based conversion is what every professional bakery does.
Mass is not weight. Mass (kg, g, lb) is the amount of matter in an object; it is intrinsic and does not change with location. Weight (newton, lbf) is the gravitational force on that mass; it depends on local gravity. A 70 kg person has 70 kg of mass on Earth, on the Moon, and on Mars — but they weigh 686 N on Earth, 114 N on the Moon, and 261 N on Mars. Your bathroom scale displays a number labelled "kg" but is actually measuring the downward force on a strain gauge and dividing by standard gravity (9.80665 m/s²). For the force quantity (newton, pound-force, kilogram-force), use the force converter; for the mass quantity, you are in the right place.
Above 1 troy ounce per item, you are almost certainly weighing precious metal — gold, silver, platinum or palladium. The bullion market quotes spot prices per troy ounce universally; gold and silver bars are stamped with troy-ounce or gram weights (a 1 kg gold bar = 32.1507 troy ounces); ETFs and futures contracts settle per troy ounce. The avoirdupois ounce simply does not appear in the precious-metals market. If you see a tag saying just "oz" on a bullion product, assume troy.
For cooking ingredient mass with density ("how many grams in 1 cup of flour"), this converter alone is not enough — flour, sugar, butter and liquid all weigh different amounts per cup because their densities differ. Use the cooking converter for those volume-to-mass conversions; this mass converter handles only the mass-to-mass conversions (g ↔ oz ↔ lb), independent of the substance.
Use the precision selector to match your audience. Casual conversions round to 0–2 decimals (a 70-kg person is just "154 lb" in a hallway conversation). Engineering and nutrition tables use 2–4 decimals. Precious-metals spot trading and pharmaceutical compounding use 4–6 decimals. Calibration metrology against a primary mass standard wants 10–15 decimals — the full IEEE-754 double mantissa this calculator exposes.

Mass Converter — Frequently Asked Questions

Is this mass converter free?

Yes. The calculator is free, requires no account, runs entirely in your browser, and is ad-free. The embeddable iframe version at /en-US/widget/mass-converter is also free and ad-free, so you can drop it into fitness sites, recipe blogs, precious-metals portals, jewelry catalogues, pharmacy reference pages or shipping calculators without exposing readers to third-party trackers.

What is the difference between a troy ounce and an avoirdupois ounce?

Both descend from the Roman uncia (1/12 of a libra), but they parted ways in medieval Europe and never reunified. The troy ounce — named after the trading fair of Troyes in Champagne, France — settled at exactly 31.1034768 g and stuck in the bullion and mint trades; the avoirdupois ounce (Old French aveir de peis, "goods of weight") settled at exactly 28.349523125 g and took over commerce, kitchens and postage. A troy ounce is about 9.7% heavier than an avoirdupois ounce. Today: gold, silver, platinum and palladium use troy; everything else uses avoirdupois.

How accurate is this converter?

Every factor in this converter is exact. The SI units (mg, g, kg, tonne) are exact by SI definition. The avoirdupois imperial units (oz, lb, stone, short ton, long ton) are exact because the 1959 International Yard and Pound Agreement fixed 1 lb = 0.45359237 kg with zero rounding error. The troy ounce is exact by ISO 80000-4 definition (1 oz t = 31.1034768 g). The grain is exact by NIST SP 811 specification (1 gr = 64.79891 mg). The metric carat is exact by CIPM 1907 (1 ct = 200 mg). For most pairs the exactness is also bit-exact in IEEE-754 double-precision arithmetic; the grain pair carries a sub-ulp rounding at the 17th digit (because 1/7000 is not a power of 2), invisible at any reasonable display precision.

Is 1 kg exactly 2.2046 pounds?

No. The exact relationship is 1 kg = 1 / 0.45359237 = 2.20462262184877581… lb, which is an infinite non-terminating decimal in base 10. Most casual references print 2.205 or 2.2046, which round at four or five significant figures; for engineering, sports nutrition or metrology work, use the full 0.45359237 kg/lb factor directly (multiply lb by 0.45359237 to get kg, or divide kg by 0.45359237 to get lb). Multiplying by the rounded 2.2046 introduces about 226 mg of drift per kilogram, invisible for daily use but compoundable over heavy shipments.

What is the difference between mass and weight?

Mass measures how much matter an object contains; weight measures the force gravity exerts on that mass. The SI unit of mass is the kilogram; the SI unit of weight is the newton (N), because weight is a force. A 70-kg person has 70 kg of mass on Earth, on the Moon, and in deep space — mass is intrinsic. Their weight is about 686 N on Earth, 114 N on the Moon, and 0 N in free fall — weight depends on local gravity. Bathroom scales display kilograms but physically measure force in newtons, dividing by standard gravity (9.80665 m/s²) internally. This calculator handles mass; for the force quantity in newtons or pound-force, use the force converter.

Is a carat the same as a karat?

No. Carat (ct) is a unit of mass for gemstones equal to exactly 200 mg (CIPM 1907). A 2-carat diamond weighs exactly 400 mg. Karat (kt or K) is a measure of gold purity expressed as a fraction of 24: 24K = 100% gold, 18K = 75% gold, 14K = 58.3% gold, 10K = 41.7% gold. They share an etymological root (Greek keration, "carob seed") but mean entirely different physical quantities — one is a mass, the other a dimensionless purity ratio. This calculator handles the metric carat only. The karat does not belong in any mass converter.

What is a stone in pounds and kilograms?

A stone (st) is exactly 14 avoirdupois pounds = 6.35029318 kg. It is the dominant UK and Ireland unit for human body weight: a typical adult will say "11 stone 4" rather than "71.7 kg" or "158 lb". The mixed stone+pounds format is also common — 11 stone 4 means 11 × 14 + 4 = 158 lb = 71.67 kg. The stone is no longer used for commercial trade after metrication in the 1970s and 1980s, but survives in UK media, professional boxing weight classes, racehorse jockey limits, Royal Navy fitness benchmarks and everyday personal-weight conversation.

What is the difference between a short ton, a long ton and a tonne?

All three are units of mass, but they differ by up to ~12%. A short ton (US) is 2000 avoirdupois pounds = 907.18474 kg, standard in US heavy industry, cranes, presses and trucking. A long ton (UK) is 2240 avoirdupois pounds = 1016.0469088 kg, standard in legacy UK shipping (the "Imperial ton") and Royal Navy displacement ratings. A tonne (also called metric ton) is exactly 1000 kg, BIPM-accepted and the standard everywhere outside the US — it is what EU automotive curb weights, international shipping under IMO standards, and any modern engineering spec mean by "ton". An unqualified "ton" on a cross-border invoice is genuinely ambiguous and should be clarified.

Why is aspirin labeled in grains on older bottles?

Because the grain (gr) was the standard pharmaceutical unit before the metric system took over. 1 grain = exactly 64.79891 mg, so a 5-grain aspirin tablet is 5 × 64.79891 = 323.99455 mg, which manufacturers conventionally round to a clean 325 mg — the canonical adult analgesic dose. The low-dose "baby aspirin" tablet of 81 mg is one quarter of that (= 1.25 grains). The grain survives today in three places: legacy US pharmacy labels, US firearm bullet and powder weights (e.g. a 150-grain rifle bullet = 9.72 g), and North American water-hardness measurements (grains per gallon).

How many grams in an ounce for cooking?

Exactly 28.349523125 g, fixed by the 1959 IYP Agreement. Most US recipes round to 28.35 g — accurate enough for any baking ratio — or to 30 g for the casual round. So 4 oz of flour = 4 × 28.35 ≈ 113 g; 8 oz of butter = 8 × 28.35 ≈ 227 g; 16 oz (1 lb) = 453.59 g. Note that this is the avoirdupois ounce (mass), not the fluid ounce (volume). And it is NOT the troy ounce (31.1034768 g), which only applies to precious metals and never appears in a recipe.

Can I use this converter for precious-metals trading?

Yes for the unit conversion itself. The troy ounce (oz t) is in its own group, set to the ISO-recommended 31.1034768 g exact value (NIST SP 811 Appendix B.8), and the (exact) badge fires for every troy-to-metric and troy-to-avoirdupois pair. So you can confidently convert 32.1507 troy ounces ↔ 1 kg of gold, or 100 troy ounces ↔ 3,110.34768 grams of silver. The converter does NOT carry spot prices or USD-per-ounce quotes — for those you use a market data feed. The converter handles the deterministic mass arithmetic only.

Can I embed this mass converter on my site?

Yes. The embeddable version lives at /en-US/widget/mass-converter; copy the iframe snippet from the embed page. The iframe is ad-free, dependency-free, mobile-responsive, and inherits no third-party trackers — useful for fitness sites, recipe blogs, precious-metals and jewelry portals, pharmacy references, shipping calculators and engineering portals that want a clean mass tool without sending readers off-site.

Key Mass Conversion Terms

Kilogram (kg)

The SI base unit of mass, defined since 20 May 2019 via the fixed value of the Planck constant h = 6.62607015 × 10⁻³⁴ J⋅s exact (combined with the defined values of the second and the metre). Replaces the International Prototype of the Kilogram (the platinum-iridium artifact in Sèvres) that held the definition from 1889 to 2019. Standard for all modern scientific publishing, engineering specifications outside the US, and physics education.

Gram (g)

An SI unit of mass equal to exactly 1 × 10⁻³ kg. The most common everyday metric mass unit for food, body parts, jewelry and chemistry. 1 gram is about 0.0353 avoirdupois ounce, or about 0.0322 troy ounce.

Pound (lb, avoirdupois)

A US customary and imperial unit of mass equal to exactly 0.45359237 kg, fixed by the 1959 International Yard and Pound Agreement signed by the US, UK, Canada, Australia, New Zealand and South Africa. Distinct from the troy pound (12 troy ounces = 373.2417216 g, used historically for precious metals). The avoirdupois pound is what every US food label, body-weight statement and shipping invoice means by "pound".

Ounce avoirdupois (oz)

A unit of mass equal to exactly 1/16 of a pound = 0.028349523125 kg = 28.349523125 g. Scope: everyday US and imperial use — food, recipes, postage, grocery labels, body weight. Because the divisor 16 is a power of 2, the conversion is bit-exact in IEEE-754 double-precision arithmetic. About 9.7% lighter than a troy ounce.

Troy ounce (oz t)

A unit of mass equal to exactly 31.1034768 g (= 480 grains × 64.79891 mg/grain). Scope: precious metals only — gold, silver, platinum, palladium spot prices, bullion bars, coins and ETF contracts. ISO-recommended symbol "oz t" (some references write "ozt"). A troy pound is 12 troy ounces, not 16. Wikidata: Q1323615.

Stone (st)

A UK and Ireland unit of mass equal to exactly 14 avoirdupois pounds = 6.35029318 kg. Standardized as the modern 14-pound stone by the British Weights and Measures Act of 1835. Historically variable (wool stones were 14 pounds, meat stones were 8 pounds). Today retained only for human body weight in UK and Irish everyday conversation, professional boxing weight classes, racehorse jockey limits and Royal Navy fitness benchmarks. A typical UK adult might quote weight as "11 stone 4" rather than 71.7 kg or 158 lb.

Tonne (t, metric ton)

A non-SI but BIPM-accepted unit of mass equal to exactly 1000 kg. Also called the metric ton. The standard unit outside the US for shipping, automotive curb weights, agricultural yields, mineral extraction, and any modern engineering spec. About 1.102 short tons or 0.984 long tons. Often written "metric ton" in US engineering documents to disambiguate from the short ton; outside the US, just "tonne" suffices.

Short ton (US ton, tn)

A US customary unit of mass equal to exactly 2000 avoirdupois pounds = 907.18474 kg. Standard in US heavy industry, cranes, presses, road trucking, coal and steel production capacity, scrap-metal pricing. About 9.3% lighter than a tonne. The default meaning of "ton" in the United States. Use the explicit label "short ton (US)" on any cross-border document.

Long ton (UK ton, imperial ton)

A UK imperial unit of mass equal to exactly 2240 avoirdupois pounds = 1016.0469088 kg. Standard in legacy UK shipping (also called the "shipping ton" or "weight ton"), Royal Navy vessel displacement ratings, and pre-1965 British industrial specifications. About 1.6% heavier than a tonne. Now uncommon in everyday UK use but persists in legacy maritime contexts; always disambiguate with the explicit "long ton (UK)" label.

Grain (gr)

A traditional mass unit equal to exactly 64.79891 mg, or 1/7000 of an avoirdupois pound, fixed by NIST SP 811 Appendix B.8 (factor 6.479891 × 10⁻⁵ kg). Originally based on the mass of a grain of barley or wheat. Survives today in three modern niches: legacy US pharmacy doses (a 5-grain aspirin = 323.99455 mg, rounded to 325 mg), US firearm bullet and propellant weights (a 150-grain .308 rifle bullet = 9.72 g), and North American water-hardness measurements (grains per US gallon). Wikidata: Q693944.

Carat (ct, metric carat)

A unit of mass for gemstones and pearls equal to exactly 200 mg = 0.2 g = 2 × 10⁻⁴ kg. Adopted at the 4th General Conference of the Metric Convention (Paris, October 1907) on a proposal from the International Committee for Weights and Measures (CIPM); the United States adopted it as the legal standard on 1 July 1913, the United Kingdom on 1 April 1914. A 1-carat diamond weighs 200 mg; a 2.5-carat ring stone weighs 500 mg. Distinct from the karat (gold purity ratio). The metric carat is divisible into 100 "points" of 2 mg each. Wikidata: Q261247.

Karat (kt, K) — NOT a mass unit

Karat is the gold-purity scale, expressed as a fraction of 24: 24K = 100% gold (pure), 22K = 91.7%, 18K = 75%, 14K = 58.3%, 10K = 41.7%. It is a dimensionless ratio, NOT a unit of mass — a 14K gold ring describes the alloy composition (14 parts gold to 24 parts total), not how much the ring weighs. Shares an etymological root with the metric carat (Greek keration, "carob seed") but measures an entirely different quantity. This calculator does not handle karat because karat is not a mass.

Mass

The amount of matter in an object — an intrinsic, scalar, extensive physical quantity that resists changes in motion (inertial mass) and attracts other masses (gravitational mass). The SI base unit of mass is the kilogram. Mass does not change with location: a 70 kg person has 70 kg of mass on Earth, on the Moon, and in deep space. Distinct from weight, which is the gravitational force on the mass and depends on local g. Wikidata: Q11423.

Weight (force)

The gravitational force exerted on a mass. Weight = mass × local gravity (W = m·g); the SI unit of weight is the newton (N), because weight is a force. A 70 kg person weighs about 686 N on Earth (under standard gravity g_n = 9.80665 m/s²), 114 N on the Moon, and 261 N on Mars. Bathroom scales display "kg" by convention but physically measure force in newtons and divide by g_n. For the force quantity directly, see the force converter. Wikidata: Q25288.

International Yard and Pound Agreement (1959)

Multilateral agreement signed 1 July 1959 by the US, UK, Canada, Australia, New Zealand and South Africa. It defined the avoirdupois pound as exactly 0.45359237 kg and the international yard as exactly 0.9144 m — eliminating the small US-vs-UK discrepancies that had complicated aviation, surveying and precision engineering. Every avoirdupois mass unit in this calculator (oz, lb, stone, short ton, long ton, grain) inherits its exact factor from this agreement.

CIPM 1907 (metric carat)

The International Committee for Weights and Measures resolution adopted at the 4th General Conference of the Metric Convention (Paris, October 1907) that fixed the metric carat at exactly 200 mg. Eliminated the regional carat variations of the 19th century, which had ranged from 195 mg to 205 mg depending on the gem-trading city. The same definition still applies and is the source of the exact factor 1 ct = 2 × 10⁻⁴ kg used by every modern jewelry and gemstone spec.

NIST SP 811 Appendix B.8

The National Institute of Standards and Technology Special Publication 811, Appendix B.8 — the authoritative US table of conversion factors between US customary, imperial and SI units. Conversion factors printed in boldface in B.8 are exact by definition (e.g. 1 lb = 0.45359237 kg, 1 grain = 6.479891e-5 kg, 1 carat = 2e-4 kg, 1 short ton = 907.18474 kg). The factors in this calculator are drawn directly from this source.

內容由 Smart Calculators 團隊審核驗證