Energy Converter

Convert between SI, electrical, and thermal energy units (joule, kilojoule, megajoule, watt-hour, kilowatt-hour, thermochemical calorie, IT calorie, kilocalorie, BTU, US therm, electronvolt) with NIST-grade exact factors anchored to the 2019 SI redefinition.

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From

Value

kWh

Result

3,412.14 BTU

1 kWh= 3,412.141633 BTU

Decimals

All units

Unit	Value
Joule (J)	3,600,000
Kilojoule (kJ)	3,600
Megajoule (MJ)	3.6
Watt-hour (Wh)	1,000
Kilowatt-hour (kWh)	1
Calorie (thermochemical) (cal_th)	860,421
Calorie (IT) (cal_IT)	859,845
Kilocalorie (food) (kcal)	860.421
British thermal unit (IT) (BTU)	3,412.14
US therm (therm)	0.0341214
Electronvolt (eV)	2.24694E25

Food-label calories are kilocalories (kcal). 1 kcal = 1000 cal_th — use the kilocalorie row when reading nutrition labels.

Oppdatert: 20. mai 2026

Energy converter. SI, electrical, thermal, and atomic energy units with NIST-exact factors.

An energy converter pivots a value through the joule between 11 SI, electrical, thermal and atomic units using NIST-exact factors. Every conversion in this set is exact — the 2019 SI redefinition fixed the elementary charge, so the electronvolt joined joules, kWh, calories, BTU and therms as a definitionally exact unit, useful for HVAC, nutrition and particle-physics work alike.

What Is an Energy Converter?

An energy converter is a tool that takes a quantity of energy in one unit and returns the equivalent value in any other supported unit. It works by routing every conversion through a single base unit, the joule, using each unit's defined factor: 1 watt-hour is exactly 3600 J, 1 kilowatt-hour is exactly 3,600,000 J, 1 thermochemical calorie is exactly 4.184 J, 1 International Table calorie is exactly 4.1868 J, 1 kilocalorie (food-label Calorie) is exactly 4184 J, 1 BTU (IT) is exactly 1055.05585262 J, 1 US therm is exactly 100,000 BTU (IT) = 1.05505585262 × 10⁸ J, and 1 electronvolt is exactly 1.602176634 × 10⁻¹⁹ J. These are not engineering approximations; they are exact by NIST SP 811 Appendix B.8, by the 2019 SI redefinition that fixed the elementary charge, and by ISO 80000-5.

This converter supports 11 units across four groups. The SI group covers the joule (coherent SI derived unit, BIPM SI Brochure 9th ed., Table 4), the kilojoule, and the megajoule. The electrical group covers the watt-hour (= 1 W × 3600 s, exact) and the kilowatt-hour, the unit on every US household electricity bill. The thermal group covers the thermochemical calorie (cal_th, the historical lab unit), the International Table calorie (cal_IT, the steam-tables unit), the kilocalorie (1000 cal_th, the energy on a US food label written with a capital C), the BTU (IT) used for HVAC and gas appliances, and the US therm used for natural-gas billing. The scientific group covers the electronvolt, the workhorse of atomic and particle physics — the rest mass-energy of an electron is exactly 0.511 MeV.

What distinguishes a serious energy converter from a marketing widget is honesty about the calorie and BTU families. Two calorie definitions exist: the thermochemical calorie (4.184 J exact, used in most chemistry references) and the International Table calorie (4.1868 J exact, used in steam tables and HVAC). They differ by 1 part in 7000 — small for cooking, fatal for reactor heat balances. Three BTU definitions are in active use (IT, thermochemical, and 59°F), differing by less than 0.1%; NIST SP 811 specifies BTU (IT) as the engineering default, which is what this calculator uses. The food-label "Calorie" with a capital C is actually a kilocalorie (1 kcal = 1000 cal_th = 4184 J exact), so a 2000-Calorie daily diet is 2000 kcal = 8.368 MJ. The converter tags every result with the exact badge because every one of the 11 unit factors is exact — the 2019 SI redefinition fixed the elementary charge as e = 1.602176634 × 10⁻¹⁹ C, which made the electronvolt exact for the first time in history.

How to Convert Between Energy Units

Every energy conversion is one multiplication and one division through the joule. 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 joules, and $a_\text{to}$ is the target unit's factor to joules. To do it by hand:

1. Look up the source-to-joule factor. For kilowatt-hours, $a_\text{from} = 3{,}600{,}000$ (exact, since 1 kWh = 1 kW × 3600 s).

2. Multiply the input by that factor to get joules. 30 kWh × 3,600,000 = 1.08 × 10⁸ J.

3. Look up the target-to-joule factor. For megajoules, $a_\text{to} = 10^6$.

4. Divide the joule value by the target factor. 1.08 × 10⁸ ÷ 10⁶ = 108 MJ.

The same procedure works for every supported unit. Going from kcal to kJ: 1 kcal × 4184 J/kcal ÷ 1000 J/kJ = 4.184 kJ — so a 2000-kcal daily diet is 8368 kJ (8.368 MJ). Going from BTU to joules: 12,000 BTU/h × 1055.05585262 J/BTU = 1.266 × 10⁷ J per hour, which is 3.517 kW — the canonical "1-ton air conditioner" rating. Going from eV to joules: a 5 MeV alpha particle is 5 × 10⁶ × 1.602176634 × 10⁻¹⁹ = 8.01 × 10⁻¹³ J.

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 exceeds 10¹² (one trillion) or falls below 10⁻³, so an eV-to-kWh conversion still renders readably as 4.45 × 10⁻²⁶ kWh. The exact badge appears on every result because all 11 unit factors are tagged exact in the source data — the SI joule, the watt-hour family by the exact-second SI definition, the calorie family by NIST SP 811 boldface, the BTU/therm family by NIST SP 811 (with the IT base), and the electronvolt by the 2019 SI redefinition that fixed e = 1.602176634 × 10⁻¹⁹ C.

Energy Conversion Formula

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

$y$ = The converted value, expressed in the target unit of energy.
$x$ = The input value, expressed in the source unit of energy.
$a_{\text{from}}$ = Factor that converts the source unit to joules (e.g. 3{,}600{,}000 for kWh, 4184 for kcal, 1055.05585262 for BTU_IT).
$a_{\text{to}}$ = Factor that converts the target unit to joules (e.g. 4.184 for cal_th, 1.602176634×10⁻¹⁹ for eV).

The formula is a two-step pivot through the joule, the SI coherent derived unit for energy (BIPM SI Brochure 9th ed., Table 4). The factor table this calculator uses is sourced from NIST SP 811 Appendix B.8 and the 2019 SI redefinition:

Joule (J): 1 J (SI coherent derived unit, exact)
Kilojoule (kJ): 10³ J (exact, SI prefix)
Megajoule (MJ): 10⁶ J (exact, SI prefix)
Watt-hour (Wh): 3600 J (exact: 1 W × 3600 s, SI)
Kilowatt-hour (kWh): 3{,}600{,}000 J (exact, SI)
Calorie, thermochemical (cal_th): 4.184 J (exact, NIST SP 811 boldface)
Calorie, International Table (cal_IT): 4.1868 J (exact, 5th Int'l Conf. on Steam Properties, London 1956; NIST SP 811 boldface; ISO 80000-5)
Kilocalorie / food-label Calorie (kcal): 4184 J (exact; = 1000 × cal_th)
BTU (IT): 1055.05585262 J (exact, NIST SP 811 specifies this as the engineering default among the multiple BTU variants)
Therm (US, IT-based): 1.05505585262 × 10⁸ J (exact, = 100,000 BTU_IT — the same convention used in NIST SP 811 and EC therm)
Electronvolt (eV): 1.602176634 × 10⁻¹⁹ J (exact since the 2019 SI redefinition fixed the elementary charge e at this value)

For the kWh → MJ direction the calculator computes 1 kWh × 3,600,000 J/kWh ÷ 10⁶ J/MJ = 3.6 MJ exact. For 2000 kcal → MJ: 2000 × 4184 ÷ 10⁶ = 8.368 MJ exact. For 1 eV → J: 1 × 1.602176634 × 10⁻¹⁹ = 1.602176634 × 10⁻¹⁹ J exact (every digit is meaningful — the SI redefinition fixed this value as a defining constant). Every conversion in this set earns the exact badge because every factor is a definitional ratio or an SI-prefix multiplier; there is no irrational constant like π or √2 in this set, so the IEEE-754 representation is exact up to the digits of the rationals themselves.

Worked Energy Conversion Examples

1 kWh to megajoules (electricity-bill arithmetic)

Set From = Kilowatt-hour, To = Megajoule, Value = 1. The formula gives 1 × 3,600,000 ÷ 1,000,000 = 3.6 MJ exact. The average US household consumed roughly 30 kWh per day in recent EIA data (about 10,500 kWh per year) — that's 108 MJ per day, or 39 GJ per year. A 1500 W space heater running for 1 hour delivers 1.5 kWh = 5.4 MJ to the room. The conversion is exact because both ends are SI rationals (kWh = 3.6 × 10⁶ J exact via the SI second; MJ = 10⁶ J by prefix).

2000 kcal to kilojoules (food-label arithmetic)

Set From = Kilocalorie, To = Kilojoule, Value = 2000. The formula gives 2000 × 4184 ÷ 1000 = 8368 kJ exact (= 8.368 MJ). This is the canonical "2000-Calorie daily diet" used as the reference value on every US Nutrition Facts label. Note the capital-C convention: the "Calorie" on a food label is exactly 1 kilocalorie = 1000 small calories — the difference matters because someone reading the European kJ value (typically printed alongside) and dividing by 4.184 instead of 4184 gets a result 1000× too high. The kcal-to-kJ ratio is 4.184 exactly, by NIST SP 811 boldface — no floating-point drift.

12,000 BTU/h to kilowatts (1-ton air conditioner)

Air-conditioner capacity in the US is rated in BTU per hour. To convert to kW, divide by 3412.142 BTU/h per kW (the inverse of 1 kW × 3600 s ÷ 1055.05585262 J/BTU). A 12,000 BTU/h unit corresponds to 12,000 ÷ 3412.142 = 3.517 kW of thermal capacity — the canonical "1-ton" air conditioner (the ton here is the cooling power needed to melt one short ton of ice per day, ≈ 12,000 BTU/h). A 24,000 BTU/h unit is a 2-ton AC, suitable for about 1100 ft² in temperate climates. The conversion is exact because BTU (IT) is exact (1055.05585262 J) and the SI second is exact.

1 therm of natural gas to kilowatt-hours (utility billing)

Set From = Therm (US), To = Kilowatt-hour, Value = 1. The formula gives 1 × 1.05505585262 × 10⁸ ÷ 3,600,000 = 29.3001 kWh exact (to printed precision). US residential natural-gas bills typically read in therms; electric bills read in kWh. To compare the energy cost of gas heating vs electric heating you need this conversion plus the appliance efficiency: a 95% AFUE gas furnace delivers 27.8 kWh of heat per therm, while a heat pump with COP 3.0 delivers 3.0 × 29.3 = 87.9 kWh of heat per therm-equivalent of electricity — which is why heat pumps beat gas furnaces on a pure-energy basis. The conversion itself is exact (BTU_IT-based therm).

Electron rest mass-energy in eV vs joules

Set From = Electronvolt, To = Joule, Value = 511000. The formula gives 511,000 × 1.602176634 × 10⁻¹⁹ = 8.18710 × 10⁻¹⁴ J (auto-precision switches to scientific notation because the result is below 10⁻³). This is the rest mass-energy of an electron, m_e × c² = 0.511 MeV exact (to the precision of the 2019-fixed electronvolt). For comparison: a 1 keV X-ray photon carries 1.602 × 10⁻¹⁶ J; a 5 MeV alpha particle from polonium-210 decay carries 8.01 × 10⁻¹³ J; the LHC's 13.6 TeV proton-proton collisions deliver 2.18 × 10⁻⁶ J per collision (a millionth of a joule, concentrated in a volume smaller than a proton). The conversion is exact because the 2019 SI redefinition fixed the elementary charge e — and therefore the joule value of 1 eV — as a defining constant.

Comparative table: 1 unit in joules

Unit	Symbol	Value in joules	Exact?
Electronvolt	eV	1.602176634 × 10⁻¹⁹	yes (2019 SI)
Calorie (thermochemical)	cal_th	4.184	yes
Calorie (International Table)	cal_IT	4.1868	yes
Joule	J	1	yes
Kilocalorie (food Calorie)	kcal	4184	yes
Watt-hour	Wh	3600	yes
Kilojoule	kJ	1000	yes
BTU (IT)	BTU	1055.05585262	yes
Megajoule	MJ	1,000,000	yes
Kilowatt-hour	kWh	3,600,000	yes
Therm (US, IT-based)	thm	1.05505585262 × 10⁸	yes

Use this table to do conversions by hand: divide source-in-joules by target-in-joules. For example, 1 kWh in kcal = 3,600,000 ÷ 4184 = 860.421 kcal exact (to printed precision). 1 therm in kWh = 1.05505585262 × 10⁸ ÷ 3,600,000 = 29.3001 kWh.

Most common energy 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.

Kilowatt-hour to megajoule: 1 kWh = 3.6 MJ (exact)
Megajoule to kilowatt-hour: 1 MJ ≈ 0.27778 kWh (exact = 1/3.6)
Kilocalorie to kilojoule: 1 kcal = 4.184 kJ (exact)
Kilojoule to kilocalorie: 1 kJ ≈ 0.23901 kcal (exact = 1/4.184)
BTU to joule: 1 BTU (IT) = 1055.05585262 J (exact)
BTU to watt-hour: 1 BTU ≈ 0.29307 Wh (exact = 1055.055.../3600)
BTU per hour to watt: 3.41214 BTU/h = 1 W (exact = 3600/1055.055...)
Therm to kilowatt-hour: 1 thm ≈ 29.3001 kWh
Therm to BTU: 1 thm = 100,000 BTU (IT) (exact)
Calorie (cal_th) to joule: 1 cal_th = 4.184 J (exact)
Calorie (cal_IT) to joule: 1 cal_IT = 4.1868 J (exact)
Electronvolt to joule: 1 eV = 1.602176634 × 10⁻¹⁹ J (exact, 2019 SI)
Megaelectronvolt to joule: 1 MeV ≈ 1.60218 × 10⁻¹³ J
12,000 BTU/h to kilowatt: 12,000 BTU/h ≈ 3.5169 kW (1-ton AC)
2000 kcal to megajoule: 2000 kcal = 8.368 MJ (exact, daily diet)

Energy Conversion Tips

Pivot through the joule. Every conversion in this tool is implemented as "input × from-factor ÷ to-factor", with the joule as the pivot. Memorising six factors gives you every cross-conversion: watt-hour (3600), kilowatt-hour (3,600,000), thermochemical calorie (4.184), kilocalorie (4184), BTU_IT (1055.05585262), and electronvolt (1.602176634 × 10⁻¹⁹).
Read the food-label "Calorie" as kilocalorie. The energy on a US Nutrition Facts label is always kcal (1000 small calories = 4184 J), even when printed as "Calories" with a capital C. A 2000-Calorie daily diet is 2000 kcal = 8.368 MJ = 2.323 kWh. European labels show both kcal and kJ side-by-side; dividing the kJ value by 4.184 (not 4184) recovers the kcal — a common 1000× error when the conventions are mixed.
Distinguish thermochemical from International Table calorie. cal_th = 4.184 J exact (used in most chemistry textbooks and food labels); cal_IT = 4.1868 J exact (used in steam tables and HVAC, defined by the 5th International Conference on the Properties of Steam, London 1956). The difference is 1 part in 7000 — irrelevant for cooking, important for reactor heat balances. NIST SP 811 prints both factors in boldface (exact).
Use BTU (IT) as the engineering default. Three BTU variants exist (IT, thermochemical, 59°F) and they differ by less than 0.1%. NIST SP 811 lists BTU (IT) = 1055.05585262 J exact as the engineering default; this calculator implements that value. If a datasheet quotes BTU without qualifying which variant, IT is almost always the intended convention.
For HVAC sizing, remember 12,000 BTU/h = 1 ton ≈ 3.517 kW. The "ton" of cooling is a legacy from ice-house economics: the cooling power needed to melt one short ton of ice per day. A 24,000 BTU/h unit is a 2-ton AC (about 7 kW thermal); a 36,000 BTU/h unit is a 3-ton AC (about 10.5 kW). Sizing rule of thumb in temperate climates: 20 BTU/h per ft² of conditioned floor space — so a 1500 ft² home wants about 30,000 BTU/h = 2.5 tons.
Use the kWh-to-therm conversion to compare gas vs electric heating. 1 therm of natural gas = 29.3 kWh of energy. A 95% AFUE gas furnace delivers 0.95 × 29.3 = 27.8 kWh of useful heat per therm; an electric resistance heater delivers 1.00 kWh of heat per kWh of electricity; a heat pump with seasonal COP 3.0 delivers 3.00 kWh of heat per kWh of electricity. Multiply by your gas and electric per-unit prices to see which is cheaper in your local market.
Use eV for atomic-scale energies, joules for everything bigger. 1 eV = 1.602 × 10⁻¹⁹ J — small enough that 1 mole of eV (Avogadro's number of them) is just 96.5 kJ, the Faraday constant in disguise. Useful scales: chemical bond energies are 1–10 eV per bond; X-rays are 100 eV – 100 keV; nuclear binding energies are 1–10 MeV per nucleon; LHC proton beams are 6.8 TeV per particle.
The exact badge applies to every conversion in this calculator because every one of the 11 unit factors is a definitional rational. There is no π or √2 in the energy unit family, so the IEEE-754 representation is exact for every factor up to the displayed digits. Compare this to the angle converter, where every π-derived factor (degree, gradian, turn, arcminute, arcsecond) is tagged approximate because π is irrational.
When the result is bigger than 10¹² (one trillion) or smaller than 10⁻³ at auto-precision, the display switches to scientific notation. This is on purpose: 1 eV in kWh is ≈ 4.45 × 10⁻²⁶ kWh, readable in scientific notation but unreadable as 0.0000…0445 kWh.

Energy Converter — Frequently Asked Questions

Is this energy converter free?

Yes. The calculator is free, requires no account, runs entirely in your browser, and is ad-free. The embeddable iframe version at /widget/energy-converter is also free and ad-free, so you can drop it into engineering portals, HVAC design tools, nutrition blogs, or physics teaching materials without exposing readers to third-party trackers.

How accurate are the conversion factors?

Every one of the 11 unit factors is exact. The SI units (joule, kilojoule, megajoule) are exact by definition. The electrical units (Wh, kWh) are exact via the SI second (1 Wh = 1 W × 3600 s). The calorie family (cal_th = 4.184, cal_IT = 4.1868, kcal = 4184) is exact per NIST SP 811 boldface. The BTU_IT (1055.05585262 J) and US therm (10⁵ BTU_IT) are exact per NIST SP 811. The electronvolt (1.602176634 × 10⁻¹⁹ J) became exact in 2019 when the SI redefinition fixed the elementary charge as a defining constant. There are no π-derived irrationals in this set, so the IEEE-754 representation is exact up to the displayed digits.

What is the difference between a calorie and a Calorie?

A small calorie (cal, lowercase) is 4.184 joules — the heat needed to raise 1 gram of water by 1 °C. A food Calorie (capital C) is actually a kilocalorie: 1000 small calories = 4184 joules. So a 2000-Calorie daily diet is 2000 kilocalories = 8.368 megajoules, not 2000 small calories.

How do I convert kWh to MJ?

Multiply by 3.6. So 1 kWh = 3.6 MJ exact, 10 kWh = 36 MJ, 30 kWh ≈ 108 MJ (a typical US household daily consumption). The conversion is exact because 1 kWh = 1000 W × 3600 s = 3,600,000 J = 3.6 MJ by SI definition.

How do I convert kilocalories to kilojoules?

Multiply by 4.184. So 100 kcal = 418.4 kJ, 1000 kcal = 4184 kJ = 4.184 MJ, and a 2000-Calorie diet = 8368 kJ = 8.368 MJ. The factor 4.184 is the thermochemical calorie convention used on US Nutrition Facts labels (NIST SP 811 boldface, exact). The IT calorie used in steam tables is 4.1868 — a 0.014% difference that does not matter for nutrition but does matter for HVAC heat-load calculations.

What is 1 BTU in joules?

One BTU (IT) equals 1055.05585262 joules, exact per NIST SP 811. The IT (International Table) definition is the engineering default; the thermochemical BTU (≈ 1054.35 J) and 59°F BTU (≈ 1054.804 J) differ by less than 0.1% and are rarely used outside niche standards work.

How many kWh in a therm of natural gas?

About 29.3001 kWh per therm. The exact computation is 100,000 BTU_IT × 1055.05585262 J/BTU ÷ 3,600,000 J/kWh = 29.30010832… kWh. This conversion is what lets you compare US natural-gas bills (in therms) with electric bills (in kWh): multiply the therm rate by 29.3 to get the equivalent kWh rate before efficiency, then divide by appliance efficiency (gas furnace ~0.95, heat pump COP ~3.0) to compare delivered heat per dollar.

Why is the electronvolt now exact?

Because the 2019 SI redefinition fixed the elementary charge e at exactly 1.602176634 × 10⁻¹⁹ coulombs as a defining constant. Before May 2019 the electronvolt was a measured quantity with a 22-parts-per-billion uncertainty (CODATA 2014); after the redefinition, 1 eV = e × 1 V is exact by construction. The rest mass-energy of an electron is exactly 0.510998950 MeV (the mass itself is now a measured quantity with the redefinition shifting the uncertainty from charge to mass).

What is the rest mass-energy of an electron in joules?

About 8.187 × 10⁻¹⁴ J. The exact computation is 511,000 eV × 1.602176634 × 10⁻¹⁹ J/eV ≈ 8.1871 × 10⁻¹⁴ J (to the precision of the electron mass, m_e c² = 0.51099895 MeV). This is the energy released when an electron and a positron annihilate into two 511 keV gamma rays — the basis of PET (positron emission tomography) imaging.

How much energy does an average US household use per day?

About 30 kWh of electricity (108 MJ), per the most recent US Energy Information Administration residential data — roughly 10,500 kWh per year. Households that also use natural gas typically add 30–80 therms per month for heating and water heating in cold climates, or roughly 880–2350 kWh-equivalent of gas energy per month. Total household energy (electric + gas) commonly lands between 40 and 100 kWh-equivalent per day depending on climate, home size and heating fuel.

Is the US therm exactly 100,000 BTU?

Yes — specifically 100,000 BTU (IT) = 1.05505585262 × 10⁸ joules exact, per NIST SP 811. A subtlety: some US natural-gas tariffs reference the older 59°F BTU instead of the IT BTU, giving a slightly different therm value (1.054804 × 10⁸ J, about 0.02% lower). This calculator uses the IT-based therm because it is the modern engineering default and the one printed in NIST SP 811.

Can I embed this converter on my site?

Yes. The embeddable version lives at /widget/energy-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 HVAC design portals, energy-audit firms, nutrition apps, and physics teaching materials that want a clean energy tool without sending readers off-site.

How big is 1 kWh in everyday terms?

1 kWh = 3.6 MJ = 860 kcal = 3412 BTU. It's about the energy used by a 1500 W space heater running for 40 minutes, a microwave oven running for 1 hour, an LED bulb (10 W) running for 100 hours, or a typical desktop computer running for 5 hours. The average US household uses about 30 kWh per day.

Key Energy Conversion Terms

Joule (J)

The SI coherent derived unit for energy, work, and heat (BIPM SI Brochure 9th ed., Table 4). One joule is the work done by a force of one newton applied through a distance of one metre, or equivalently the energy dissipated by a one-watt power source in one second. It is the base unit through which this calculator routes every conversion.

Kilowatt-hour (kWh)

An SI-accepted unit of energy equal to exactly 3,600,000 joules (1 kW × 3600 s, via the exact SI second). Standard on US household electricity bills; the average US home consumes about 30 kWh per day per recent EIA data. 1 kWh = 3.6 MJ exact.

Calorie, thermochemical (cal_th)

Exactly 4.184 joules, per NIST SP 811 (boldface) and the original 1925 thermochemistry convention. Used in most chemistry references and US food labels (where the kcal version is written as "Calorie" with a capital C). 1 kcal = 1000 cal_th = 4184 J exact.

Calorie, International Table (cal_IT)

Exactly 4.1868 joules, defined by the 5th International Conference on the Properties of Steam (London, July 1956) and codified in NIST SP 811 (boldface) and ISO 80000-5. Used in steam tables, HVAC engineering, and refrigeration calculations. It differs from the thermochemical calorie by 1 part in 7000.

Kilocalorie (kcal) / food Calorie

Exactly 1000 thermochemical calories = 4184 joules. The energy on a US Nutrition Facts label is always in kilocalories, written as "Calories" with a capital C. A 2000-Calorie reference daily diet equals 2000 kcal = 8.368 MJ = 2.323 kWh.

British thermal unit, IT (BTU_IT)

The International Table BTU, exactly 1055.05585262 joules per NIST SP 811. The heat needed to raise 1 lb of water by 1 °F. Used in US and UK HVAC nameplates and natural-gas billing. The thermochemical (≈ 1054.35 J) and 59°F (≈ 1054.80 J) BTU variants differ by less than 0.1% and are rarely used outside niche standards work.

US therm (thm)

Exactly 100,000 BTU (IT) = 1.05505585262 × 10⁸ joules ≈ 29.3 kWh. The standard unit on US residential natural-gas bills. A typical US household burns 30–80 therms per month for heating and water heating in cold climates.

Electronvolt (eV)

The kinetic energy gained by a single electron accelerated through a potential difference of one volt. Exactly 1.602176634 × 10⁻¹⁹ joules since the 2019 SI redefinition fixed the elementary charge e as a defining constant. Standard unit in atomic, nuclear, and particle physics: chemical bonds are 1–10 eV; X-rays 100 eV – 100 keV; nuclear binding 1–10 MeV per nucleon.

NIST SP 811 Appendix B.8

The NIST Guide to the SI, Appendix B.8, lists conversion factors for non-SI units in alphabetical order. Factors printed in boldface are exact by definition (cal_th 4.184 J, cal_IT 4.1868 J). The BTU (IT) family appears in regular type in the table but is anchored to the cal_IT via NIST footnote 9, which sets 1 BTU_IT = 1.05505585262 × 10³ J exact; the US therm is then exactly 100,000 BTU_IT. This calculator implements the boldface-exact factors plus the footnote-9 BTU/therm chain.

2019 SI redefinition

The major revision to the International System of Units that took effect 20 May 2019, fixing the values of seven defining constants (Planck constant h, elementary charge e, Boltzmann constant k, Avogadro number N_A, speed of light c, hyperfine frequency of caesium Δν_Cs, and luminous efficacy K_cd). The fixing of e = 1.602176634 × 10⁻¹⁹ C made the electronvolt exact for the first time.

Significant figures

The digits in a numerical result that carry meaning about its precision. Auto-precision in this calculator targets 6 significant figures, the default for engineering tables. NIST SP 811 quotes conversion factors to 6 or 7 significant figures unless they are exact (boldface), in which case all digits are meaningful.

IEEE-754 double

The 64-bit floating-point format used by JavaScript and most calculators. It carries about 15–17 significant decimal digits. Because every energy unit factor in this calculator is a definitional rational (no π, no √2), the IEEE-754 representation is exact up to the displayed digits — which is why every conversion in this tool earns the exact badge.

Kilder og referanser

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