Length Converter

Convert between metric, imperial, and astronomical length units (millimetre, metre, kilometre, inch, foot, yard, mile, nautical mile, ångström, light-year, AU, parsec) with NIST-grade exact factors.

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From

Value

Result

0.3048 m

1 ft= 0.3048 m

Decimals

All units

Unit	Value
Millimeter (mm)	304.8
Centimeter (cm)	30.48
Meter (m)	0.3048
Kilometer (km)	3.048E-4
Inch (in)	12
Foot (ft)	1
Yard (yd)	0.333333
Mile (mi)	1.89394E-4
Nautical mile (nmi)	1.64579E-4
Ångström (Å)	3,048,000,000
Astronomical unit (AU)	2.03746E-12
Light-year (ly)	3.22174E-17
Parsec (pc)	9.8779E-18

更新日: 2026年5月18日

Length converter. Metric, imperial, and astronomical units with NIST-exact factors.

A length converter translates a value from one unit to another using exact factors from NIST SP 811, the 1959 international yard, and the IAU. It tags every result as exact or approximate, so engineers and students can tell rounded conversions from definitionally exact ones.

What Is a Length Converter?

A length converter is a tool that takes a measurement in one unit of length and returns the equivalent value in any other supported unit. It works by routing every conversion through a single base unit, the metre, using each unit's defined factor: 1 inch is exactly 0.0254 m, 1 foot is exactly 0.3048 m, 1 mile is exactly 1609.344 m, 1 nautical mile is exactly 1852 m, and 1 astronomical unit is exactly 149,597,870,700 m. These are not engineering approximations; they are exact definitions agreed by the 1959 International Yard and Pound Agreement, the 1929 International Hydrographic Conference (Monaco), and IAU Resolution B2 (2012).

This converter supports 13 units across three groups. The metric group covers the millimetre, centimetre, metre and kilometre, all derived from the SI definition of the metre as the distance light travels in vacuum in 1/299,792,458 of a second (in force since the 2019 SI revision). The imperial group covers the inch, foot, yard, mile and nautical mile. The astronomical group covers the ångström (10⁻¹⁰ m, used for atomic radii and visible-light wavelengths from about 4000 to 7000 Å), the astronomical unit, the light-year, and the parsec.

What distinguishes a serious length converter from a marketing widget is honesty about precision. The factor 1 metre = 3.28084 feet is not exact; it is a six-significant-figure rounding of the irrational quotient 1/0.3048 = 3.28083989501312…. The exact relationship is the inverse: 1 foot = 0.3048 m. This converter labels every result with an "exact" tag when both the source and target are defined by exact factors (e.g. metre to inch, mile to kilometre), and treats astronomical units like the light-year — whose IAU value of 9,460,730,472,580,800 m is exact by definition but does not round cleanly to a 15-digit IEEE-754 double — as approximate to avoid false confidence.

How to Convert Between Length Units

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

1. Look up the source-to-metre factor. For feet, $a_\text{from} = 0.3048$.

2. Multiply the input by that factor to get metres. 100 ft × 0.3048 = 30.48 m.

3. Look up the target-to-metre factor. For yards, $a_\text{to} = 0.9144$.

4. Divide the metre value by the target factor. 30.48 ÷ 0.9144 = 33.333… yd.

The same procedure works for every supported unit. Going from astronomical units to kilometres: 1 AU × 149,597,870,700 m/AU ÷ 1000 m/km = 149,597,870.7 km. Going from inches to ångström: 1 in × 0.0254 m/in ÷ 1e−10 m/Å = 2.54 × 10⁸ Å.

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 for values outside the 10⁻³ to 10¹² range so that an angström-to-parsec result is still readable. The "exact" badge appears next to the result whenever the source and target units are both definitionally exact and the conversion does not lose information; light-years and parsecs are flagged as approximate because their floating-point representation rounds the last few digits.

Length Conversion Formula

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

$y$ = The converted value, expressed in the target unit.
$x$ = The input value, expressed in the source unit.
$a_{\text{from}}$ = Factor that converts the source unit to metres (e.g. 0.3048 for feet, 1609.344 for miles).
$a_{\text{to}}$ = Factor that converts the target unit to metres (e.g. 1000 for kilometres, 0.0254 for inches).

The formula is a two-step pivot through the metre base unit. The factor table this calculator uses is sourced directly from NIST SP 811 Appendix B.8 and matching primary references:

Millimetre: 1e−3 m (exact, SI)
Centimetre: 1e−2 m (exact, SI)
Metre: 1 m (exact, SI base unit since 2019)
Kilometre: 1e3 m (exact, SI)
Inch: 0.0254 m (exact, International Yard and Pound Agreement 1959)
Foot: 0.3048 m (exact, 1959 agreement)
Yard: 0.9144 m (exact, 1959 agreement)
Mile: 1609.344 m (exact, 1959 agreement; equal to 5280 ft)
Nautical mile: 1852 m (exact, Monaco 1929)
Ångström: 1e−10 m (exact)
Astronomical unit: 149,597,870,700 m (exact, IAU Resolution B2 2012)
Light-year: 9,460,730,472,580,800 m (IAU; exact by definition but stored as a rounded double in this calculator)
Parsec: 648000/π AU = 30,856,775,814,913,673 m (IAU 2015; defined exactly but the metre value is irrational, stored as a rounded double)

For the metre→feet direction the calculator displays the irrational quotient 1/0.3048 = 3.280839895013… ft/m. Most casual converters print 3.28084 and stop there; this one will print as many digits as the precision selector requests, up to 15.

Worked Length Conversion Examples

100 metres to feet (engineering precision)

Set From = Meter, To = Foot, Value = 100. The formula gives 100 × (1/0.3048) = 328.0839895013… ft. At auto-precision the result reads 328.084 ft (6 significant figures). Set precision to 15 decimals to see 328.083989501312336 ft — the longer string is the full IEEE-754 double, useful when you are double-checking a CAD drawing against a datasheet. Because metre and foot are both exact units (1 ft = 0.3048 m exact), the conversion is tagged exact even though the printed digits look irrational; the irrationality is just the form of the answer, not a measurement uncertainty.

1 mile to kilometres (round-trip exactness)

Set From = Mile, To = Kilometer, Value = 1. The result is 1.609344 km exact (since 1 mi = 1609.344 m and 1 km = 1000 m, both by definition since 1959). The reverse 1 km → mile gives 0.6213711922… mi, which is irrational in form but still tagged exact. Compare to typical "1.61 km" approximations you find in driving handbooks: that rounding accumulates 224 m of drift over 1000 km — small for road navigation, fatal for surveying.

1 nautical mile to land miles

Set From = Nautical mile, To = Mile, Value = 1. By definition 1 NM = 1852 m exact and 1 mi = 1609.344 m exact, so the conversion is 1852 / 1609.344 = 1.150779448… land miles. This is why aviation and maritime charts use nautical miles: at the equator, one minute of arc of longitude is almost exactly one nautical mile, and the difference vs the statute mile is the 15% that separates a sector chart from a road map. Both ends of the conversion are exact.

1 astronomical unit to kilometres

Set From = Astronomical unit, To = Kilometer, Value = 1. The IAU defined the AU as exactly 149,597,870,700 m in 2012 (replacing the older Gaussian-gravitational-constant definition), so the conversion is exact: 1 AU = 149,597,870.7 km. Earth–Sun distance at perihelion is about 0.983 AU = 147 million km; at aphelion about 1.017 AU = 152 million km. The Voyager 1 probe is currently about 165 AU from the Sun.

1 parsec to light-years

Set From = Parsec, To = Light-year, Value = 1. Parsec = 648000/π AU exactly (IAU 2015 Resolution B2). Light-year = Julian year × c, also exact by IAU definition. The ratio is 30,856,775,814,913,673 / 9,460,730,472,580,800 ≈ 3.26156 ly. This is the conversion every astronomy textbook quotes as "~3.26 light-years" — useful when you read that Proxima Centauri is 1.30 pc away and want it in the familiar light-year scale (4.24 ly). The result is tagged approximate because both ends are stored as rounded doubles inside the calculator, not because the underlying definition is fuzzy.

Comparative table: 1 unit in metres

Unit	Value in metres	Exact?
Ångström (Å)	1e−10	yes
Millimetre (mm)	1e−3	yes
Inch (in)	0.0254	yes
Foot (ft)	0.3048	yes
Yard (yd)	0.9144	yes
Metre (m)	1	yes
Kilometre (km)	1000	yes
Mile (mi)	1609.344	yes
Nautical mile (NM)	1852	yes
Astronomical unit (AU)	149,597,870,700	yes
Light-year (ly)	9,460,730,472,580,800	yes by def., rounded as double
Parsec (pc)	30,856,775,814,913,673	yes by def., rounded as double

Use this table to do conversions by hand: divide source-in-metres by target-in-metres. For example, 1 mile in ångström = 1609.344 / 1e−10 = 1.609344 × 10¹³ Å.

Most common length 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.

Metres to feet: 1 m = 3.28084 ft
Feet to metres: 1 ft = 0.3048 m (exact)
Metres to inches: 1 m = 39.3701 in
Inches to centimetres: 1 in = 2.54 cm (exact)
Centimetres to inches: 1 cm = 0.393701 in
Millimetres to inches: 1 mm = 0.0393701 in
Yards to metres: 1 yd = 0.9144 m (exact)
Metres to yards: 1 m = 1.09361 yd
Miles to kilometres: 1 mi = 1.60934 km
Kilometres to miles: 1 km = 0.621371 mi
Nautical miles to kilometres: 1 NM = 1.852 km (exact)
Nautical miles to miles: 1 NM = 1.15078 mi
Astronomical units to kilometres: 1 AU = 149,597,870.7 km
Light-years to kilometres: 1 ly ≈ 9.461 × 10¹² km
Parsecs to light-years: 1 pc ≈ 3.26156 ly

Length Conversion Tips

Pivot through the metre. Every conversion in this tool is implemented as "input × from-factor ÷ to-factor", so memorising the factor of each unit to the metre gives you every cross-conversion for free. The five exact factors worth memorising are inch (0.0254), foot (0.3048), yard (0.9144), mile (1609.344) and nautical mile (1852) — all defined exactly, all by international agreement.
Treat 3.28 feet per metre as a back-of-envelope approximation. The exact relationship is the inverse direction: 1 foot is exactly 0.3048 m. Going the other way, 1 m = 1/0.3048 = 3.28083989501… ft is irrational and any printed value is a rounding. When you need engineering accuracy, work in metres first and convert to feet only at the end.
Use the precision selector to match your domain. Auto-precision (6 significant figures) is fine for everyday conversions. Engineers checking against a datasheet usually want 10 decimals; metrologists comparing against a primary standard want 15 decimals (the full IEEE-754 double). Construction trades almost never need more than 4.
An "approximate" tag does not mean the underlying definition is fuzzy. Light-year and parsec are mathematically exact (IAU definitions, fixed values in metres) but their integer metre values are too large to fit a 64-bit double without rounding the last few digits. The calculator labels them approximate to be honest with you, not because Einstein left a typo somewhere.
Nautical miles are not the same as statute miles. 1 NM = 1852 m exact; 1 mi = 1609.344 m exact. The 15% difference is why aviation, marine and military charts use nautical miles — one minute of latitude on the Earth's surface is almost exactly one nautical mile, so a chart reading in nautical miles maps directly to angular position.
Ångström is alive and well in physics but not on the BIPM accepted list. Use it for visible-light wavelengths (4000–7000 Å), bond lengths (C–C single bond ≈ 1.54 Å), and X-ray crystallography. For everything modern (semiconductors, photonics), the SI unit is the nanometre, where 1 nm = 10 Å.
Confirm exactness when stakes are high. Surveying, aerospace tolerancing and primary metrology depend on knowing whether your conversion factor is a definition (no error) or a rounding (bounded error). NIST SP 811 Appendix B.8 prints the canonical list; this calculator reproduces the same boldface-exact convention with the "exact" badge.
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: writing 1 light-year in millimetres as 9,460,730,472,580,800,000 mm is unreadable, while 9.46073e18 mm is parseable at a glance.

Length Converter — Frequently Asked Questions

Is this length 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/length-converter is also free and ad-free, so you can drop it into engineering portals, school materials or technical blogs without exposing readers to third-party trackers.

How accurate are the conversion factors?

The metric, imperial and nautical-mile factors are exact, sourced from NIST SP 811 Appendix B.8 and the 1959 International Yard and Pound Agreement. The astronomical unit is exact per IAU 2012 Resolution B2. The light-year and parsec are exact by IAU definition but stored as 64-bit doubles, so the last few digits of their metre value are rounded — the calculator labels them approximate for that reason. At 15-decimal precision you are reading the full IEEE-754 mantissa.

Why does 1 metre to feet give an irrational decimal?

Because the exact definition runs the other way: 1 foot is exactly 0.3048 metres. The inverse 1/0.3048 = 3.28083989501312… ft/m is mathematically irrational — there is no last digit, only a longer truncation. Engineering and surveying tables almost always print 3.28084 or 3.281; this calculator will show whatever precision you ask for, up to 15 decimal digits.

What is the difference between an exact and an approximate conversion?

An exact conversion uses factors fixed by international definition (1 in = 0.0254 m, 1 mi = 1609.344 m, 1 AU = 149,597,870,700 m). The result has zero conversion error; any imprecision is in your input value. An approximate conversion uses a factor that is itself a rounding (3.28084 ft/m), which adds a small error of around 1 part in 10⁵ to every result. NIST SP 811 prints exact factors in boldface; this calculator tags them with an "exact" badge.

How many feet are in a metre?

One metre equals 3.280839895013… feet. The commonly quoted 3.28084 is a six-significant-figure rounding. The exact direction is the inverse: one foot is exactly 0.3048 metres, fixed by the 1959 International Yard and Pound Agreement. For most practical work, 3.28084 ft/m is enough; for surveying or aerospace, take more decimals.

Is 1 inch exactly 2.54 cm?

Yes. Since 1 July 1959 the inch is defined as exactly 25.4 mm, which is exactly 2.54 cm. This is one of the cleanest exact relationships in the whole imperial-to-metric table: it is the only common imperial unit whose metric value is a finite decimal. The American Standards Association adopted the 25.4 mm inch as early as 1933, well before the international agreement made it universal.

What is 1 light-year in miles?

About 5.878625 × 10¹² miles, or roughly 5.88 trillion miles. The exact computation is 9,460,730,472,580,800 m ÷ 1609.344 m/mi = 5,878,625,373,183.61 miles. The light-year itself is defined exactly by the IAU as the distance light travels in vacuum in one Julian year (365.25 days × 86400 s × 299,792,458 m/s), but the kilometre or mile value of that distance is a rounded double in any 64-bit computation.

How many astronomical units in a light-year?

Approximately 63,241.077… astronomical units. The IAU fixed both endpoints (1 AU = 149,597,870,700 m exact, 1 ly = 9,460,730,472,580,800 m exact), so the ratio is well-defined. Astronomers typically write it as ~63,241 AU or ~63,000 AU when context is loose. By comparison, Voyager 1 is currently about 165 AU from the Sun, or 0.0026 light-years.

What is a parsec and how many light-years is it?

A parsec is the distance at which one astronomical unit subtends an arcsecond of angular separation. IAU 2015 Resolution B2 fixed it as exactly 648,000/π AU, which works out to 3.26156 light-years (3.0857 × 10¹⁶ m). Astronomers prefer parsecs because the unit is geometrically tied to parallax measurements; the kiloparsec and megaparsec are the standard units for galactic and extragalactic distance scales.

Why use nautical miles instead of statute miles?

Because one nautical mile (1852 m exact) is almost exactly one minute of arc of latitude on the Earth's surface. A navigator reading a position fix can convert directly between angular position on a chart and distance over ground without a separate scale. A statute mile (1609.344 m) has no such geometric relationship; it is a land-survey legacy from medieval England.

Can I embed this converter on my site?

Yes. The embeddable version lives at /widget/length-converter; copy the iframe snippet from the embed page. The iframe is ad-free, dependency-free, mobile-responsive, and inherits no third-party trackers — unlike most length-converter embeds available today, which serve ads or tracking pixels inside the iframe.

How do I convert a measurement with mixed units like 5 ft 9 in?

Convert each part to a common unit first, then add. 5 ft 9 in = 5 × 0.3048 + 9 × 0.0254 = 1.524 + 0.2286 = 1.7526 m. For height work, the international convention is to convert to metres or to a single imperial unit (e.g. 69 inches), then enter that single value in the converter.

Key Length Conversion Terms

SI base unit

One of the seven units fixed by the International System of Units. The metre is the SI base unit for length, defined since 2019 as the distance light travels in vacuum in 1/299,792,458 of a second.

Exact factor

A conversion factor fixed by international definition, with zero rounding error. Examples: 1 inch = 0.0254 m, 1 mile = 1609.344 m, 1 AU = 149,597,870,700 m. NIST SP 811 Appendix B.8 prints exact factors in boldface.

International yard (1959)

The yard fixed as exactly 0.9144 metres by the International Yard and Pound Agreement of 1 July 1959, signed by the US, UK, Canada, Australia, New Zealand and South Africa. This single agreement defined the modern foot, inch, yard and mile in terms of the metre.

Nautical mile

A unit of length equal to exactly 1852 m, fixed by the First International Extraordinary Hydrographic Conference (Monaco, 1929). One nautical mile is approximately one minute of arc of latitude on the Earth's surface, which is why it remains standard for aviation and maritime navigation.

Astronomical unit (AU)

A unit of length equal to exactly 149,597,870,700 m, fixed by IAU Resolution B2 at the 2012 General Assembly. It replaced the older 1976 definition that tied the AU to the Gaussian gravitational constant.

Light-year

The distance light travels in vacuum in one Julian year (365.25 days). Per the IAU, exactly 9,460,730,472,580,800 m, derived from the exact speed of light (299,792,458 m/s) and the exact number of seconds in a Julian year (31,557,600 s).

Parsec

The distance at which one astronomical unit subtends an angle of one arcsecond. IAU 2015 Resolution B2 fixed the parsec as exactly 648,000/π astronomical units, equal to approximately 3.26156 light-years.

Ångström

A unit of length equal to exactly 10⁻¹⁰ metres (0.1 nanometres), named after the Swedish physicist Anders Jonas Ångström. Standard in atomic physics, X-ray crystallography and visible-light spectroscopy (visible spectrum: ~4000–7000 Å).

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 conversion factors are quoted to 6 or 7 significant figures.

Scientific notation

A way of writing very large or very small numbers as a coefficient times a power of ten, e.g. 1 light-year = 9.46073 × 10¹⁵ m. The calculator switches to this notation automatically when results fall outside the 10⁻³ to 10¹² range.

IEEE-754 double

The 64-bit floating-point format used by JavaScript and most calculators. It carries about 15–17 significant decimal digits. The 15-decimal precision setting in this tool exposes the full mantissa for double-checking against a primary reference.

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