Length Unit Converter — 60+ Units
Convert any length between metric, imperial, US survey, nautical, astronomical, atomic and historical units. Enter a value, pick a source unit, and every other unit is computed live — with step-by-step working, a cosmic log-scale ruler, and reference tables down to the Planck length.
Every unit is a fixed multiple of the metre
Since 1983 the metre has been defined as the distance light travels in vacuum in 1/299,792,458 of a second. Every other length unit in this tool — from the ångström to the megaparsec — is defined as an exact or measured multiple of that one number. Converting between any two of them is therefore a single ratio: the source factor divided by the target factor. There is no separate conversion path for each pair, which is why 63 units only need 63 numbers rather than 3,969.
Some definitions are exact, some are measured
The inch, foot, yard, mile, nautical mile, chain, furlong and rod are exact multiples of the metre by international agreement, so conversions among them carry no error beyond floating-point roundoff. The astronomical unit is exact too: 149,597,870,700 m by IAU definition since 2012. But the parsec, light-year, solar radius, lunar distance and Earth radius involve measured or derived quantities, and their factors carry the uncertainty of the underlying measurement — typically a few parts in 10⁹ for the parsec and rather more for the lunar distance.
The scale spans 62 orders of magnitude
A Planck length is about 1.6 × 10⁻³⁵ m; the observable universe is about 8.8 × 10²⁶ m across. Between them lie 62 decades, and the units in this tool cover essentially all of them. That range is why the interface formats very large and very small numbers in scientific notation and why the slider is logarithmic: a linear scale from the Planck length to the universe would need 10⁶¹ tick marks to show even a single metre.
Watch the survey foot and the nautical mile
Two traps catch people regularly. First, the US survey foot is defined as 1200/3937 m ≈ 0.3048006096 m, which is about 2 parts per million longer than the international foot of 0.3048 m exactly — invisible on a house plot, but a metre of error over 500 km. Second, the nautical mile is 1,852 m exactly, not 1,609.344 m; using the statute-mile factor on a nautical distance gives an answer that is wrong by about 15 %. Both units are kept separate in this tool.
How to Use This Converter
Length conversion is a single ratio: the source unit's factor to the metre divided by the target unit's factor. Pick a source unit, enter a value, then pick any target — every other unit updates instantly.
1. Choose the unit you have
Filter by category — metric, imperial and US, nautical, astronomical, atomic, historical — or type in the search box. Matching units appear immediately in the grid below.
2. Enter the value
Type any number including scientific notation such as 6.371e6 or 1.5e-9, or drag the logarithmic slider to sweep from femtometres to petametres.
3. Pick a target in the big table
Every one of the 63 units is converted live in the full table beneath. Click any row to make that unit the featured target shown in the result card at the top right.
4. Read the working
The step-by-step panel shows the value converted to metres and then to the target, plus a round-trip check and a note on whether the definitions involved are exact or measured.
5. See where it sits on the cosmic ruler
The scale schematic places your length on a 62-decade logarithmic axis running from the Planck length to the observable universe, with named reference objects marked along it.
6. Switch to Advanced
Advanced mode adds a reference ladder chart, a sensitivity check, and companion metrics including the value in SI notation, orders of magnitude, and the nearest named object.
What This Tool Offers
Everything resolved through SI metres, so any of the 63 units converts to any other in one step — with no missing pairs and no rounding chains.
63 units, 6 categories
Metric prefixes from yocto to peta, imperial and US customary including the survey foot, nautical units, astronomical units up to the megaparsec, atomic units down to the Planck length, and fourteen historical units.
Live all-unit output
The full conversion table recomputes on every keystroke, so you never have to pick the right target before seeing the answer. Click any row to feature it.
62-decade cosmic ruler
A logarithmic schematic from 10⁻³⁵ m to 10²⁷ m with the Planck length, proton, atom, virus, hair, human, mountain, Earth, solar system, galaxy and observable universe all marked.
Searchable unit picker
Filter by category or type a name or symbol. Both "micrometre" and "µm" find the same unit, as do "thou", "mil" and "mils".
Exact vs measured flagged
The step panel tells you whether the conversion you just did rests on exact international definitions or on measured quantities with real uncertainty.
Reference tables
SI prefixes, exact imperial definitions, astronomical distances, approximate physical lengths, and a worked-example gallery.
Reference Tables & Conversions
A quick converter card, the SI prefix table, exact definitions, and physical reference lengths.
Quick converter — metres as the hub
SI Prefixes — the Metric Ladder
Every metric length unit is a power of ten times the metre. The prefixes below are the complete SI set recognised since 2022.
| Prefix | Symbol | Factor | Example unit | Typical object |
|---|---|---|---|---|
| quecto | q | 10⁻³⁰ | quectometre | Below the Planck length — no physical meaning |
| ronto | r | 10⁻²⁷ | rontometre | Far below any measurable length |
| yocto | y | 10⁻²⁴ | yoctometre | Neutrino wavelength scale |
| zepto | z | 10⁻²¹ | zeptometre | High-energy particle wavelengths |
| atto | a | 10⁻¹⁸ | attometre | LHC collision scale |
| femto | f | 10⁻¹⁵ | femtometre | Proton radius ≈ 0.84 fm |
| pico | p | 10⁻¹² | picometre | Atomic radii ≈ 50–300 pm |
| nano | n | 10⁻⁹ | nanometre | DNA width ≈ 2 nm |
| micro | µ | 10⁻⁶ | micrometre | Bacteria ≈ 1–10 µm |
| milli | m | 10⁻³ | millimetre | Raindrop, small screw |
| centi | c | 10⁻² | centimetre | Fingernail width |
| deci | d | 10⁻¹ | decimetre | Edge of a 1-litre cube |
| — | — | 1 | metre | SI base unit of length |
| deca | da | 10¹ | decametre | Height of a 3-storey building |
| hecto | h | 10² | hectometre | Length of a football pitch |
| kilo | k | 10³ | kilometre | 10-minute walk |
| mega | M | 10⁶ | megametre | Radius of a large planet |
| giga | G | 10⁹ | gigametre | Earth–Moon distance ≈ 0.384 Gm |
| tera | T | 10¹² | terametre | Earth–Sun distance ≈ 149.6 Tm |
| peta | P | 10¹⁵ | petametre | About 0.1 light-year |
| exa | E | 10¹⁸ | exametre | About 106 light-years |
| zetta | Z | 10²¹ | zettametre | About 106,000 light-years |
| yotta | Y | 10²⁴ | yottametre | About 106 million light-years |
| ronna | R | 10²⁷ | ronnametre | Comparable to the observable universe |
| quetta | Q | 10³⁰ | quettametre | Beyond the observable universe |
Exact Definitions — Imperial, US and Nautical
These units are exact multiples of the metre by international agreement, so conversions among them introduce no rounding error at all.
| Unit | Exact value in metres | Defined as | Notes |
|---|---|---|---|
| inch (in) | 0.0254 | 25.4 mm exactly | International agreement, 1959 |
| foot (ft) | 0.3048 | 12 inches | International foot |
| yard (yd) | 0.9144 | 3 feet | International yard |
| rod (rd) | 5.0292 | 16½ feet | Also called pole or perch |
| chain (ch) | 20.1168 | 66 feet = 4 rods | Gunter's chain |
| furlong (fur) | 201.168 | 220 yards = 10 chains | ⅛ of a mile |
| mile (mi) | 1,609.344 | 5,280 feet = 8 furlongs | International statute mile |
| league (lea) | 4,828.032 | 3 statute miles | Rarely used today |
| fathom (ftm) | 1.8288 | 6 feet | Water depth |
| cable (cb) | 185.2 | 1/10 nautical mile | International cable |
| nautical mile (nmi) | 1,852 | 1 minute of arc of latitude | International agreement, 1929 |
| US survey foot (ftUS) | 1200/3937 ≈ 0.3048006096 | 1200/3937 m | About 2 ppm longer than the international foot |
| US survey mile (miUS) | 5,280 × 1200/3937 ≈ 1,609.3472187 | 5,280 US survey feet | Being phased out; 2023 deadline for most uses |
| angstrom (Å) | 1 × 10⁻¹⁰ | 0.1 nanometre | Not an SI unit but widely used in chemistry |
Astronomical Distances
| Unit | Value in metres | Equivalent | Where it is used |
|---|---|---|---|
| light-second | 299,792,458 | exact | Earth–Moon ≈ 1.28 ls |
| light-minute | 1.7987547 × 10¹⁰ | 60 light-seconds | Sun–Earth ≈ 8.317 lm |
| astronomical unit (AU) | 149,597,870,700 | exact by IAU 2012 | Distances inside the solar system |
| light-hour | 1.07925285 × 10¹² | 3,600 light-seconds | Beyond Jupiter |
| light-day | 2.59020684 × 10¹³ | 86,400 light-seconds | Beyond the heliopause |
| light-year (ly) | 9.4607304725808 × 10¹⁵ | c × Julian year | Interstellar distances in popular science |
| parsec (pc) | 3.0856775814913673 × 10¹⁶ | 648,000/π AU | The preferred unit in professional astronomy |
| kiloparsec (kpc) | 3.0856775814913673 × 10¹⁹ | 1,000 pc | Distances within the galaxy |
| megaparsec (Mpc) | 3.0856775814913673 × 10²² | 10⁶ pc | Galaxy clusters and cosmology |
| solar radius (R☉) | 6.957 × 10⁸ | ≈ 0.00465 AU | Stellar sizes |
| Earth radius (mean) | 6.371 × 10⁶ | ≈ 3,959 mi | Planetary science |
| lunar distance (LD) | 3.844 × 10⁸ | ≈ 1.28 light-seconds | Mean Earth–Moon separation |
Physical Reference Lengths
Approximate sizes across the whole range of the calculator, useful for sanity-checking a result.
| Object | Approximate length | In metres | In other units |
|---|---|---|---|
| Planck length | 1.616 × 10⁻³⁵ m | 1.616e-35 | Quantum gravity scale |
| Proton radius | 0.84 fm | 8.4e-16 | 0.00084 pm |
| Hydrogen atom diameter | 106 pm | 1.06e-10 | 1.06 Å |
| Carbon–carbon bond | 154 pm | 1.54e-10 | 1.54 Å |
| DNA double helix width | 2 nm | 2e-9 | 20 Å |
| Typical virus | 100 nm | 1e-7 | 0.1 µm |
| E. coli bacterium | 2 µm | 2e-6 | 2,000 nm |
| Human red blood cell | 7 µm | 7e-6 | 7,000 nm |
| Human hair width | 70 µm | 7e-5 | 0.07 mm |
| Grain of fine sand | 0.25 mm | 2.5e-4 | 250 µm |
| Credit card thickness | 0.76 mm | 7.6e-4 | 0.030 in |
| AA battery length | 50.5 mm | 5.05e-2 | 1.99 in |
| Human height (mean) | 1.71 m | 1.71 | 5.61 ft |
| Football pitch length | 105 m | 105 | 115 yd |
| Eiffel Tower height | 330 m | 330 | 1,083 ft |
| Mount Everest height | 8,848.86 m | 8848.86 | 5.50 mi |
| Marathon distance | 42.195 km | 42195 | 26.219 mi |
| Earth diameter | 12,742 km | 1.2742e7 | 7,918 mi |
| Earth–Moon distance | 384,400 km | 3.844e8 | 1.28 light-seconds |
| Earth–Sun distance | 149.6 million km | 1.496e11 | 1 AU |
| Light-year | 9.4607 trillion km | 9.4607e15 | 0.3066 pc |
| Nearest star (Proxima) | 4.2465 ly | 4.017e16 | 1.302 pc |
| Milky Way diameter | ≈ 105,700 ly | 1.0e21 | ≈ 32 kpc |
| Andromeda distance | ≈ 2.537 million ly | 2.4e22 | ≈ 778 kpc |
| Observable universe diameter | ≈ 93 billion ly | 8.8e26 | ≈ 28.5 Gpc |
What Is a Length Unit Conversion?
A length unit conversion changes the number you use to describe a distance without changing the distance itself. A runway that is 3,000 metres long is also 3 kilometres long and 9,842.5 feet long — the same strip of tarmac, described three ways. What changes is only the size of the unit you are counting with.
Because every length unit in common use is defined as a fixed multiple of the metre, converting between any two of them is a single ratio. That is why this tool needs only one number per unit — its factor to the metre — rather than a separate conversion path for every possible pair. With 63 units there would otherwise be 3,969 pairs to define; with the hub-and-spoke approach there are 63.
The Length Conversion Formula
Converting a value from one unit to another is a two-step calculation through the SI metre:
target = metres ÷ factorto
which collapses to a single ratio:
target = value × (factorfrom ÷ factorto)
Two consequences follow directly. First, converting a unit to itself always returns the original value, because the ratio is 1. Second, converting and then converting back returns the original value exactly, because the two ratios are exact reciprocals — provided you do not round in between.
For the most common pairs the ratio has a familiar name:
1 in = 2.54 cm exactly · 1 ft = 30.48 cm exactly · 1 yd = 91.44 cm exactly
1 mi = 1.609344 km exactly · 1 nmi = 1.852 km exactly
1 Å = 10⁻¹⁰ m · 1 AU = 1.495978707 × 10¹¹ m · 1 ly = 9.4607304725808 × 10¹⁵ m
1 pc = 3.0856775814913673 × 10¹⁶ m = 3.26156 ly
How Each Category of Unit Behaves
Metric units form a perfectly regular ladder. Each prefix is a power of ten, so converting within the metric system is just moving the decimal point. A kilometre is a thousand metres, a millimetre is a thousandth, and there are no awkward factors anywhere. That regularity is the main practical argument for the metric system: 1 km = 1,000 m = 100,000 cm = 1,000,000 mm, with no fractions to remember.
Imperial and US customary units are historical accumulations and do not form a clean ladder. There are 12 inches in a foot, 3 feet in a yard, 5,280 feet in a mile and 8 furlongs in a mile. Because these ratios are integers, they are exact — but they are memorable only by repetition. Since 1959 all of them have been anchored to the metre by fixed definitions, which removed the last ambiguity between the British and American versions of each unit.
Nautical units are built around the geometry of the Earth. A nautical mile is one minute of arc of latitude along a meridian, fixed at exactly 1,852 metres in 1929. That definition makes it the natural unit for navigation: one minute of latitude on a chart corresponds to one nautical mile on the surface, so a navigator can read distance directly off the latitude scale without any arithmetic.
Astronomical units span an enormous range and are chosen for convenience at each scale. The astronomical unit covers the solar system, the light-year covers the neighbourhood of the Sun, and the parsec — the distance at which one astronomical unit subtends one arcsecond — covers everything beyond. Because the parsec is defined geometrically it is the preferred unit in professional astronomy, and the light-year survives mainly in popular writing.
Atomic and quantum units are all defined in terms of physical constants. The ångström is exactly 10⁻¹⁰ m by definition; the Bohr radius is derived from the electron mass, charge and Planck's constant; the Planck length combines the gravitational constant, Planck's constant and the speed of light. Each is the natural yardstick at its own scale, and each is measured rather than agreed.
Worked Examples
metres = 5 × 1000 = 5,000 m
miles = 5,000 ÷ 1609.344 = 3.10686 mi
feet: 6 × 30.48 = 182.88 cm
inches: 2 × 2.54 = 5.08 cm
total = 187.96 cm
metres = 100 × 10⁻⁶ = 0.0001 m
inches = 0.0001 ÷ 0.0254 = 0.0039370 in
metres = 9.4607304725808 × 10¹⁵ m
kilometres = 9.4607304725808 × 10¹² km
parsecs = 9.4607304725808 × 10¹⁵ ÷ 3.0856775814913673 × 10¹⁶ = 0.306601 pc
26 mi = 26 × 1609.344 = 41,842.944 m
385 yd = 385 × 0.9144 = 352.044 m
total = 42,194.988 m ≈ 42.195 km
metres = 3.0856775814913673 × 10¹⁶ m
light-years = 3.0856775814913673 × 10¹⁶ ÷ 9.4607304725808 × 10¹⁵ = 3.26156 ly
Frequently Asked Questions
Sixty-three units in six categories. Metric runs from the yoctometre (10⁻²⁴ m) to the petametre (10¹⁵ m) across all eighteen SI prefixes plus the base metre. Imperial and US customary covers thou, inch, hand, foot, yard, rod, chain, furlong, mile, league and the US survey foot and mile. Nautical covers fathom, cable and nautical mile. Astronomical runs from the Earth radius through the light-second, lunar distance, solar radius, astronomical unit, light-year and up to the megaparsec. Atomic covers the Planck length, x-unit, Bohr radius and ångström. Historical covers fourteen older units including the cubit, span, pace, ell, arshin, sazhen, verst, shaku and li.
It depends entirely on the pair. Conversions among metric units are exact because every prefix is a power of ten and the arithmetic is a shift of the decimal point. Conversions among imperial, US and nautical units are also exact because all of those units have been defined as exact multiples of the metre since 1959 — the inch is exactly 0.0254 m, the mile exactly 1,609.344 m, the nautical mile exactly 1,852 m. Astronomical conversions involving the light-year, parsec, solar radius or lunar distance are not exact: those factors rest on measured quantities, and the parsec in particular carries an uncertainty of about one part in 10¹¹. The step panel tells you which situation applies to the conversion you just did.
The international foot is exactly 0.3048 metres, agreed in 1959. The US survey foot is exactly 1200/3937 metres, which is about 0.3048006096 m — roughly two parts per million longer. The difference is invisible on a house plot but accumulates: over a mile it is about 3.2 mm, and over 500 kilometres it reaches about a metre. The survey foot was historically used for land surveying and mapping in the United States; in 2022 the National Institute of Standards and Technology retired it in favour of the international foot for all new work, though it remains in some older records and in a few state plane coordinate systems.
Because it was built for a different purpose. The statute mile is a historical land measure fixed at 1,609.344 m. The nautical mile is one minute of arc of latitude along a meridian, fixed at exactly 1,852 m by international agreement in 1929. Its advantage is navigational: on a Mercator chart the latitude scale is graduated in minutes, and one minute of latitude equals one nautical mile on the surface. A navigator can therefore measure a distance on the chart with dividers and read it directly off the latitude scale without any conversion. Speed in nautical miles per hour is a knot.
Because it falls directly out of the measurement astronomers actually make. Stellar distances are found by parallax: photograph a star six months apart and measure how far it appears to shift against the background. That shift is an angle in arcseconds. A parsec is defined as the distance at which one astronomical unit subtends one arcsecond, so the distance in parsecs is simply the reciprocal of the parallax in arcseconds — d = 1/p. No conversion factor is needed at all. The light-year, by contrast, is defined from the speed of light and the length of a year, which is a more familiar idea but less convenient to compute with. Professional astronomy therefore uses parsecs and its multiples; popular writing uses light-years.
Convert the two parts separately and add them. Five feet is 5 × 0.3048 = 1.524 m; eight inches is 8 × 0.0254 = 0.2032 m. The total is 1.7272 m, or 172.72 cm. In this tool you can do it in one step by converting 5 feet to metres and 8 inches to metres, or by entering 68 inches (5 × 12 + 8) directly, since the inch is the common sub-unit. The same approach works for any mixed-unit measurement: convert each part to a common unit and sum.
The Planck length is about 1.616 × 10⁻³⁵ m, built from the gravitational constant, Planck's constant and the speed of light. It is not a measured quantity but a natural scale: it is the length at which quantum gravitational effects are expected to become important, and it is the smallest length at which the ordinary notion of distance is thought to be meaningful. It is often described as the smallest possible length, but that phrasing is informal — current physics simply has no description of what distance means below that scale, and several candidate theories of quantum gravity suggest that spacetime itself may be discrete at that level.
All arithmetic uses IEEE-754 double-precision floating point, which carries about 15 to 16 significant decimal digits. Because every unit is stored as a single factor to the metre and every conversion is one multiplication and one division, at most one rounding error is introduced beyond the representation of the factors themselves. That means the result is accurate to the limit of double precision for any conversion among exactly defined units. For units whose factors are themselves measured — the parsec, the lunar distance, the solar radius — the dominant uncertainty is in the physical measurement, not in the computation.
Variables
| Symbol | Quantity | Unit |
|---|---|---|
| miles | Distance in miles | mi |
| km | Distance in kilometers | km |
What it means
This formula converts distances from kilometers (km) to miles (mi). The conversion factor is approximately 0.621371 (exactly 1 km = 0.621371192 miles). The mile is a unit of length used primarily in the United States, United Kingdom, and a few other countries, while kilometers are used in most other countries and in scientific contexts. This conversion is commonly used in road signs, car speedometers (mph vs. km/h), and for running/cycling distances. Accurate conversion is important for travel planning, fuel efficiency calculations (miles per gallon vs. litres per 100 km), and for comparing performance data. The inverse conversion (miles to km) uses 1.60934. Understanding this factor helps in navigation and in communicating distances across different measurement systems.
Worked example
Kilometers to Miles – Two Examples
Real‑World| Parameter | Value |
|---|---|
| km | 10 |
| Parameter | Value |
|---|---|
| km | 50 |
Common mistakes
- Conversion factor: 1 mile = 1.60934 km – multiply km by 0.621371 to get miles.
- Significant figures: Use appropriate precision for the context.
- Direction: This converts km to miles – for miles to km, use 1.60934.
- Nautical miles: Do not confuse with statute miles – this formula is for statute miles.
- Applications: Used in travel, sports, and mapping.
Applications
The conversion from kilometres to miles, miles = km × 0.621371, is essential for travel, navigation, and international trade. Many countries use kilometres, while the US and UK use miles for road distances and speed limits. Travelers, logistics planners, and pilots frequently convert between these units to plan routes and fuel consumption. In automotive engineering, vehicle speedometers often display both units. This conversion is also used in sports, such as running and cycling events, where distances may be given in either unit. Accurate conversion ensures safety, efficiency, and clear communication in global transport and commerce.
- Route planning and navigation for road trips and air travel
- Vehicle speedometer calibration and driver information systems
- Logistics and supply chain management for international shipping
- Sports event planning and performance tracking (marathons, cycling)
- Geographic information systems (GIS) and mapping applications
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