Length Converter
Convert between 13 length units from nanometres to light-years. Live ruler shows relative lengths at a glance.
From
To
Result
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in metres
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in feet
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in km
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Quick Presets
Length Comparison
All Equivalent Lengths
Enter a value above to see all equivalents.
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Length Units Reference
| Unit | Symbol | = metres |
|---|---|---|
| Nanometre | nm | 1 × 10⁻⁹ |
| Micrometre | µm | 1 × 10⁻⁶ |
| Millimetre | mm | 0.001 |
| Centimetre | cm | 0.01 |
| Metre | m | 1 |
| Kilometre | km | 1,000 |
| Inch | in | 0.0254 |
| Foot | ft | 0.3048 |
| Yard | yd | 0.9144 |
| Mile | mi | 1,609.344 |
| Nautical Mile | nmi | 1,852 |
| Astronomical Unit | AU | 1.496 × 10¹¹ |
| Light-Year | ly | 9.461 × 10¹⁵ |
How to use this calculator
Two fields. One swap button. The formula line below tells you exactly what math ran.
FROM field — Type your value. The converter accepts whole numbers, decimals, and scientific notation (useful for nanometres and astronomical distances). Any number you type updates the result instantly.
Unit dropdown (FROM) — Pick the unit your value is in. 13 options: nanometre, micrometre, millimetre, centimetre, metre, kilometre, inch, foot, yard, mile, nautical mile, astronomical unit, and light-year.
Unit dropdown (TO) — Pick the unit you want the result in. The converted value appears in the TO field in blue, with full precision.
Formula line — Below the input fields, the calculator shows exactly what conversion factor it applied. For metres to feet: ft = m × 3.2808399. This is your audit trail. If a result looks off, check the formula line to confirm the direction of conversion is correct.
Swap button (↕) — Flips FROM and TO units in one click. Useful when you’ve just converted m to ft and now need ft to m.
Result panel — The blue output card shows the primary result in large type, plus parallel readouts in metres, feet, and km simultaneously. Saves you running three conversions when you just need a quick orientation across unit systems.
Length comparison bars — A scaled visual showing the relative lengths of your FROM and TO values. When the two units are close in scale (like metres and yards), the bars look nearly identical. When they’re far apart (metres and nanometres), the scale difference becomes obvious immediately.
Quick example — 1 metre to feet
FROM: 1, unit: m / TO unit: ft
Formula shown: ft = m × 3.2808399
Result: 3.2808399 ft
Also shown in result panel: 1.0000000 m / 3.2808399 ft / 0.0010000 km
The formula line uses the full precision factor, not a rounded one. If you’re doing chained conversions (metres to inches, then inches to points for typography), use the full unrounded result from each step as your input to the next. Rounding early compounds into meaningful error over several steps.
The 13 units and where each one actually gets used
Most people use 4 or 5 length units throughout their entire life. The others exist for contexts most people never encounter. Here’s the full set.
| Unit | Symbol | In metres | Used for |
|---|---|---|---|
| Nanometre | nm | 0.000000001 m | Chip features, light wavelengths, DNA structure |
| Micrometre | µm | 0.000001 m | Cells, bacteria, machining tolerances, dust |
| Millimetre | mm | 0.001 m | Engineering drawings, rainfall, small hardware |
| Centimetre | cm | 0.01 m | Body measurements, clothing sizes, furniture |
| Metre | m | 1 (base unit) | Room dimensions, athletics, general measurement |
| Kilometre | km | 1,000 m | Road distances, geography, running races |
| Inch | in | 0.0254 m | US construction, screen sizes, pipe fittings |
| Foot | ft | 0.3048 m | Altitude, US building dimensions, human height |
| Yard | yd | 0.9144 m | American football, fabric, golf |
| Mile | mi | 1,609.344 m | Road distances in US and UK |
| Nautical mile | nmi | 1,852 m | Aviation, maritime navigation, weather charts |
| Astronomical unit | AU | 1.496 × 10¹¹ m | Solar system distances, planetary orbits |
| Light-year | ly | 9.461 × 10¹⁵ m | Interstellar and galactic distances |
The metre is the SI base unit. Every other unit in this table is defined by a fixed ratio to the metre — not the other way around. A foot is exactly 0.3048 metres by international definition since 1959. An inch is exactly 25.4 mm. A nautical mile is exactly 1,852 metres. These are exact values, not approximations.
The conversion formula
Every length conversion is a single multiplication by a fixed factor.
For metres to feet: factor = 1 / 0.3048 = 3.2808399…
For kilometres to miles: factor = 1000 / 1609.344 = 0.6213712…
For inches to centimetres: factor = 0.0254 / 0.01 = 2.54 (exact)
The metre-as-base-unit approach means you can find any conversion factor by dividing one unit’s metre value by another’s. You don’t need a lookup table. You need two numbers from the table above and one division.
Inches to centimetres is one of the few conversions with an exact integer-adjacent factor. 1 inch = 2.54 cm exactly. This makes it a useful anchor. A 6-inch phone screen is exactly 15.24 cm. A 12-inch ruler is exactly 30.48 cm. When in doubt about whether a metric-imperial conversion is running correctly, check a known inch-to-cm value first.
Common conversions and their exact factors
Quick reference for the conversions people actually search for.
| Convert from | Convert to | Multiply by | Reverse (divide by) |
|---|---|---|---|
| Metres | Feet | 3.28084 | 0.30480 |
| Metres | Inches | 39.3701 | 0.02540 |
| Kilometres | Miles | 0.62137 | 1.60934 |
| Miles | Kilometres | 1.60934 | 0.62137 |
| Feet | Metres | 0.30480 | 3.28084 |
| Inches | Centimetres | 2.54000 | 0.39370 |
| Yards | Metres | 0.91440 | 1.09361 |
| Nautical miles | Kilometres | 1.85200 | 0.53996 |
| Nautical miles | Miles | 1.15078 | 0.86898 |
| Feet | Centimetres | 30.4800 | 0.03281 |
The “reverse” column is just 1 divided by the forward factor. Kilometres to miles: × 0.62137. Miles to kilometres: × 1.60934 (which is 1 / 0.62137). You only ever need to remember one direction.
Real examples where the right conversion matters
Construction and architecture
A UK architect receives a US contractor’s drawing with dimensions in feet and inches. The main span reads 24’6” (24 feet 6 inches).
24 feet 6 inches = 24.5 feet
24.5 ft × 0.30480 = 7.4676 metres
Structural drawings submitted to UK building control need this in mm: 7,467.6 mm or 7.468 m.
The 6-inch fraction is the part people drop. 24 feet converts to 7.3152 m. The extra half-foot adds 152.4 mm. On a structural span, that matters.
Aviation altitude
An aircraft is flying at 35,000 feet. The weather report gives cloud base in metres.
35,000 ft × 0.30480 = 10,668 metres = 10.668 km
Cloud base reported at 2,400 m = 2,400 / 0.30480 = 7,874 feet
Clearance between aircraft and cloud base: 35,000 - 7,874 = 27,126 feet
Aviation in most of the world uses feet for altitude (a legacy of US aviation dominance in the post-war period). Meteorology uses metres. Pilots and dispatchers run this conversion constantly.
Screen size in different units
A laptop is advertised as a 15.6-inch display. Dimensions for a case need to be in centimetres.
15.6 in × 2.54 = 39.624 cm
Screen diagonal: 39.624 cm. Actual screen dimensions depend on aspect ratio (16:9 for most laptops):
Width = 39.624 × (16 / √(16² + 9²)) = 39.624 × 0.8720 = 34.55 cm Height = 39.624 × (9 / √(16² + 9²)) = 39.624 × 0.4905 = 19.43 cm
Case interior needs at least 35 × 20 cm. The diagonal-to-dimension calculation happens before the unit conversion, but the inch-to-cm step is exact and simple.
Nautical distance
A yacht is sailing from Gibraltar to the Canary Islands. The chart shows the distance as 630 nautical miles.
630 nmi × 1.85200 = 1,166.76 km
At 8 knots average speed (8 nautical miles per hour): 630 / 8 = 78.75 hours sailing time
Nautical miles and knots are inseparable. 1 knot = 1 nautical mile per hour. Speed and distance in the same unit means time calculations stay clean. Converting to km would require converting speed to km/h simultaneously, which adds unnecessary steps.
Semiconductor manufacturing
A chip architect specifies a transistor gate length of 3 nanometres (modern leading-edge process node).
3 nm in other terms:
3 nm × 0.001 = 0.003 µm (3 thousandths of a micrometre)
3 nm in inches = 3 × 3.93701 × 10⁻⁸ = 1.18 × 10⁻⁷ inches
A human hair is roughly 70,000 nm wide. A 3nm transistor gate is about 23,000 times narrower than a single human hair. The nanometre scale is where the converter’s scientific notation handling matters. Standard calculators either can’t express these values or lose precision.
The metric-imperial divide: why two systems still coexist
The United States, Myanmar, and Liberia are the only countries that haven’t officially adopted the metric system as their primary measurement standard. Practically, this means engineers, product designers, scientists, and anyone working internationally needs to convert constantly.
The situation is more nuanced than “US uses imperial, everyone else uses metric.” Aviation uses feet globally. Nautical navigation uses nautical miles globally. UK road distances use miles. Screens and TVs are measured in inches worldwide. Carpentry in the US uses feet and inches, but sheet goods are sold in 4x8 foot sheets that translate to approximately 1220x2440mm for export specs.
No single system is dominant across all domains. The converter exists because the domain-by-domain patchwork is genuinely complicated.
A metre is defined as the distance light travels in 1/299,792,458 of a second. A foot is exactly 0.3048 metres. An inch is exactly 25.4 millimetres. These aren't approximations set by committee. They're internationally agreed exact values that have been fixed since 1959. Every conversion this tool runs is exact within floating-point precision limits.
Units people confuse most often
Miles vs nautical miles. A nautical mile (1,852 m) is longer than a statute mile (1,609 m) by about 15%. A ship traveling 100 nautical miles has gone 115 statute miles. Mixing them up in range or fuel calculations is a real navigational error.
Metres vs yards. 1 yard = 0.9144 m. Close enough that casual estimates work, but a 100-yard American football field is only 91.44 metres. “About 90 metres” is fine for conversation. An engineering spec at 91.44 m needs the exact figure.
Feet vs feet and inches. 5’10” is not 5.10 feet. It’s 5 feet and 10 inches = 5 + (10/12) = 5.833 feet. The fractional notation trips up any calculation that treats the “10” as a decimal. Convert feet-and-inches to decimal feet or to inches first, then convert to metric.
Micrometres vs millimetres. Machining tolerances and biological cell sizes sit in the micrometre range. A tolerance of ±0.05 mm is 50 µm. A human red blood cell is about 8 µm. These are two orders of magnitude apart and people write them interchangeably by accident.
The feet-and-inches problem trips up spreadsheets and manual calculations constantly. 6’2” entered as 6.2 feet gives 188.98 cm. The correct answer is 187.96 cm. The 1 cm error comes from treating the inches portion as a decimal fraction of a foot. Always convert inches to the fraction of a foot (divide by 12) before using the decimal foot value in any formula.
The astronomical units: when normal numbers stop working
Astronomical Unit (AU) and light-year exist because the distances they describe are too large for any other unit to express usefully.
Earth to Sun: 1 AU = 149,597,870,700 metres. Written in km that’s 149,597,870.7 km. In miles: 92,955,807 miles. All of those are technically correct and all are too unwieldy to use in calculations. “1 AU” is cleaner in every way.
Proxima Centauri, the nearest star system: 4.243 light-years. In km that’s 4.014 × 10¹³ km. In AU it’s 268,000 AU. The light-year collapses interstellar distances into numbers a human can process.
The converter handles these. Entering 1 light-year and converting to metres returns 9.461 × 10¹⁵ m. Entering 1 AU in nanometres returns 1.496 × 10²⁰ nm. These are exact within double-precision floating-point limits.
The bottom line
Length conversion is multiplication by a fixed factor. The converter automates that multiplication, shows you the factor, and outputs the result with full precision.
The units that catch people are the ones that look similar but aren’t: miles and nautical miles, feet-and-inches vs decimal feet, micrometres and millimetres. The formula line below the converter shows which direction the conversion ran and what factor was applied. If a result looks wrong, that line is where you check first.
For anything technical, use the full-precision result rather than a rounded one. 1 metre is 3.2808399 feet, not 3.28. That difference is 0.0008399 feet per metre. At 1,000 metres, it’s 0.8399 feet of accumulated error. At the scale of engineering and construction, it’s the difference between drawings that align and drawings that don’t.
Frequently Asked Questions
How do I convert feet and inches to centimetres?
First convert feet to inches: multiply feet by 12. Add extra inches. Then multiply total inches by 2.54 to get centimetres. Example: 5 ft 10 in = (5 × 12 + 10) × 2.54 = 70 × 2.54 = 177.8 cm. The reverse: divide cm by 2.54 to get total inches, then divide by 12 for feet with the remainder as inches.
What is a nautical mile and why is it different from a regular mile?
A nautical mile (nmi) = 1852 m = 1.15078 statute miles. It was originally defined as one arc-minute of latitude along the Earth's meridian, making navigation calculations simpler: a ship traveling 60 nautical miles covers exactly 1° of latitude. Today it is used universally in aviation and maritime navigation. The speed unit "knot" is 1 nautical mile per hour.
How far is one astronomical unit (AU)?
1 AU = exactly 149,597,870,700 m ≈ 150 million km ≈ 93 million miles. It is defined as the average distance from Earth to the Sun. Mars is about 1.52 AU from the Sun; Jupiter is 5.2 AU; Neptune is 30 AU. The edge of the solar system (heliopause) is about 100–120 AU from the Sun. The Voyager 1 probe is now over 165 AU away.
What is a light-year?
A light-year is the distance light travels in one year in a vacuum: exactly 9,460,730,472,580,800 m ≈ 9.461 × 10¹⁵ m ≈ 63,241 AU. The nearest star system (Alpha Centauri) is 4.37 light-years away. The Milky Way galaxy is about 100,000 light-years in diameter. The observable universe is about 93 billion light-years in diameter.
How small is a nanometre?
A nanometre (nm) = 1 billionth of a metre (10⁻⁹ m). A human hair is about 80,000–100,000 nm wide. A red blood cell is about 8,000 nm. A DNA double helix is 2 nm wide. Modern computer chips have features as small as 3–7 nm. Visible light wavelengths range from 380 nm (violet) to 700 nm (red).
What is the difference between a mile and a kilometre?
1 mile (statute mile) = 1,609.344 m = 1.609 km. 1 km = 0.621371 miles. Quick mental math: to convert km to miles, multiply by 0.6 (rough) or divide by 1.6. To convert miles to km, multiply by 1.6. Highway speed limits: 60 mph ≈ 97 km/h; 70 mph ≈ 113 km/h; 100 km/h ≈ 62 mph; 130 km/h ≈ 81 mph.
What is a micrometre (micron)?
A micrometre (µm, colloquially called a "micron") = 10⁻⁶ m = 0.001 mm = 1000 nm. Human hair: 50–100 µm. Pollen: 10–100 µm. A human cell: 10–30 µm. Red blood cell: 6–8 µm. Bacteria: 1–10 µm. PM2.5 air pollution particles: 2.5 µm. Micrometres are the standard unit in manufacturing tolerances and semiconductor lithography.
How does the metric system's length units relate to each other?
Each SI prefix multiplies by 1000: 1 km = 1000 m; 1 m = 1000 mm; 1 mm = 1000 µm; 1 µm = 1000 nm. Between mm and m: 1 m = 100 cm = 1000 mm. The SI standard is built on powers of 10, making conversions straightforward. A centimetre is 0.01 m; a decimetre is 0.1 m (less common but used in some countries).
What is the relationship between yards and metres?
1 yard (yd) = 3 feet = 36 inches = exactly 0.9144 m. 1 m = 1.09361 yd. A metre is about 9.4% longer than a yard. The 100-metre dash is approximately 109 yards. American football fields are 100 yards (91.44 m) between goal lines. The yard was historically defined as the distance from the tip of the king's nose to the outstretched middle finger.
What is the most precise length measurement possible?
The most precise length measurements use laser interferometry. The International Bureau of Weights and Measures (BIPM) can measure lengths with uncertainties below 1 picometre (10⁻¹² m). The metre is defined by fixing the speed of light at exactly 299,792,458 m/s. The smallest measurable length (the Planck length) is about 1.616 × 10⁻³⁵ m.