Length is the most-converted physical quantity, and the units span an enormous range — from nanometres in chip design to astronomical units in space science. This reference lets you enter a value in any one unit and instantly read it in every other, using exact conversion factors rather than rounded constants.
How it works
Every unit is stored as an exact number of metres per unit, so the metre acts as a common base. Converting from one unit to another is two steps:
metres = value × metresPerUnit[from]
result = metres ÷ metresPerUnit[to]Because all units route through the same metre base, there is no error from chaining conversions. The imperial units use their exact legal definitions: 1 inch = 0.0254 m, 1 foot = 0.3048 m, 1 yard = 0.9144 m, 1 mile = 1609.344 m. The nautical mile is exactly 1852 m (exactly one minute of arc on the Earth’s meridian), and the astronomical unit is the IAU 2012 definition of 149,597,870,700 m.
Quick reference table
| Unit | Exact factor (metres) | Common use |
|---|---|---|
| Nanometre (nm) | 0.000000001 | Chip fabrication, wavelengths |
| Millimetre (mm) | 0.001 | Engineering tolerances |
| Centimetre (cm) | 0.01 | Everyday measurement |
| Metre (m) | 1 | SI base unit |
| Kilometre (km) | 1000 | Road distances |
| Inch (in) | 0.0254 | US/UK engineering |
| Foot (ft) | 0.3048 | Aviation altitude, US heights |
| Yard (yd) | 0.9144 | Sports fields (American football) |
| Mile | 1609.344 | US/UK road distances |
| Nautical mile (nmi) | 1852 | Maritime, aviation |
| Astronomical unit (AU) | 149,597,870,700 | Solar-system distances |
Worked examples
Converting 5 miles to kilometres: 5 × 1609.344 = 8046.72 m = 8.04672 km. A quick mental check: multiply miles by 1.609 for km, or roughly 1.6 for a fast estimate.
Converting 6 feet to metres: 6 × 0.3048 = 1.8288 m. The inch-to-centimetre factor is exact and worth memorising: 1 in = 2.54 cm, so 1 ft = 30.48 cm.
Converting 250 nautical miles to kilometres: 250 × 1852 = 463,000 m = 463 km. A knot (nautical mile per hour) is about 1.852 km/h, which is useful for fuel-burn calculations.
Scale reference — when to use which unit
- Chip features and DNA: nanometres
- Manufacturing tolerances, screw threads: millimetres
- Room dimensions, fabric, body measurements: centimetres or inches
- Running, swimming distances: metres
- Road navigation: kilometres or miles
- Ocean and air navigation: nautical miles
- Planetary distances: astronomical units
Values far above or below the metre scale appear in scientific notation in the output (for example a nanometre as 1e-9 m) so the numbers remain readable at all scales.
Why exact factors matter
Using rounded constants (for example 1 in = 2.54001 cm) introduces systematic error that compounds when you convert a design across several units. The international standards specify exact legal definitions — the inch has been exactly 25.4 mm since 1959 — so exact arithmetic is not only possible, it is the correct approach.