Length & Distance Converter
Convert between metric and imperial length units — millimetres, centimetres, metres, kilometres, inches, feet, yards, and miles.
How to use this converter
Pick the unit you're starting from on the left, the unit you want on the right, and type a value. The converted figure updates as you type. The swap button (⇄) flips the two sides instantly — handy when you want to convert in the other direction.
The eight units covered
Metric: millimetre (mm), centimetre (cm), metre (m), kilometre (km). Each is a power-of-ten step from the next, which is why metric arithmetic is nearly trivial. Imperial:inch (in), foot (ft = 12 in), yard (yd = 3 ft), mile (mi = 1760 yd = 5280 ft). The imperial relationships are historical artefacts (a yard was once a stride), so they don't scale neatly — just memorise them once.
The most useful conversions to know cold
1 inch = 2.54 cm exactly. This is the keystone — every other inch-to-metric conversion derives from it. 1 foot ≈ 30 cm. Close enough for everyday estimation; the exact figure is 30.48 cm. 1 metre ≈ 3.28 ft. Multiply metres by 3 to estimate feet, then add ~10%. 1 mile ≈ 1.6 km. Marathons are 26.2 mi or 42.2 km — same race. 1 km ≈ 0.62 mi. A 5K run is roughly 3.1 miles.
Where these conversions matter
Travel: posted speed limits in km/h vs mph, distance to the next exit in km vs mi, and altitude in m vs ft on aviation apps. Cooking and recipes: cm-based pan sizes vs imperial inches. Fitness: race distances (5K, 10K, half marathon, full marathon) and treadmill mph vs outdoor min/km. Construction and DIY: stud spacing (16" standard in the US, 400 mm in most of the world), board lengths, fittings.
The 1959 international agreement
Until 1959, the US, UK, Canada, Australia, and New Zealand each defined the inch and foot slightly differently. The International Yard and Pound Agreement standardised them at exactly 25.4 mm per inch — meaning every imperial-metric conversion is now globally consistent. The calculator uses the post-1959 definitions everywhere, so a metre converted to feet matches what a US carpenter, a UK joiner, and an Australian builder all see.
Frequently asked questions
How accurate are the conversions?▾
Exact, within standard floating-point precision. Imperial-to-metric conversions use the international yard and pound definitions adopted in 1959 — 1 inch = exactly 25.4 mm, 1 foot = 304.8 mm, 1 mile = 1609.344 m. These are agreed-upon definitions, not approximations, so a result of 5.08 cm from 2 inches is mathematically exact.
What's the difference between a US mile and a UK mile?▾
Identical for everyday purposes. The 1959 international agreement standardised the mile at exactly 1609.344 m for both. The historical US survey mile was a tiny bit longer (about 3 mm per mile) and was officially retired in 2022. Marine and air contexts use the nautical mile, which is a different unit (1852 m) — not included here.
Why isn't there a fathom, league, or rod option?▾
We stuck to the eight units that cover ~99% of practical needs: mm, cm, m, km on the metric side; inches, feet, yards, miles on the imperial side. Niche units like fathoms (6 ft, marine), leagues (3 mi, archaic), and rods (5.5 yd, surveying) are deliberately omitted to keep the dropdown clean. If you need them, the conversion factors are stable and easy to look up.
Does this work with very small or very large values?▾
Yes. The math runs in 64-bit floating point, accurate to about 15 significant digits. You can convert nanometre-scale (input as fractions of a mm) up to interplanetary-scale (input as kilometres). Display rounds to 6 significant digits — enough for any everyday use without confusing trailing noise.
How do I convert between feet and inches with mixed input?▾
Enter the value in one unit at a time — 5 feet 6 inches becomes either 66 inches or 5.5 feet. To convert mixed feet-and-inches, do it in two steps: convert the inches portion to feet (6 in ÷ 12 = 0.5 ft), add to the feet portion (5.5 ft total), then convert. Many architectural and construction calcs work this way.
Is centimetre or centimeter correct?▾
Both — "centimetre" is the international/British spelling and "centimeter" is the US spelling. The unit is identical. SI itself uses the "-metre" spelling. We use the international spelling in labels for consistency with the metric system's origin.
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