How to use the Length Converter
- Type the number you want to convert into the value field.
- Choose the unit that number is currently expressed in.
- Choose the unit you want it expressed in.
- Read the headline conversion, plus three common alternatives shown underneath.
- For heights in feet and inches, use the dedicated height converter instead — it takes both parts separately.
How the calculation works
Every unit here is stored as an exact multiple of the metre, which is itself defined by the distance light travels in a vacuum in 1/299,792,458 of a second. The conversion runs in two steps: the input is multiplied by its factor to reach metres, then divided by the target unit's factor. Doing it through a single base unit rather than a table of pairwise ratios eliminates the rounding drift that appears when conversions are chained.
The imperial factors are exact rather than approximate. Since the 1959 international yard and pound agreement, the yard is defined as exactly 0.9144 metres, which makes the inch exactly 25.4 mm, the foot exactly 0.3048 m and the statute mile exactly 1609.344 m. Older US survey units differ in about the eighth significant figure and were officially retired at the end of 2022, so unless you are working with pre-2023 land surveys the international definitions are the ones you want.
The nautical mile is the odd one out: at exactly 1852 metres it derives from one minute of arc along a meridian rather than from any land-based unit. It survives because it makes chart work trivial — one minute of latitude is one nautical mile anywhere on the globe — which is why aviation and marine navigation kept it while everything else standardised.
value_target = (value_source × factor_source) ÷ factor_target, where factor is the unit's exact length in metres (in = 0.0254, ft = 0.3048, mi = 1609.344, nmi = 1852)Source: International Yard and Pound Agreement (1959); NIST Special Publication 811; BIPM SI Brochure, 9th edition.
Worked example
A build drawing specifies a 12 ft 6 in beam and the fabricator works in millimetres.
- Convert the whole span to inches: 12 × 12 + 6 = 150 in.
- Enter 150 and select inches as the source unit.
- Select millimetres as the target: 150 × 25.4 = 3,810 mm.
- Cross-check in metres: 3.81 m, or 3.81 ÷ 0.3048 = 12.5 ft, confirming the round trip.
12 ft 6 in is exactly 3,810 mm — an exact figure, not a rounded one, because the inch is defined in millimetres.
Frequently asked questions
Is the inch exactly 25.4 mm?+
Yes. Since 1959 it is a definition, not a measurement, so conversions between inches and millimetres are exact rather than approximate.
What is the difference between a statute mile and a nautical mile?+
A statute mile is 1,609.344 m; a nautical mile is 1,852 m, about 15% longer. Nautical miles are used at sea and in the air because one equals one minute of latitude.
Should I use US survey feet?+
Almost never. The survey foot was deprecated on 31 December 2022. It differs from the international foot by roughly 2 parts per million, which only matters across very long survey baselines.
Why do results show more decimals for small numbers?+
The display adapts precision to magnitude so that small conversions keep useful significant figures instead of rounding to zero.
Last reviewed September 1, 2026. We review this page whenever the underlying formula, tax year, published rate or standard changes.