Length Converter
Length is the conversion asked for most often, and the one where a stray factor of ten does the most damage. This converter moves between metric and imperial units in both directions, and the notes below cover the factors that are exact by definition rather than rounded.
Formula
metres = value × factor for the unit
- 1 inch = 25.4 mm exactly
- 1 foot = 0.3048 m
- 1 mile = 1,609.344 m
5 km in miles: 5,000 ÷ 1,609.344 = 3.107 miles
The Factors Are Exact, Not Approximate
Since the international yard and pound agreement of 1959, imperial lengths have been defined in terms of the metre. These are not measurements with error bars; they are exact:
- 1 inch = 25.4 mm exactly
- 1 foot = 0.3048 m exactly
- 1 yard = 0.9144 m exactly
- 1 mile = 1,609.344 m exactly
The awkward-looking decimals in the other direction — 1 m = 3.280839895 ft — are the reciprocals, and those genuinely do not terminate. Convert in the direction that uses the exact factor when precision matters.
Shortcuts Worth Memorising
For everyday estimates these get you within a couple of percent:
- Kilometres to miles: multiply by 5 and divide by 8 (0.625 against the true 0.621371)
- Metres to feet: multiply by 3.28, or add a bit over a quarter
- Centimetres to inches: divide by 2.5 and take off 2%
- A metre is 3 ft 3⅜ in, so a two-metre doorway is a shade over 6 ft 7 in
There is also a pleasing accident of arithmetic: consecutive Fibonacci numbers convert kilometres to miles. 5 km is about 3 miles, 8 km about 5, 13 km about 8. The ratio of consecutive Fibonacci numbers approaches 1.618, and the true km-to-mile ratio is 1.609.
Where Conversions Go Wrong
Three mistakes account for nearly all bad answers.
Mixing units inside one calculation. Measuring a room in metres and the skirting in centimetres, then multiplying, is invisible until the delivery arrives. Convert everything first, then calculate.
Reusing a linear factor for area or volume. A metre is 3.2808 feet, but a square metre is 10.764 square feet and a cubic metre is 35.315 cubic feet. The factor is squared or cubed, not reused.
Rounding too early. Rounding 2.54 to 2.5 looks harmless and introduces a 1.6% error that compounds through every subsequent step. Round once, at the end.
Quick Reference
| Unit | In metres | Reverse |
|---|---|---|
| 1 inch | 0.0254 | 1 m = 39.37 in |
| 1 foot | 0.3048 | 1 m = 3.2808 ft |
| 1 yard | 0.9144 | 1 m = 1.0936 yd |
| 1 mile | 1,609.344 | 1 km = 0.6214 mi |
| 1 nautical mile | 1,852 | 1 km = 0.5400 nmi |
Frequently Asked Questions
- How many feet are in a metre?
- 3.280839895 feet. The exact definition runs the other way: a foot is 0.3048 metres exactly, and 3.28084 is its reciprocal.
- What is the quickest way to convert kilometres to miles in my head?
- Multiply by 5 and divide by 8. That gives 0.625 against the true 0.621371, so you are within half a percent.
- Why is 1 square metre not 3.28 square feet?
- Because area units are squared. A square metre is 3.280839895 squared, which is 10.764 square feet. Cubic units cube the factor.
- Is an inch exactly 2.54 cm?
- Yes. It has been defined as exactly 25.4 millimetres since 1959, so the conversion is a definition rather than a measurement.