Length Converter: Metric, Imperial & Tape Measure
Convert between millimetres, centimetres, metres, kilometres, inches, feet, yards, miles and nautical miles. See every unit at once, plus the nearest tape-measure fraction of an inch.
Written by Suhaib Hassan. Figures are estimates you can edit. Our methodology.
It converts through the metre using the exact international definitions, so there is no rounding until the display.
Most converters give a decimal. On a job you want a tape-measure reading, and a quick look at the same length in every unit.
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Length conversion factors
Every length here is defined exactly in terms of the metre. Since 1959 the international yard has been exactly 0.9144 m, and so the inch is exactly 25.4 mm.
| Unit | Symbol | Metres in one | Status |
|---|---|---|---|
| Millimetres | mm | 0.001 | exact |
| Centimetres | cm | 0.01 | exact |
| Metres | m | 1 | exact |
| Kilometres | km | 1,000 | exact |
| Micrometres | µm | 1e-06 | exact |
| Inches | in | 0.0254 | exact |
| Feet | ft | 0.3048 | exact |
| Yards | yd | 0.9144 | exact |
| Miles | mi | 1,609.344 | exact |
| Nautical miles | nm | 1,852 | exact |
Formulas
How the numbers are worked out.
Convert through metres
Multiply by the source unit's length in metres, then divide by the target unit's length in metres.
result = value × (metres per source ÷ metres per target)Tape measure reading
Turn the length into inches and round to the nearest fraction.
fraction = round(inches × N) ÷ N, N = 8, 16, 32 or 64Exact definitions
These relationships are exact by definition, not approximations.
1 in = 25.4 mm
1 mi = 1,609.344 m
1 NM = 1,852 mWorked examples
Illustrative figures, each checked by hand. The first is the default setup.
240 mm to inches
The default.
tape: 9 7/16 in (9.4375), 0.0113 in under
Marathon
26.2 miles to kilometres.
Construction
100 feet to metres.
How to convert lengths
- Enter the value and choose the units to convert from and to.
- Read the converted length and the tape-measure reading.
- Check the table for the same length in every other unit.
- Use decimal places to control precision.
Conversions between these units are exact. Tape readings are rounded to the nearest fraction you choose, so they can differ from the true length by a fraction of that size.
By Suhaib Hassan · Reviewed 29 September 2026 · How CalculatePilot verifies formulas →
Sources
- NIST, Special Publication 811, Guide for the Use of the International System of Units (SI), Appendix B.
- BIPM, The International System of Units (SI Brochure), 9th edition.
- International Yard and Pound Agreement, 1959.
Frequently asked questions
How many millimetres are in an inch?
Exactly 25.4. This has been the international definition since 1959.
How many feet are in a mile?
5,280 international feet, which is exactly 1,609.344 metres.
What is a nautical mile?
Exactly 1,852 metres. It is about one minute of latitude and is used in aviation and at sea.
How do I read a tape measure fraction?
The result is the whole inches plus the nearest fraction, such as 9 7/16. Choose 1/32 or 1/64 for finer work.
Why does the converter use metres in the middle?
Converting every unit to metres first and then to the target avoids compounding rounding errors across steps.
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