Length Converter
Convert between meters, feet, inches, centimeters, kilometers, miles, and more length units instantly.
100 Centimeters (cm) =
39.370079
Inches (in)
What is Length and Distance Conversion?
A conversion factor is a statement that two different descriptions name the same physical thing. Saying "1 inch = 25.4 mm" does not mean two quantities are similar; it means they are the same length written two ways. That is why you can multiply by a conversion factor freely — you are multiplying by one.
This framing makes unit conversion mechanical rather than memorised. To convert 5 metres to feet, multiply by a fraction equal to 1 that has metres on the bottom and feet on top. The metres cancel like any algebraic term and feet remain.
It also explains why some conversions are exact and others are not, which turns out to be a matter of historical definition rather than measurement precision.
The formula — how to calculate Length and Distance Conversion
- The factor
- = always arranged so the unit you are leaving cancels and the unit you want survives
- Exact vs approximate
- = imperial-metric conversions have been exact by international agreement since 1959
Write the factor as a fraction and check that units cancel before multiplying. If the unit you wanted to remove is still there, the fraction is upside down — which is the single most common conversion error.
Step-by-step example
- 01Convert 5 metres to feet.
- 02Start from the exact definition: 1 ft = 0.3048 m, so 1 = 1 ft / 0.3048 m.
- 035 m × (1 ft / 0.3048 m) — the metres cancel — = 5 / 0.3048 = 16.404 ft.
- 04Convert 6 feet 2 inches to centimetres.
- 05Work in a single unit first: 6 ft × 12 = 72 in, plus 2 = 74 in.
- 0674 in × 2.54 cm/in = 187.96 cm.
- 07Convert 100 kilometres to miles: 100 km × (1 mile / 1.609344 km) = 62.137 miles.
- 08Notice the pattern: the factor always goes in whichever orientation cancels the unit you are leaving. There is nothing to memorise beyond the factor itself.
Why the inch is exactly 25.4 millimetres
Until 1959 the inch was defined slightly differently in different countries, and the discrepancies mattered for precision engineering and international trade.
That year, the national standards bodies of the English-speaking countries agreed the International Yard and Pound: the yard would be exactly 0.9144 metres, which makes the inch exactly 25.4 millimetres.
The consequence is that imperial-metric length conversions are exact rather than approximate. 25.4 is not a rounded measurement — it is a definition, in the same way that 12 inches to the foot is a definition. Any imprecision in a converted figure comes from your original measurement, never from the factor.
The United States retained a separate survey foot for land surveying, differing by about two parts per million, precisely because redefining it would have shifted every existing property boundary. It was officially retired in 2022, though historic survey records still use it.
Common length conversions (all exact)
| From | To | Multiply by |
|---|---|---|
| Inches | Centimetres | 2.54 |
| Feet | Metres | 0.3048 |
| Yards | Metres | 0.9144 |
| Miles | Kilometres | 1.609344 |
| Nautical miles | Kilometres | 1.852 |
The nautical mile is defined differently again — as 1,852 metres, chosen to approximate one minute of latitude, which is why it is used in navigation and aviation.
Why unit errors are expensive
Dimensional analysis — tracking units through a calculation as though they were algebra — is the discipline that catches these mistakes, and skipping it has a costly history.
The Mars Climate Orbiter was lost in 1999 because one team supplied thrust data in pound-force seconds while the receiving software expected newton-seconds. Neither number was wrong; they simply described different quantities, and nothing in the pipeline checked the units.
The practical habit is simple: carry units through every step and confirm the final unit is the one you wanted. If you calculate a speed and the units come out as metres, something upstream is wrong regardless of how reasonable the number looks.
Check the direction before you multiply
Ask whether the answer should be a bigger or smaller number. Converting metres to feet must give a larger number, because a foot is smaller than a metre and it takes more of them. If your answer moved the wrong way, your factor is inverted — the fastest way to catch the most common conversion error.
Precision and significant figures
Converting does not create precision. A room measured to the nearest foot as "12 ft" is not suddenly known to the millimetre because 3.6576 m appears on a screen.
The sensible convention is to report a converted figure to roughly the precision of the original measurement. Twelve feet, measured with a tape to the nearest inch, converts honestly to about 3.66 m — not 3.6576.
This matters most in construction and manufacturing, where a spuriously precise converted dimension implies a tolerance nobody actually achieved, and can send someone chasing accuracy that was never present in the source measurement.
Key considerations
- Write conversion factors as fractions and confirm the units cancel.
- Imperial-metric length conversions are exact by definition, not approximations.
- Convert compound measurements to a single unit before converting systems.
- Report converted values to the precision of the original measurement.
- Nautical miles are a separate definition (1,852 m) used in navigation and aviation.
- Check whether the answer should get larger or smaller as a direction test.
Common mistakes to avoid
- Inverting the conversion factor, which produces an answer wrong by the square of the factor.
- Converting feet and inches without first combining them into one unit.
- Reporting a converted figure to more decimal places than the original measurement supports.
- Confusing nautical miles with statute miles in navigation contexts.
- Failing to track units through multi-step calculations.
- Assuming 25.4 is a rounded value when it is an exact definition.
Frequently asked questions
Sources & references
Written and fact-checked by the CalcProLabs Editorial Team. Read our calculation methodology and editorial policy.
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