Length Calculator
What the length calculator does
The length calculator converts a measurement between eight common units: metres, centimetres, millimetres, kilometres, inches, feet, yards and miles. Type the value, pick the unit you have and the unit you want, and press Calculate. The result is shown to four decimal places.
How the conversion works
Every unit is first converted to metres, then from metres to the target unit. The factors are the international definitions fixed by the 1959 agreement between English-speaking countries, in which the yard was set as exactly 0.9144 m:
| Unit | Symbol | Metres |
|---|---|---|
| Millimetre | mm | 0.001 |
| Centimetre | cm | 0.01 |
| Metre | m | 1 |
| Kilometre | km | 1000 |
| Inch | in | 0.0254 (exact) |
| Foot | ft | 0.3048 (exact) |
| Yard | yd | 0.9144 (exact) |
| Mile | mi | 1609.344 (exact) |
So 12 inches × 0.0254 = 0.3048 m, which is one foot, and 5,280 feet make one mile.
Worked examples
| Input | Output | Working |
|---|---|---|
| 180 cm | 70.87 in | 180 × 0.01 ÷ 0.0254 |
| 6 ft | 1.8288 m | 6 × 0.3048 |
| 10 km | 6.2137 mi | 10,000 ÷ 1609.344 |
| 100 yd | 91.44 m | 100 × 0.9144 |
| 2.5 mi | 4.0234 km | 2.5 × 1.609344 |
| 3 mm | 0.1181 in | 0.003 ÷ 0.0254 |
Quick mental conversions
- 1 inch is about 2.5 cm; 4 inches is about 10 cm.
- 1 foot is about 30 cm; 10 feet is about 3 m.
- 1 metre is about 1.1 yards, or 3 feet 3 inches.
- 1 mile is about 1.6 km; 5 miles is about 8 km.
- To go from km to miles, multiply by 0.6; from miles to km, multiply by 1.6.
Where each unit is used
Metric units are standard in science, engineering and most countries’ everyday life. Inches and feet remain common for height, screen sizes and construction in the United States, and for some building materials elsewhere. Yards are used on golf courses and American football fields, and miles for road distances in the U.S. and the U.K. Mixed usage is why conversion errors are common: a plan drawn in millimetres read as inches is 25 times too small.
Precision and significant figures
The calculator shows four decimal places, which is more precision than most measurements justify. A tape measure reads to about 1 mm, so a 2.4 m wall converted to feet should be quoted as 7.87 ft, not 7.8740 ft. Keep the extra digits only during intermediate steps, and round the final answer to match the precision of the original measurement.
Units this calculator does not cover
For very small or very large lengths, other tools on the site fill the gaps: the gauge to mm converter for wire and sheet metal, the pixel to inches converter for screens and print, the nautical mile to km converter for sea and air distances (1 nautical mile = 1,852 m), and the light year converter for astronomy. Feet and inches in mixed form, such as 5 ft 11 in, are handled by the feet to inches calculator and the height converter.
Common mistakes
- Choosing the same unit twice. The result simply equals the input.
- Confusing the survey foot. U.S. land surveys used a foot of 0.3048006 m until 2022. The difference is 2 parts per million, negligible except over many kilometres.
- Entering feet and inches as a decimal. 5 ft 6 in is 5.5 ft, not 5.6 ft.
- Mixing statute and nautical miles. A nautical mile is about 15% longer.
External references
Frequently asked questions
Are the conversion factors exact?
Yes for inch, foot, yard and mile, which are defined in terms of the metre. Metric prefixes are exact by definition.
How many centimetres are in an inch?
Exactly 2.54. To convert inches to centimetres, multiply by 2.54; to go the other way, divide by 2.54.
How do I convert a height like 5 ft 9 in?
Convert the inches part first: 9 in = 0.75 ft, so the height is 5.75 ft. Entering 5.75 ft gives 1.7526 m. The height converter does this in one step.
Why does 1 km show as 0.6214 miles and not 0.6?
0.6 is a mental shortcut. The exact factor is 1 ÷ 1.609344 = 0.621371.
Can I convert areas or volumes with this tool?
No. Area and volume factors are the length factor squared or cubed. Use the dedicated converters in the converters section.