Length Converter
Convert between metric and imperial lengths, from millimetres to nautical miles.
The one number that defines the rest
One inch is exactly 2.54 centimetres. That is not a rounded measurement, it is a definition agreed in 1959 by the countries still using imperial units. Every foot, yard and mile conversion follows from it: a foot is 12 inches so 30.48 cm, a yard is 3 feet so 91.44 cm, and a mile is 1,760 yards so 1,609.344 metres.
Before 1959 the US and British inches differed very slightly, which mattered for surveying and engineering. The US survey foot was only retired at the end of 2022, and older land records may still use it.
Common conversions
| From | To | Multiply by |
|---|---|---|
| Inches | Centimetres | 2.54 |
| Centimetres | Inches | 0.3937 |
| Feet | Metres | 0.3048 |
| Metres | Feet | 3.2808 |
| Miles | Kilometres | 1.6093 |
| Kilometres | Miles | 0.6214 |
| Yards | Metres | 0.9144 |
Estimating without a calculator
For miles to kilometres, multiply by 1.6 or add about 60 percent. For kilometres to miles, multiply by 0.6. A 5K run is close to 3.1 miles, and 100 km/h is roughly 62 mph.
For centimetres to inches, divide by 2.5 and the answer will be within two percent. A 30 cm ruler is 11.8 inches, and the quick estimate gives 12.
Common questions
Length Unit Conversion Reference
Length conversions use exact defined values. The international inch is exactly 2.54 centimetres by definition. All other conversions in the metric and imperial systems follow from this anchor point and from the internal relationships within each system.
Key reference values
- 1 inch = 2.54 cm (exact, by definition)
- 1 foot = 30.48 cm (12 × 2.54)
- 1 yard = 91.44 cm (3 feet)
- 1 mile = 1,609.344 m (5,280 feet)
- 1 metre = 100 cm
- 1 kilometre = 1,000 m
- 1 nautical mile = 1,852 m
Metric system prefixes
The metric system uses the metre as its base unit. Prefixes scale it up and down by powers of ten:
- mm (millimetre): 0.001 m — used for small mechanical parts, thread sizes, rainfall depth
- cm (centimetre): 0.01 m — everyday measurements: height, screen size, fabric
- m (metre): 1 m — room dimensions, fabric lengths, athletic distances
- km (kilometre): 1,000 m — road distances, geographic coordinates
Imperial and US customary units
The imperial system uses inches, feet, yards, and miles. The US customary system is nearly identical for length. A common point of confusion is that the US survey foot (used in land measurement) differs very slightly from the international foot, but this difference matters only in high-precision land surveying — not in everyday conversions.
Common conversion tasks
- Height in cm to feet and inches: Divide centimetres by 2.54 to get total inches. Then divide by 12 to get feet and the remainder is inches. For example: 175 cm ÷ 2.54 = 68.9 in = 5 ft 8.9 in.
- Screen sizes: Screen diagonals are given in inches in most markets. A 15.6 inch laptop screen is about 39.6 cm.
- Running distances: A 5K race is 5,000 metres or 3.107 miles. A marathon is 42.195 km or 26.219 miles.
- Fabric and paper: Rolls of fabric are often sold in metres or yards. One yard is slightly shorter than one metre (0.9144 m).
Precision note
When you convert a measurement, the number of significant figures in your result should not exceed the number in your original measurement. A measurement of 10 cm (two significant figures) converts to 3.9 inches, not 3.93701 inches, because the original precision does not support six figures.