The Two Major Length Systems
Metric System (SI): Used by all countries except the USA, Liberia, and Myanmar. Based on powers of 10 — easy to calculate.
Imperial/US Customary System: Used primarily in the United States. Uses inches, feet, yards, and miles with irregular conversion factors.
Metric Length Units
- Kilometre (km): 1,000 metres. Used for road distances.
- Metre (m): The base unit of length in SI. About 3.28 feet.
- Centimetre (cm): 1/100 of a metre. Used for body measurements.
- Millimetre (mm): 1/1,000 of a metre. Used in engineering and manufacturing.
- Micrometre (μm): 1/1,000,000 of a metre. Used in science.
Imperial Length Units
- Mile (mi): 1,760 yards = 1.609 km. Road distances in the USA and UK.
- Yard (yd): 3 feet = 0.914 m. Used in football fields and fabric.
- Foot (ft): 12 inches = 30.48 cm. Used for human height in the USA.
- Inch (in): 2.54 cm. Used for screen sizes, pipe diameters, and clothing.
Key Conversion Factors to Remember
- 1 inch = 2.54 cm
- 1 foot = 30.48 cm = 0.3048 m
- 1 mile = 1.609 km
- 1 km = 0.621 miles
- 1 metre = 39.37 inches = 3.281 feet
How to Convert Length Units Online
1Open the Length Converter tool.
2Type a value in any unit field.
3All other units update instantly in real time.
💡 Quick Tip: To convert cm to inches mentally: divide by 2.54. To convert km to miles: multiply by 0.621 (or multiply by 5 and divide by 8).
FAQ
What is a nautical mile?
A nautical mile (nmi) is 1,852 metres (about 1.151 regular miles). It's based on the circumference of the Earth and is used in maritime and aviation navigation.