How to Convert Length Units

Formula

result = value × factor, where factor = meters in one from unit ÷ meters in one to unit.

• value: the length you want to convert, in the from unit.
• meters in one from unit: the size of the from unit in meters; 0.0254 for the inch.
• meters in one to unit: the size of the to unit in meters; 0.01 for the centimeter.
• factor: how many to units make one from unit.

The sizes are exact by definition (NIST SP 811, Appendix B; NIST Handbook 44, Appendix C):

The inch, foot, yard and mile have been tied to the meter since the 1959 international yard and pound agreement. That is why these factors are exact, not rounded.

Step-by-step example

Convert the 55-inch diagonal of a TV screen to centimeters:

1. The from unit is the inch: 1 in = 0.0254 m. The to unit is the centimeter: 1 cm = 0.01 m.
2. Find the factor: 0.0254 ÷ 0.01 = 2.54. One inch is 2.54 centimeters.
3. Multiply the value by the factor: 55 × 2.54 = 139.7.
4. The result: 55 in = 139.7 cm. The same length is 1.397 m and about 4.58 feet.

Common conversions

Common mistakes

• Mixing up the mile and the nautical mile. The mile is 1,609.344 m; the nautical mile is 1,852 m, or about 1.1508 miles. Distances at sea and in the air use the nautical mile.
• Multiplying in the wrong direction. To go from centimeters to inches, do not multiply by 2.54; divide by it. The factor is 0.01 ÷ 0.0254, about 0.3937. Always divide the size of the from unit by the size of the to unit.
• Using a rounded factor. 1 m ≈ 3.28 ft is fine for a guess, but the error grows with the distance. The length converter uses the exact factors and keeps 12 significant digits.
• Expecting US survey units. The converter uses the international foot and mile. The old US survey foot and mile differ very slightly and are not included.