Length Converter

How length units are defined

Every unit in this converter is defined from the meter, the SI unit of length. Since 1983 the meter has been the distance light travels in a vacuum in 1/299,792,458 of a second. Metric units step up and down in powers of ten: a kilometer is 1,000 m, a meter is 100 cm or 1,000 mm, and a millimeter is 1,000 micrometers.

US customary and British imperial lengths are the same units, and they too are defined from the meter. In 1959 the national standards laboratories of the United States, the United Kingdom, Canada, Australia, New Zealand and South Africa agreed on an international yard of exactly 0.9144 m. Everything else follows from that figure with no rounding:

• 1 foot = ⅓ yard = 0.3048 m
• 1 inch = 1/12 foot = 2.54 cm
• 1 mile = 1,760 yards = 5,280 feet = 1,609.344 m

The nautical mile is separate: it is exactly 1,852 m, chosen because it is close to one minute of latitude, which makes it convenient on charts. Ships and aircraft measure distance in nautical miles and speed in knots (nautical miles per hour).

The conversion formula

Converting a length means multiplying by the ratio of the two unit sizes:

value

the length you enter

size

one unit expressed in meters, e.g. 0.0254 m for an inch and 0.01 m for a centimeter

For inches to centimeters the factor is 0.0254 ÷ 0.01 = 2.54, a short exact number, so the calculator multiplies by it. In the other direction the factor would be 1 ÷ 2.54 = 0.393700787…, a decimal that never ends, so the calculator divides by 2.54 instead. That is why the working shows “÷ 2.54” for centimeters to inches: the figure you see is the defined one, not a rounded copy.

Results that come out as a finite decimal are shown in full. Others are rounded to 10 significant digits, and very small or very large numbers switch to scientific notation (for example 1 micrometer in miles is about 6.2 × 10⁻¹⁰ mi).

Worked examples

10 centimeters to inches. An inch is exactly 2.54 cm, so divide: 10 ÷ 2.54 = 3.937007874 in, just under 3 ¹⁵⁄₁₆ inches on a ruler marked in sixteenths.

5 miles to kilometers. A mile is 1,609.344 m and a kilometer 1,000 m, so the factor is 1.609344: 5 × 1.609344 = 8.04672 km, an exact answer. (A “5K” race is 5 km, about 3.106855961 miles.)

5 feet 9 inches to centimeters. Mixed feet and inches have to be turned into one unit first: 5 × 12 + 9 = 69 inches, and 69 × 2.54 = 175.26 cm. The BMI calculator accepts feet and inches directly and does this step for you.

Common length conversions

Every conversion from US units to metric is exact. The rounding only appears when going from metric to US units, because 2.54 does not divide evenly into powers of ten.

Mistakes to avoid

• Decimal feet are not feet and inches. 5.9 ft is 5 ft 10.8 in, not 5 ft 9 in; 5 ft 9 in is 5.75 ft. Convert heights to inches (or use the inch field) before converting.
• Rounded factors drift. Using 3.28 ft per meter on a 400 m running track gives 1,312 ft; the exact figure is 1,312.335958 ft. Over long distances or large quantities, use the full factor.
• Old US land surveys may use a different foot. The US survey foot is 1200/3937 m, about two parts per million longer than the international foot, a difference of roughly 3.2 mm per mile. NIST retired it on 1 January 2023, but older deeds and state plane coordinates can still be based on it. This converter uses the international foot.
• A mile at sea is longer. Nautical miles are about 15% longer than statute miles, so a boat’s “10 miles” on a chart is 18.52 km, not 16.09 km.