Length Conversion Chart
Every common length conversion in one place, with the exact factors behind them.
Key length conversions: 1 inch = 2.54 cm exactly, 1 foot = 30.48 cm, 1 yard = 0.9144 m, 1 mile = 1.609344 km. Reverse: 1 cm = 0.3937 in, 1 m = 3.28084 ft, 1 km = 0.621371 mi. Metric conversions are powers of 10; imperial ones use the exact defined factors.
The chart
Imperial to metric: 1 inch = 2.54 cm (exact). 1 foot = 30.48 cm. 1 yard = 0.9144 m. 1 mile = 1.609344 km. Metric to imperial: 1 millimeter = 0.03937 in. 1 centimeter = 0.393701 in. 1 meter = 3.28084 ft (or 39.3701 in). 1 kilometer = 0.621371 mi.
Within systems: metric is powers of 10 (10 mm = 1 cm, 100 cm = 1 m, 1,000 m = 1 km). Imperial: 12 in = 1 ft, 3 ft = 1 yd, 1,760 yd = 1 mi, 5,280 ft = 1 mi.
Why 2.54 is exact
The inch is defined as exactly 2.54 centimeters by the 1959 international yard and pound agreement. That makes every imperial-to-metric length conversion exact, not measured: feet, yards, and miles all derive from the inch definition.
This matters in engineering and machining, where thousandths of an inch convert cleanly to hundredths of a millimeter. A 1/4-inch bolt is exactly 6.35 mm, which is why metric and imperial fasteners almost but never quite interchange.
Mental-math rules
Inches to cm: multiply by 2.5 (close to 2.54). Feet to meters: divide by 3.3 (or multiply by 0.3). Miles to km: multiply by 1.6. Km to miles: multiply by 0.6. Cm to inches: divide by 2.5.
For running: a 5K is 3.1 miles, a 10K is 6.2 miles, a half marathon is 13.1 miles, a marathon is 26.2 miles. For driving: 60 mph is about 97 km/h, 100 km/h is about 62 mph.
When precision matters
Everyday use: two decimals are plenty (1.61 km per mile). Construction: work in the units the plans use rather than converting back and forth, since each rounded conversion adds error. Machining and 3D printing: convert once with full precision at the start.
The golden rule: convert once, at the end, from source to target directly. Chaining rounded intermediates (miles to feet to inches to cm) accumulates rounding error that a single direct conversion avoids.
Skip the arithmetic
Convert any length precisely with the free unit converter.
Length conversion questions
How many centimeters are in an inch?
Exactly 2.54 centimeters per inch. The inch has been defined as exactly 25.4 millimeters since the 1959 international yard and pound agreement, so this conversion is exact, not approximate.
How many kilometers are in a mile?
Exactly 1.609344 kilometers per international mile. For everyday estimates, multiplying miles by 1.6 gets you within half a percent.
How many feet are in a meter?
About 3.28084 feet per meter, which is roughly 3 feet and 3.37 inches. Going the other way, a foot is exactly 0.3048 meters.