When to use this converter
Use it for anything measured in a line: a person’s height in feet and inches versus centimeters, furniture that must fit a space listed in metric, the distance of a race or a road trip, or a nautical chart that measures in nautical miles. Choose the unit you have, the unit you need, and read the result instantly — no button press required. Measuring a room? The square footage calculator multiplies length by width for you.
Where the numbers come from
Since the 1959 international yard and pound agreement, the inch has been defined as exactly 25.4 millimeters. Every other US customary length follows from it: 12 inches to the foot, 3 feet to the yard and 5,280 feet to the mile, which makes a mile exactly 1,609.344 meters. The nautical mile is defined separately as exactly 1,852 meters. Because these are definitions rather than measurements, the conversions have no uncertainty at all.
Tips for accurate results
Keep full precision until the last step: round only the number you actually report. For height, convert the total in inches (for example 5 ft 10 in = 70 in) rather than converting feet and inches separately. Lengths are magnitudes, so this converter accepts zero and positive values only. For floor space and land, use the area converter.
Worked examples
Height: someone who is 5 ft 10 in tall is 70 inches, and 70 × 2.54 = 177.8 cm. Going the other way, 180 cm ÷ 2.54 is about 70.87 inches, or roughly 5 ft 10.9 in. See centimeters to feet and feet to centimeters for full tables.
Running: a 10K is 10 km ÷ 1.609344 ≈ 6.21 miles, and a marathon is 42.195 km, about 26.2 miles. The kilometers to miles page lists the standard race distances.
Hardware: a 1/2-inch board is 0.5 × 25.4 = 12.7 mm thick, so a 12 mm metric sheet is slightly thinner. Check the inches to millimeters table before mixing metric and inch parts.