straighten Length Unit Converter star

How It Works

Type a value into any field — millimeters, centimeters, meters, kilometers, inches, feet, yards, miles, or nautical miles — and the rest update instantly from exact conversion factors (1 inch = 2.54 cm exactly, 1 mile = 1609.344 m exactly), not rounded rules of thumb. There's no need to pick a "from" and "to" unit first; every unit is always visible at once.

Worked example: a 6-foot person is 182.88 cm; a 100-meter dash is 109.36 yards; a 10-inch tablet screen, measured diagonally, is 25.4 cm; a 26.2-mile marathon is 42,164.99 m.

What to Know

• The meter itself has been redefined. From 1889 to 1960 it was the length of a specific platinum-iridium bar kept near Paris — a physical object that could theoretically wear, shrink, or be damaged. Since 1983, the meter is defined as the distance light travels in a vacuum in 1/299,792,458 of a second. Because the speed of light is a universal physical constant, this definition can be reproduced in any properly equipped lab on Earth, with no risk of drift over time.
• Inches and centimeters convert exactly, not approximately: 1 inch has been defined as precisely 2.54 cm since a 1959 international agreement, so results in this tool carry no accumulated rounding error until the final display.
• Nautical miles are not the same as statute miles — this converter includes both because they serve different purposes: statute miles (1609.344 m) for roads and land distances, nautical miles (1852 m) for sea and air navigation.
• Feet and yards are also fixed against the meter, not independently defined: 1 foot is exactly 0.3048 m and 1 yard (3 feet) is exactly 0.9144 m, both set by the same 1959 international agreement that fixed the inch. That's why architectural drawings, lumber sizes, and sports field dimensions convert cleanly between metric and imperial without any hidden rounding.
• Showing every unit at once avoids re-rounding. Converting inches to meters and then meters to feet, as a two-step manual process, introduces a small rounding error at each step. This tool converts every unit directly from the entered value in one pass, so the displayed figures are as accurate as floating-point math allows, not compounded across intermediate steps.