Length Converter

How to use the length converter

Type a value, pick the unit you have on the left and the unit you want on the right, and the answer updates as you type. Every pairing in the list works: millimeters to inches, yards to meters, kilometers to nautical miles, inches to kilometers if you have a reason. The swap control flips the two dropdowns and keeps your number, which is the fast way to sanity-check a conversion in both directions.

Results show six significant figures, so a small number keeps its decimals and a large one does not become a wall of zeros. Nine units are available — millimeter, centimeter, meter, kilometer, inch, foot, yard, mile and nautical mile — and every one of them is priced in meters internally.

Every imperial figure here traces back to one document. On 1 July 1959, standards bodies in the United States, United Kingdom, Canada, Australia, New Zealand and South Africa signed the International Yard and Pound Agreement, which defined the yard as exactly 0.9144 meters. That single number fixes the rest by simple division.

None of these are measurements with error bars — they are definitions. That is why 12 inches converts to 30.48 cm with nothing left over, and why the factors have not moved in over sixty years.

The nautical mile is the odd unit in the list, because it is not a land measure at all. It is exactly 1,852 meters, set at the First International Extraordinary Hydrographic Conference in Monaco in 1929, and that value was chosen to approximate one minute of arc of latitude.

The link is the whole point: one minute of latitude is one nautical mile, so a navigator reads distance straight off the latitude scale printed down the side of a chart. Because the Earth is not a sphere, a real minute of latitude runs from roughly 1,842 m near the equator to about 1,861 m near the poles, and 1,852 m is the agreed average. In land terms that is 1.150779 statute miles, or 6,076.12 feet.

Which American foot is in your deed?

A second foot existed in the United States until recently, and it is worth knowing about if you touch land records. The US survey foot was the older 1893 value, defined as 1200/3937 meter — about 0.30480061 m, two parts per million longer than the international foot. On a tape measure the gap is invisible. On a State Plane coordinate running into the millions of feet it is about two feet per million, enough to move a property line.

The formula, worked line by line

Nine units make thirty-six possible pairings, but there is only one calculation behind them. Every unit declares how many meters it is worth, so any conversion goes into meters first and back out into the target unit second. The meter is the pivot, and no pairwise lookup table is needed.

Written out, that is one multiplication followed by one division. Because the factors are exact definitions rather than measured approximations, the pivot introduces no error of its own — the only rounding happens when the answer is printed.

meters = value × (meters per from-unit)
result = meters ÷ (meters per to-unit)
result = value × from.perBase ÷ to.perBase
mi → nmi: result = value × 1609.344 ÷ 1852

Two factors are worth carrying in your head, because they are exact and everything else derives from them: an inch is 25.4 mm and a yard is 0.9144 m. The metric-to-imperial numbers people quote are not exact and should be treated as rounded — a meter is 3.280839895 feet, a kilometer is 0.621371192 miles. Multiplying by a truncated version drifts over distance: using 3.28 instead of 3.280840 across 10,000 meters loses about 8.4 feet.

Questions people ask