Length

Nine units, and most of the relationships between them are exact by international agreement rather than measured. Since the International Yard and Pound Agreement took effect on 1 July 1959, one inch is exactly 25.4 mm and one yard is exactly 0.9144 m. Every imperial figure in this converter follows from those two definitions with no rounding at all.

The converter opens on 1 mile, which is 1,609.344 m — again exact, since a mile is 5,280 feet and a foot is 0.3048 m by definition. Type into any box and the rest update.

The metre itself is the one unit that is not defined against another length: since 1983 it has been the distance light travels in vacuum in 1/299,792,458 of a second, which turned the speed of light into a defined constant rather than something to be measured.

The nine units, and where the odd numbers come from

Metric. Millimetre, centimetre, metre and kilometre, each a factor of ten or a thousand from the next. Engineering drawings almost always dimension in millimetres precisely to avoid the decimal-point ambiguity that centimetres invite.

Imperial. Twelve inches to a foot, three feet to a yard, 5,280 feet to a mile. That last figure looks arbitrary until you follow it back through surveying: a chain is 66 feet, ten chains make a furlong of 660 feet, and eight furlongs make a mile. Eighty chains to the mile, and the same 66-foot chain is why an acre is 43,560 square feet — see the Area Converter.

Nautical mile. Exactly 1,852 m, fixed at the International Hydrographic Conference in Monaco in 1929 and adopted by the United States in 1954. It approximates one minute of arc of latitude: the meridian runs about 40,008 km, and dividing by 21,600 minutes gives 1,852.2 m. That approximation is the reason marine and air navigation still use it — on a paper chart the latitude scale down the side doubles as a distance ruler.

The US survey foot, and where two parts per million bites

Before 1959 the American foot was defined as 1200/3937 of a metre, or 0.3048006096 m. The 1959 agreement adopted 0.3048 exactly for general use but let the older value survive for land surveying under the name US survey foot. The two differ by about two parts per million, which is a millionth of nothing over a room and a serious problem over a state.

State Plane Coordinate System values run into the millions of feet. At a coordinate of 1,000,000 ft the two definitions of the foot differ by roughly 2 ft — enough to put a parcel corner, a pipeline centreline or a bridge abutment in the wrong place. Surveyors who mixed the two definitions in a single project have moved property boundaries by more than the width of a driveway.

NIST and the National Geodetic Survey retired the US survey foot on 31 December 2022; from 1 January 2023 the international foot is the only official foot in the United States. Deeds and plats recorded before that date may still carry survey-foot coordinates and should be read as such. This converter uses the international foot throughout; to convert legacy survey feet, multiply by 1.000002.

The factors, all referenced to the metre

• 1 mm = 0.001 m, 1 cm = 0.01 m, 1 km = 1,000 m
• 1 in = 0.0254 m (exact)
• 1 ft = 0.3048 m (exact)
• 1 yd = 0.9144 m (exact)
• 1 mi = 1,609.344 m (exact)
• 1 nautical mile = 1,852 m (exact)

Two useful reciprocals: one metre is 39.3701 inches, and one kilometre is 0.621371 miles. The second one has a well-known shortcut. Consecutive Fibonacci numbers converge on the golden ratio, 1.618, and a mile is 1.609 km — close enough that 5 miles is about 8 km (true: 8.05), 8 miles about 13 km (12.87), and 13 miles about 21 km (20.92). The error stays under one percent all the way up the sequence.

Running distances, altitudes and timber

A 5K is not three miles. Five kilometres is 3.10686 miles, so a runner used to a flat three-mile loop is adding about 190 yards. A 10K is 6.2137 miles. A marathon is 42.195 km, which comes from 26 miles 385 yards — the distance of the 1908 London Olympic course, made the official standard in 1921 — and works out to 26.2188 miles.

Flight levels. Aviation altitudes are quoted in hundreds of feet, so FL350 is 35,000 ft, exactly 10,668 m. Countries that use metric flight levels, notably China and Russia historically, work in metres directly, which is where conversion errors in flight planning have historically crept in.

Nominal versus actual. A North American 2×4 stud measures 1.5 in × 3.5 in after drying and planing, which is 38 mm × 89 mm. European equivalents are cut to 38 × 89 mm outright. Screen diagonals, pipe diameters and wire gauges are similarly nominal: converting the label gives you a number that no part of the object actually measures.

Lengths to calibrate your intuition against

The football field row is exact rather than approximate: 100 yards is 91.44 m by definition, one of the few American sporting dimensions that lands on a clean metric figure.

Where a rounded conversion is fine, and where it is not

For travel, running and rough sizing, two decimal places is more than enough. For anything that has to fit, it is not. One sixteenth of an inch is 1.5875 mm, so rounding a metric drawing to whole millimetres can drift a full sixteenth over a run of eight joints — the difference between a cabinet door that closes and one that binds. Machinists work in thousandths of an inch, where one thou is 25.4 microns exactly.

Convert once, at the start, and work in a single system from there. Converting back and forth at each step accumulates rounding, and it is where mixed-unit projects go wrong. For speeds derived from these distances see the Speed Converter, and for the squares of them the Area Converter.

Frequently asked questions

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