Length & Distance Converters

72 converters, all running in your browser.

The metre is the SI base unit of length, defined since 1983 as the distance light travels in vacuum in 1/299,792,458 of a second. Every other length unit on this page is a fixed multiple of it, which is why these conversions are exact rather than approximate.

The imperial units were themselves redefined in terms of the metre by the 1959 international yard and pound agreement: the inch became exactly 25.4 mm, the yard exactly 0.9144 m. Before that, a US inch and a UK inch differed slightly, and surveying in the United States still uses the older "survey foot" for legacy records.

The nautical mile is the exception that is not derived from the metre at all. It is one minute of latitude — 1,852 m by international agreement — which is why it survives in aviation and shipping where position is worked out from angles rather than distances.

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Conversion factors

From	To	Multiply by
Meters	Kilometers	0.001
Meters	Centimeters	100
Meters	Millimeters	1000
Meters	Miles	6.2137e-4
Meters	Yards	1.0936133
Kilometers	Meters	1000
Kilometers	Centimeters	100000
Kilometers	Millimeters	1000000
Kilometers	Miles	0.62137119
Kilometers	Yards	1093.6133
Centimeters	Meters	0.01
Centimeters	Kilometers	1.0000e-5
Centimeters	Millimeters	10
Centimeters	Miles	6.2137e-6
Centimeters	Yards	0.010936133
Millimeters	Meters	0.001
Millimeters	Kilometers	1.0000e-6
Millimeters	Centimeters	0.1
Millimeters	Miles	6.2137e-7
Millimeters	Yards	0.0010936133

Choosing the Right Precision

Most length conversions are exact, so the question is how many digits to keep rather than how accurate the factor is. An inch is exactly 25.4 mm, so 12 inches is exactly 304.8 mm — no rounding is involved at all.

Where rounding matters is in the reverse direction. One millimetre is 0.0393700787... inches, a repeating decimal. Quoting it to three places is fine for a shelf and not fine for a machined part, where thousandths of an inch are the working unit and the accumulated error across twenty measurements becomes visible.

A practical rule: keep the same number of significant figures you were given. A measurement of "about 5 metres" converted to "16.4041994751 feet" claims a precision the original never had, and stating it that way is a small dishonesty that spreadsheets propagate.

The Mental Shortcuts Worth Knowing

Several length conversions have fraction approximations close enough for everyday use and easy to do in your head:

Kilometres to miles: multiply by 5, divide by 8. Off by 0.6%.
Centimetres to inches: divide by 2.5. Off by 1.6%.
Metres to feet: multiply by 3, add 10%. Off by 0.3%.
Inches to centimetres: multiply by 2.5, add 2%. Near exact.

Each is worth remembering for the situations where you are checking whether a number is plausible rather than computing one.

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