Length converter

Enter one length and see it in every unit at the same time: metric from millimetres to kilometres, imperial from inches to miles, the nautical mile, and the Taiwanese chi and tsun that Taiwanese carpentry and interior work still price in. If you only need one specific pair, the single-direction pages are quicker — this page is for when you want the whole picture, or a unit the quick pages do not cover.

Two systems, one exact bridge

Metric length is defined by the metre, and since 1983 the metre has been defined by the speed of light: the distance light travels in vacuum in 1/299,792,458 of a second. Everything else in the metric column is a power of ten from there, which is the entire reason metric arithmetic is easy.

Imperial length is now defined by reference to the metre rather than independently. The 1959 International Yard and Pound Agreement fixed the yard at exactly 0.9144 m, which makes the foot exactly 0.3048 m and the inch exactly 0.0254 m. So the bridge between the two systems is exact, and every conversion on this page is a multiplication by a defined constant.

The nautical mile is the exception that is worth knowing about. It is exactly 1852 m, and unlike the statute mile it was not derived from a body part or a field boundary — it is one minute of latitude, which is why it is still standard in aviation and shipping and why a knot means one nautical mile per hour.

The Taiwanese chi, and the three-centimetre mistake

The chi (台尺) is legally 10/33 of a metre. That is 30.303 cm, not 30 cm, and the difference is the single most common measuring error in Taiwanese interior work. Over a ten chi run — a normal wardrobe or a kitchen counter — assuming 30 cm loses you three centimetres, which is wide enough to see and wide enough to stop a door closing.

The tsun (台寸) is one tenth of a chi: 3.0303 cm, not 3 cm. Both units came in with the Japanese colonial period, where the same measure is the shaku, and they stayed because the trades never switched. Doors, window frames, cabinet carcasses, tatami-derived room proportions and timber stock in Taiwan are all still quoted in chi.

One more trap that belongs here rather than on a page about pipes: the hardware trade also uses 分, but there it means one eighth of an inch of nominal pipe size, so a 4分 pipe is half an inch. The traditional Taiwanese 分 is one tenth of a tsun, or 3.03 mm. Because that single character carries two meanings an order of magnitude apart, this tool stops and asks rather than guessing when you type it.

Choosing the right unit for what you are doing

Use millimetres for anything you will make or machine. Engineering drawings across the metric world are dimensioned in millimetres with no unit suffix at all, precisely so that no one has to think about decimal points between centimetres and metres.

Use centimetres for bodies, furniture and parcels, and metres for rooms, buildings and land. Centimetres for a room and metres for a screw are both technically correct and both make the number harder to read, which is the only real criterion once the arithmetic is exact.

Use inches when the specification, the tool or the counterparty is American — screen sizes, fastener standards, pipe threads, bicycle wheels, luggage limits, US paper sizes. Converting the specification to metric and then working in metric is usually a mistake if you will have to hand the result back in inches, because you round twice.

Why converters disagree, and what this one does about it

Most disagreements between length converters are not about the constants — those are defined and identical everywhere. They are about display. Some tools round to a fixed number of decimals, which turns 0.0393701 inches into 0.04 and destroys the information; some print fifteen digits of floating point noise; some insert thousands separators into a number you are about to paste somewhere.

This page rounds to six significant figures by default, which behaves sensibly at both ends of the scale and is enough that a value converted out and back returns exactly what you typed. You can fix the display to a set number of decimals if you prefer, and if that would print a non-zero value as 0 the page switches to scientific notation and tells you why instead of quietly showing zero.

You can also type the unit directly into the value box — 10 cm, 5'10", 3/4 in — and the source unit follows what you typed. If a unit is genuinely ambiguous, the page refuses to guess and explains the options, because a plausible-looking wrong number is worse than an error message.

Nothing is uploaded

Conversion is arithmetic. It happens in your browser, the reference tables are rendered as static HTML, and the Content-Security-Policy this site sends restricts network connections to our own origin — a restriction your browser enforces regardless of what our code tries to do.

The practical proof takes ten seconds: disconnect your network and reload this page. The converter still works, because there was never anything on the other end to ask.