Length Converter

Details

About this tool

Converts between metric, imperial, typographic and astronomical lengths: millimetres to inches, metres to feet, kilometres to miles, points to millimetres, light-years to kilometres. Type a value, pick the two units, and either side can be edited.

The imperial units are not approximations. Since the 1959 international yard and pound agreement, the yard is defined as exactly 0.9144 metres, which makes an inch exactly 25.4 mm and a mile exactly 1609.344 m. So a conversion here is limited only by how far you choose to round it, not by the definition - a distinction that matters in engineering drawings, where a rounded factor accumulates across a chain of dimensions.

Miles are the trap. A statute mile is 1609.344 m and a nautical mile is 1852 m - a difference of 15 per cent, which is why a boat's speed in knots and a car's in mph are not comparable. The nautical mile is not arbitrary: it is one minute of latitude, which is what makes it useful on a chart. Both are here, named separately.

Feet and inches are handled as a pair, because that is how heights and room dimensions are actually written. Enter 5'10" and it converts as one measurement rather than two, and a metric height converts back into feet and inches rather than into a decimal number of feet that nobody uses.

Pixels are a special case and worth understanding: a pixel has no physical size until a resolution is fixed. The conversion here uses the DPI you set - 96 is the CSS reference, print work is usually 300 - which is why the same pixel count is a different number of millimetres on screen and on paper.

Good to know

Questions people actually ask

Is an inch exactly 2.54 cm?

Yes, exactly. The 1959 international yard and pound agreement defines the yard as exactly 0.9144 m, which makes an inch exactly 25.4 mm. The conversion is limited only by how far you round it, not by any uncertainty in the definition.

What is the difference between a mile and a nautical mile?

About 15 per cent. A statute mile is 1609.344 m; a nautical mile is 1852 m, defined as one minute of latitude, which is what makes it useful for navigation. A speed in knots is nautical miles per hour, so it is not directly comparable with mph - a boat doing 20 knots is doing 23 mph.

How do pixels convert to millimetres?

Only once a resolution is fixed, because a pixel has no physical size on its own. This converter uses the DPI you set: 96 is the CSS reference used by browsers, and print work is usually 300. The same image is therefore a different physical size on screen and on paper.

How do I convert a height like 5'10"?

Type it as it is written and it converts as a single measurement - 5'10" is 177.8 cm. Going the other way, a metric height comes back as feet and inches rather than as a decimal number of feet, because 5.83 feet is not how anybody states a height.

What is a light-year, exactly?

A distance, not a time: how far light travels in a Julian year, which is 9,460,730,472,580,800 m by definition. Astronomers more often use the parsec, about 3.26 light-years, because it falls directly out of the parallax measurement rather than needing a conversion.