Length Converter

What this calculates

Enter a length in millimetres, centimetres, metres, kilometres, inches, feet, yards or miles, and this shows it in all eight at once, with each conversion worked through. It also places the length on a scale running from a tenth of a millimetre to ten thousand kilometres, so an inch and a mile can be read against each other.

Every figure comes from exact definitions — nothing is converted with a rounded shortcut factor.

The formula

Every conversion goes through metres: into metres with one unit's size, then out again with another's.

The metric units are powers of ten: a kilometre is 1,000 m, a metre is 100 cm or 1,000 mm. The imperial units are exact too, and each one builds on the last.

Because every factor is exact, a conversion is exact until it is rounded for display — and the calculator rounds each result once, from the exact figure.

Worked example

Not every conversion comes out exact. 100 m is 100 ÷ 0.3048 = 328.084 ft and 109.3613 yd, each shown to seven significant figures, because a metre is not a whole number of feet.

Going the other way is exact again. 6 ft is 6 × 0.3048 = 1.8288 m, which is also 72 in and 2 yd — multiplying by an exact factor gives an exact answer.

When to use it, and the mistakes to avoid

Use it when a plan, a product listing or a set of instructions gives a length in a unit you do not think in, when checking a height or a room size, and when a distance is quoted in kilometres and you think in miles, or the other way round.

The mistakes that cost the most:

• Using the 2.54 the wrong way round. An inch is 2.54 cm, so inches to centimetres multiplies and centimetres to inches divides. 10 in is 25.4 cm; dividing by 2.54 instead gives 3.937008 — which is 10 centimetres in inches, the conversion run backwards.
• Reading feet and inches as a decimal. 5′8″ means 5 feet and 8 inches, which is 68 in or 172.72 cm. It is not 5.8 ft: 5.8 ft is 69.6 in, or 176.784 cm. Convert the feet to inches and add the inches before doing anything else.
• Treating kilometres and miles as the same. A mile is about 1.6 km — exactly 1.609344 — so they are far apart. A 10 km route is 6.213712 mi, and 10 miles is 16.09344 km. The quick "times 1.6" rule gives 16 km for 10 miles: close for a rough sense of distance, but not an answer.
• Expecting every digit of every result. Results are shown to seven significant figures — 1 km is 0.6213712 mi — which is more precision than any tape measure gives. A length far too small for a unit, such as 0.0000000001 mm in miles, reads as 0, and a note under the result says so rather than leaving a silent zero.
• Entering a negative length. A length cannot be less than zero, so the calculator refuses one with a message instead of converting it.

FAQ