Length Converter

The formula

Units per metre

How many of that unit make one metre. Every conversion goes through metres as an intermediate, which is why any unit converts to any other without needing a separate factor for each pair.

Precision

Results are shown to 12 significant figures with trailing zeros removed, so exact conversions stay exact and inexact ones do not sprout floating-point noise.

Worked example

Converting 5 miles to kilometres.

• Metres = 5 ÷ 0.000621371 = 8,046.72
• Km = 8,046.72 × 0.001 = 8.04672

5 mi = 8.04672 km.

Which of these factors are exact

The relationship between inches and metres has been defined exactly since the international yard agreement: one inch is exactly 25.4 mm, which makes a foot, a yard and a mile exact too. So 5 miles is exactly 8.04672 km, not an approximation. The metric relationships — centimetres and kilometres to metres — are exact by definition as well. That means every conversion here is exact arithmetic, and any residual digits you see are floating-point representation rather than measurement uncertainty.

It converts as you type

There is no calculate button. Changing the value, the source unit or the target unit recalculates immediately, which makes it fast for stepping through a list of measurements. It also means a partially typed number briefly produces a partial answer, which is expected rather than a fault.

Feet and inches together

The tool takes one unit at a time, so a height or a room dimension given as 5 ft 9 in has to be converted to a single unit first. Multiply the feet by 12 and add the inches — 5 ft 9 in is 69 in — then convert that. Converting the feet and the inches separately and adding the two metric results works too and gives the same answer.

Reference points worth carrying

One inch is 2.54 cm, one foot is 30.48 cm, one metre is about 3 ft 3.4 in, one mile is 1.609344 km and one kilometre is about 0.621 miles. Those five let you sanity-check almost any result in your head before trusting it — useful when a decimal point lands in the wrong place.

Common questions

How many kilometres are in a mile?

Exactly 1.609344. The relationship is defined rather than measured, because the international yard was fixed at exactly 0.9144 metres, so mile-to-kilometre conversions are exact arithmetic with no rounding involved.

Why does the result have so many decimal places?

Because the conversion is carried to 12 significant figures before trailing zeros are stripped. That keeps exact conversions exact and preserves precision on small values. Round the displayed figure yourself to whatever precision your measurement actually justifies — a length measured with a tape does not deserve six decimals.

How do I convert feet and inches?

Combine them into a single unit first. Multiply feet by 12 and add the inches to get total inches, then convert. Six foot two is 74 inches, which is 187.96 cm.

Do I need to press a button to convert?

No. The result updates as soon as you change the number or either unit, so you can leave it open and run through a list of measurements without clicking anything.