Length Converter

About the Length Converter

Lengths turn up everywhere: a person's height, a room's dimensions, the size of a screen or a pipe, the distance of a run or a drive. Depending on where you are and what you are measuring, the same length might be given in millimetres, centimetres, metres or kilometres, or in inches, feet, yards or miles — and sailors and pilots use nautical miles as well. Moving between them by memory is error-prone, especially with feet and inches, which do not divide by ten.

This length converter converts between nine units of length, from millimetres to nautical miles. It gives the converted value, the exact factor in both directions, the length in every other unit, and a practical imperial form — feet and inches for everyday lengths, or miles and feet for long distances.

How to Use the Length Converter

Enter the value you want to convert.

Choose the unit to convert from and the unit to convert to.

The result shows at once, together with feet and inches (or miles) and the value in all the other units.

How Length Conversion Works

Every length unit is defined as an exact number of metres:

  1 mm   = 0.001 m          1 in  = 0.0254 m   (exactly 2.54 cm)
  1 cm   = 0.01 m           1 ft  = 0.3048 m   (12 in)
  1 km   = 1,000 m          1 yd  = 0.9144 m   (3 ft)
  1 nmi  = 1,852 m          1 mi  = 1,609.344 m (1,760 yd)

  result = value × (from-unit in metres) ÷ (to-unit in metres)

Step-by-Step Example: Kilometres to Miles

10 km to miles.

  To metres:   10 km × 1,000 = 10,000 m
  To miles:    10,000 ÷ 1,609.344 = 6.21371 mi

A 10K race is about 6.21371 miles. A handy approximation is to multiply kilometres by 5/8: 10 × 5/8 = 6.25, close enough for rough estimates.

Step-by-Step Example: Height in Feet and Inches

178 cm to feet and inches.

  To inches:   178 ÷ 2.54 = 70.0787 in
  Feet:        70.0787 ÷ 12 = 5 whole feet (60 in)
  Remainder:   70.0787 − 60 = 10.0787 in

So 178 cm is about 5 ft 10.1 in. Going the other way, multiply feet by 30.48 and inches by 2.54 and add: 5 ft 10 in is 152.4 + 25.4 = 177.8 cm.

Quick Reference

  1 cm   = 0.393701 in        1 in  = 2.54 cm
  1 m    = 3.28084 ft         1 ft  = 30.48 cm
  1 m    = 1.09361 yd         1 yd  = 91.44 cm
  1 km   = 0.621371 mi        1 mi  = 1.609344 km
  1 nmi  = 1.15078 mi         1 nmi = 1.852 km

Where Each Unit Is Used

Millimetres are standard for engineering drawings, tools, screws, rainfall and many product dimensions. Centimetres are common for body measurements and clothing in metric countries. Metres measure rooms, buildings and sports distances. Kilometres measure road and running distances almost everywhere except the US and UK.

Inches remain common worldwide for screen sizes, tyre rims, pipe fittings and, in the US, most everyday measurements. Feet are used for heights and room sizes in the US and UK and for aircraft altitude worldwide. Yards survive in American football and golf, and miles on road signs in the US and UK. Nautical miles, based on a minute of latitude, are used in shipping and aviation, where speeds are given in knots.

Mental Shortcuts

For everyday estimates, a few round figures go a long way. A metre is a little more than a yard, about 10 percent more. A foot is roughly 30 centimetres, and an inch about two and a half. Five miles is almost exactly 8 kilometres, since 8 ÷ 1.609344 is 4.97. And a kilometre takes about 10 to 12 minutes to walk, a mile about 15 to 20. These shortcuts are fine for directions and conversation, but use the exact factors for building, sewing, engineering or anything that has to fit.

Precision and Tolerances

A converted length can never be more precise than the original measurement. If a plank is measured as 2.4 m to the nearest centimetre, converting it gives 7.87402 ft, but only 7.87 ft is meaningful. For parts that must fit together, work in one system from start to finish where possible: converting back and forth and rounding each time can build up errors of a millimetre or more.

A Short History of the Units

The metre was defined in France in the 1790s as one ten-millionth of the distance from the North Pole to the equator, and today it is fixed by the speed of light: the distance light travels in 1/299,792,458 of a second. The older units grew from the human body and farming — the foot, the inch as the width of a thumb, the yard as a stride, and the furlong as the length of a ploughed furrow, 220 yards, eight of which make a mile. Since 1959 all of them have been tied exactly to the metre, which is why this converter can give exact factors rather than approximations.

Converting Very Large and Small Lengths

For microscopic sizes, such as a human hair at about 0.07 mm, or astronomical ones, such as the 384,400 km to the Moon, the same factors apply; the converter switches to powers of ten when numbers become very large or very small, so they stay readable.

Understanding Your Result

The headline is the converted length.

The conversion factor gives the exact factor in both directions.

The in every unit line shows the same length in all nine units.

The also line gives feet and inches for lengths under a mile, or miles and feet for longer distances.

When Should You Use This Calculator?

Use it to convert heights between centimetres and feet and inches.

Use it for running, cycling and driving distances in kilometres and miles.

Use it with plans, furniture and product dimensions from other countries.

Use it for school maths and science, where lengths must be in metres.

Common Mistakes

Treating feet and inches as a decimal. 5.10 feet is not 5 ft 10 in; 5 ft 10 in is 5.83 feet.

Using 2.5 instead of 2.54. The shortcut is 1.6 percent out, which adds up over long lengths.

Confusing nautical and statute miles. A nautical mile is about 15 percent longer.

Rounding before converting. Convert the full measurement, then round.

Mixing units in one sum. Convert every length to the same unit before adding.