Length Converter

How the Length Converter works

Length conversion looks like the most settled arithmetic there is, and mostly it is. Type an amount, pick the two units and the answer appears with the conversion factor and its inverse underneath. The useful knowledge is where those factors come from, the one place they still disagree, and how many digits of the answer you should actually keep.

How the length converter works

Every unit is stored as a number of metres, and the converter divides one by the other. The formula is simply: result = amount × (metres per "from" unit ÷ metres per "to" unit). That single rule covers all 81 pairs of the nine units, including odd ones like nautical miles to inches, without a separate table for each.

Why the factors are exact. Since the international yard and pound agreement of 1959, the yard has been defined as exactly 0.9144 metres. Everything else follows:

• a foot is a third of a yard, 0.3048 m exactly
• an inch is a twelfth of a foot, 25.4 mm exactly
• a mile is 1,760 yards, 1,609.344 m exactly
• a nautical mile is 1,852 m exactly, set by international agreement in 1929

None of these are measurements that might be refined later. They are definitions, so 2.54 cm to the inch is exact rather than convenient, and results terminate instead of drifting.

Feet and inches in one box. When converting from feet you can type 5'10", 5 ft 10 or 5'10 as well as a decimal. Heights and room sizes are almost always written that way, and turning 5 ft 10 in into 5.8333 ft by hand is where errors creep in.

Where length conversion matters

• Heights and body sizes. Medical forms, passports and dating profiles swap between centimetres and feet and inches. For example, 5 ft 10 in is 177.8 cm, and 6 ft is 182.88 cm.
• Building and DIY. UK and Australian trades work in millimetres, but timber is still sold by the foot in places, and US plans are in feet and inches. A 2.4 m sheet is about 7 ft 10.5 in, not quite the 8 ft that US sheets measure.
• Running and sport. A 5 km race is 3.107 miles and a marathon is 42.195 km, or 26.219 miles. A 100 m sprint is about 109.4 yards.
• Aviation and shipping. Charts use nautical miles because one nautical mile is roughly one minute of latitude, so sixty of them make a degree and a chart distance converts straight to an angle. The knot is one nautical mile per hour.

The US survey foot is a different foot

Before 1959 the United States defined the foot as 1200/3937 of a metre, about two parts per million longer than the international foot. That is only 2 mm over a kilometre, but state plane coordinates run to hundreds of kilometres from their origin, where the difference grows to metres. The US kept the old unit as the "US survey foot" for mapping and only retired it at the end of 2022. This converter uses the international foot, which is right for everything except historical survey plats and legal land descriptions that name the survey foot.

Reading the result

• "… equals" is the converted amount, shown to up to ten significant figures so no precision is thrown away before you decide what to keep.
• Conversion is the factor for one unit, for example 1 ft = 0.3048 m. Paste it into a spreadsheet to convert a whole column.
• Inverse is the factor the other way, 1 m = 3.28084 ft. It exists so you can check the direction by eye.

Worked example. Converting 12 ft to metres gives 3.6576 m, with the factor 0.3048 and the inverse 3.28084. Converting 3.6576 m back gives exactly 12 ft, because the factors are exact.

Tips and pitfalls

• Multiply or divide? The most common conversion mistake is using the factor the wrong way round. Converting to a smaller unit must give a bigger number, and converting to a bigger unit a smaller one. If it doesn't, you have the inverse.
• Precision is not accuracy. A tape measure read to the nearest inch gives a converted figure only good to about 25 mm, however many decimals appear. Round to the precision you actually measured, especially on drawings and order forms.
• Round at the end. Rounding 1 in to 2.5 cm and multiplying by 100 is 4 cm out on a 100-inch run. Convert the full figure, then round once.
• Watch the mile. "Mile" on a boat or a plane means the nautical mile, 15 % longer than the statute mile on a road sign.

Length converter quick reference

The exact factors are 1 in = 2.54 cm, 1 ft = 0.3048 m, 1 yd = 0.9144 m and 1 mi = 1.609344 km. Some common conversions, rounded to a sensible precision:

For single pairs with their own tables, see metres to feet, cm to inches and km to miles.

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