How to Convert Length Units Without Getting It Wrong
Metric length conversion is moving a decimal point. Imperial is remembering that twelve inches make a foot, three feet make a yard and 1,760 yards make a mile. Moving between the two systems is where the mistakes live. The Length Converter handles all three cases.
What is worth memorising
An inch is exactly 25.4 millimetres. That is a definition, not an approximation, and it was agreed internationally in 1959. Everything else follows from it: a foot is exactly 304.8 mm, a yard exactly 0.9144 m, a mile exactly 1,609.344 m.
For rough work, a metre is a bit over a yard, a kilometre is about six tenths of a mile, and a centimetre is a shade under half an inch. Those three will get you through most of daily life.
Where it actually goes wrong
Mixed units in one measurement. Five foot nine is not 5.9 feet. It is 5.75 feet, because nine inches is three quarters of a foot. Typing 5.9 into a converter gives you an answer that is wrong by four centimetres, and it looks plausible.
Rounding too early. Convert, do the arithmetic, round at the end. Rounding an inch to 25 mm before multiplying by a hundred leaves you 40 mm short of where you should be.
Assuming a unit means one thing. A nautical mile is 1,852 metres, not 1,609 - it is a minute of latitude, which is why it is the unit at sea and in the air. A US survey foot differed from the international foot by two parts per million, which sounds like nothing and mattered enormously over the length of a state boundary; the US finally retired it in 2022.
The most expensive version of this was the Mars Climate Orbiter, lost in 1999 because one team worked in pound-force seconds and another assumed newton-seconds. Nobody made an arithmetic error. The units were simply never agreed.
Say the unit out loud
The habit that prevents nearly all of this is writing the unit next to every number, in every note, every spreadsheet column header and every message. A column headed "length" is an invitation; a column headed "length (mm)" is not.
Common questions
How many centimetres in an inch?
2.54 exactly. Multiply inches by 2.54 for centimetres, divide for the other direction.
Is a US inch the same as a UK inch?
Yes, since 1959. Before that they differed very slightly, which matters only for historical surveying records.
What about converting an area or a volume?
The factor is squared or cubed, which surprises people. A square metre is about 10.8 square feet, not 3.3. The Area Converter and the Volume Converter do those properly.
How precise should I be?
As precise as the measurement that went in. Converting a figure you measured to the nearest centimetre and reporting it to four decimal places claims accuracy you do not have.
Got a measurement to convert?