Length is the conversion people look up most, and it is also the one with the tidiest history. Since 1959, the inch has been defined as exactly 25.4 millimetres — not approximately, exactly. Before that, the US and the UK each had their own slightly different inch, and precision engineering across the Atlantic was an exercise in mutual suspicion.
That single definition cascades. A foot is 12 inches, so exactly 0.3048 m. A yard is three feet, exactly 0.9144 m. A mile is 1,760 yards, exactly 1,609.344 m. None of these involves rounding, which is why the reference table below is exact rather than approximate.
Quick reference
The table follows whichever units you select above.
The conversions worth memorising
1 in = 2.54 cm— exact, and the root of everything else1 ft = 30.48 cm1 m ≈ 3.28 ft— a metre is a long stride1 mi ≈ 1.609 km— a 5K is about 3.1 miles1 km ≈ 0.621 mi
The mile-to-kilometre ratio sits close to the golden ratio, 1.618, which produces a pleasing party trick: consecutive Fibonacci numbers approximate mile-kilometre pairs. 5 miles is about 8 km, 8 miles about 13 km, 13 miles about 21 km. It is a coincidence of the definitions rather than anything deep, but it is accurate to within about half a per cent and it is genuinely useful on a road sign.
Where people go wrong
The US survey foot
Until recently the United States maintained a second, marginally longer foot for land surveying — bigger than the international foot by two parts per million. Irrelevant over a room, roughly 3 cm over 15 km. It was officially retired at the end of 2022, but it persists in older deeds, state plane coordinate systems and legacy survey data. If you are working with historical land records, check which foot they used.
Nautical miles are not miles
A nautical mile is 1,852 m exactly, about 15 per cent longer than a statute mile. It was defined as one minute of latitude, which is why it is the natural unit at sea and in the air: one minute of arc on a chart is one nautical mile, no arithmetic required. A knot is one nautical mile per hour.
Feet-and-inches is not decimal feet
A height of 5 ft 9 in is 5.75 ft, not 5.9 ft. The most common conversion error in spreadsheets is treating the inches as a decimal fraction. Divide the inches by 12 first.
Fractional inches. Imperial trades work in halves, quarters, eighths and sixteenths rather than decimals. 3/8 in is 9.525 mm; 1/16 in is 1.5875 mm. A sixteenth of an inch is a little over one and a half millimetres, which is a handy anchor when reading a dual-scale tape.
Common questions
How many centimetres in an inch?
Exactly 2.54. This is a definition, not a measurement, so it carries no error.
Is a metre longer than a yard?
Yes, by about 9.4 per cent. A metre is 1.0936 yards, so a 100 m sprint is a little longer than 100 yards.
How do I convert my height to centimetres?
Multiply the feet by 12, add the inches, then multiply by 2.54. So 5 ft 9 in is 69 in, which is 175.26 cm.