Metric and Imperial: The Conversions People Get Wrong

Most conversions are exact by definition

Since the 1959 international yard and pound agreement, an inch is exactly 25.4 mm and a pound is exactly 0.45359237 kg. These are not measurements with uncertainty — they are definitions, so any rounding you see comes from the display, never from the conversion itself.

A few units are genuinely awkward. The US survey foot, retired for most purposes at the end of 2022, was longer than the international foot by two parts per million: irrelevant for a room, and about 3 metres across a state.

The factors worth memorising

Temperature is the odd one out

Every conversion in the table above is a single multiplication, because both units share a zero point. Celsius and Fahrenheit do not: water freezes at 0°C and 32°F, so the conversion needs an offset as well as a scale. °F = °C × 9/5 + 32.

The mental shortcut is to double the Celsius figure and add 30 — accurate to within a few degrees across normal weather, and wrong enough at oven temperatures that you should use the real formula there. The two scales cross at −40°, which is the same temperature in both.

Kelvin and Rankine are absolute scales starting at absolute zero, so converting to them is an offset with no scale change (K = °C + 273.15) or a scale change with no offset (°R = K × 9/5).

Convert anything

Every pair in the table has its own page with the exact factor, a table of common values and definitions of both units — length, area, volume, weight and temperature are all covered individually.