The conversions worth memorising
Most unit conversions are one multiplication, and most of the factors are exact rather than approximate. An inch is exactly 25.4 millimetres — defined that way by international agreement in 1959, not measured and rounded.
A small number of factors covers nearly everything anyone needs day to day, and one conversion behaves completely differently from all the others.
The ones worth knowing
An inch is 25.4mm exactly. A mile is 1.609 kilometres. A kilogram is 2.205 pounds. A litre is 0.264 US gallons — or, more usefully in the other direction, a US gallon is 3.785 litres and an imperial gallon is 4.546.
Two of those have handy approximations. A metre is a bit over a yard — 1.094, so add about ten per cent. And a kilometre is about five eighths of a mile, which is a genuinely useful one for reading road signs abroad: multiply by six and drop a tenth, or just remember that 80 km/h is about 50 mph.
The exactness matters more than it sounds. Because the inch is defined as 25.4mm rather than approximated, conversions chain without accumulating error. Feet, yards and miles all inherit that definition, so the whole imperial length system is exactly specified in metric terms.
1 in = 25.4 mm (exact by definition)
1 m = 39.37 in = 3.281 ft = 1.094 yd
1 mile = 1.609 km 1 km = 0.621 miles
1 kg = 2.205 lb 1 US gal = 3.785 L
Temperature is the exception
Every other unit family shares a zero. No length is a different length in another unit, and zero metres is zero feet. Temperature scales do not share a zero, so each conversion needs an offset as well as a factor: multiply by nine fifths and add thirty-two to go from Celsius to Fahrenheit.
The consequence catches people out constantly: temperature differences do not scale the way values do. Twenty degrees Celsius is not twice as warm as ten in any meaningful sense, and 68°F is not twice 50°F. The ratio between two temperatures is an artefact of where somebody put the zero.
Kelvin is the exception to the exception, because its zero is absolute rather than chosen. Ratios in kelvin do mean something, which is why physics uses it.
One number worth carrying: minus forty is the same in both Celsius and Fahrenheit. It is the only place the two scales meet, and it makes a good sanity check on any conversion you have done in your head.
Work it out: Temperature Converter →
Two gallons, two tons, two pints
The imperial and US customary systems share names and not sizes, which is a reliable source of confusion in anything read online. A US gallon is 3.785 litres, an imperial gallon is 4.546 — twenty per cent larger. That difference propagates into every fuel economy figure, which is why the same car reads about twenty per cent better in UK mpg.
A US pint is 16 fluid ounces and an imperial pint is 20, so recipes cross the Atlantic badly. And "ton" means 2,000 pounds in the United States, 2,240 in the imperial system, and 1,000 kilograms as a metric tonne — three different weights, of which the middle one is now rare enough to be forgotten just when it matters.
The rule when reading anything: identify the system before trusting a number. A quantity quoted in gallons, pints or tons without a nationality attached is not fully specified.
Speed is a ratio, so it converts twice
Speed carries a distance unit and a time unit, and both may need converting. Miles per hour to kilometres per hour is just the distance factor because the hours match. Metres per second to kilometres per hour is both — times 3.6, which is 3,600 seconds in an hour over 1,000 metres in a kilometre.
That 3.6 is worth knowing because it is the conversion that appears in physics homework and weather forecasts alike. Ten metres per second is 36 km/h, which is a brisk wind.
The rule generalises to any compound unit. Convert each part separately and be careful about which way up each factor goes — a unit on the bottom of a fraction converts in the opposite direction from one on the top, which is the step most often got backwards.
Work it out: Speed Converter →
Questions
How many millimetres in an inch?
Exactly 25.4. It is a definition agreed internationally in 1959, not a rounded measurement, which is why conversions through it never drift.
Why is temperature conversion different?
Because Celsius and Fahrenheit do not share a zero, so the conversion needs an offset as well as a multiplication. Every other unit family shares a zero and needs only a factor.
Is 20°C twice as warm as 10°C?
No. Ratios between temperatures are meaningless on Celsius or Fahrenheit because their zeros are arbitrary. Only kelvin, which starts at absolute zero, supports that kind of comparison.
What temperature is the same in Celsius and Fahrenheit?
Minus forty. It is the only point where the two scales cross, and it makes a good check on a conversion done in your head.
How different are US and imperial gallons?
A US gallon is 3.785 litres and an imperial gallon is 4.546 — about twenty per cent larger. It is why the same car appears more efficient in UK mpg figures.
How do I convert metres per second to km/h?
Multiply by 3.6 — that is 3,600 seconds in an hour divided by 1,000 metres in a kilometre. Ten metres per second is 36 km/h.
How many pounds in a ton?
It depends which ton. A US short ton is 2,000 pounds, an imperial long ton is 2,240, and a metric tonne is 1,000 kilograms — about 2,205 pounds.