The metric-imperial conversions actually worth memorising
You do not need conversion tables for everything. A handful of anchors, plus two mental tricks, cover almost every day-to-day conversion.
Most conversion tables are exhaustive and useless — nobody needs furlongs. In practice a handful of anchor figures and two arithmetic tricks handle nearly everything you meet while travelling, cooking or reading a recipe from another country.
The anchors worth knowing
| To convert | Use | Close enough for |
|---|---|---|
| Miles → km | × 1.6 | Road signs, running distances |
| km → miles | × 0.6 | Driving abroad |
| Inches → cm | × 2.5 | Screen sizes, furniture |
| Feet → metres | × 0.3 | Room dimensions, heights |
| Pounds → kg | ÷ 2.2 | Body weight, luggage |
| °C → °F | × 2, + 30 | Weather forecasts |
Trick one: double and subtract a tenth
For miles to kilometres, exact is × 1.609. Doubling overshoots, so double then take off a fifth: 30 miles → 60, minus 12 → 48 km. The true answer is 48.3. For the reverse, halve and add a tenth: 100 km → 50, plus 10 → 60 miles, against a true 62.1.
Trick two: the Fahrenheit shortcut
The textbook formula is °F = °C × 9/5 + 32. In your head, double and add 30. At 20°C you get 70°F against a true 68°F. At 30°C you get 90°F against 86°F. Fine for deciding on a jacket, not for a fever or an oven.
Where approximating goes wrong
Rough conversions are fine for distance and weather. They are not fine for three things: baking, where ratios decide whether something rises; medication, where a rounding error is a dosing error; and engineering or construction, where tolerances are small and errors accumulate across a run. Use exact figures there.
The other trap is cups. A US cup is 236.6 ml, a metric cup is 250 ml, and an imperial cup is 284 ml. A recipe that says "a cup" without saying whose can be off by 20%, which matters enormously in baking and not at all in soup.
Length Converter Convert length exactly, with the working shown Temperature Converter Convert °C, °F and Kelvin Cooking Converter Handle cups, spoons and millilitres