Almost every country measures in metric. Three do not, and one of them publishes most of the internet — so anyone reading a recipe, buying timber or following instructions online ends up converting between the two whether they meant to or not.
The conversions worth memorising
There are only about eight. Everything else is either rare or close enough to estimate.
| From | Multiply by | In your head |
|---|---|---|
| Inches → cm | 2.54 | Two and a half |
| Feet → metres | 0.3048 | Divide by three, take off a bit |
| Miles → km | 1.609 | Add 60% |
| Pounds → kg | 0.4536 | Halve it, add back 10% |
| Ounces → grams | 28.35 | Thirty, near enough |
| US cups → ml | 236.6 | About 240 |
| Sq feet → sq metres | 0.0929 | Divide by eleven |
| °F → °C | −32, × 5/9 | Subtract 30, halve |
The mile rule is the neat one: kilometres and miles sit almost exactly at the golden ratio, so consecutive Fibonacci numbers convert. 5 miles is about 8 km, 8 miles about 13, 13 about 21. Accurate to under 1%.
Three that catch people out
A US gallon is not a UK gallon
This is the big one. A US gallon is 3.79 litres; an imperial gallon is 4.55 — about 20% larger. The same split runs through every volume unit derived from them: pints, quarts and fluid ounces all differ.
- US pint — 473 ml. UK pint — 568 ml.
- US fluid ounce — 29.6 ml. UK fluid ounce — 28.4 ml.
Note the fluid ounces run the other way round: the US one is larger even though the US pint is smaller, because a US pint is 16 fl oz and a UK pint is 20. Miles per gallon figures are not comparable between the two countries for the same reason.
Weight ounces and fluid ounces are unrelated
An ounce is a mass; a fluid ounce is a volume. They share a name for historical reasons and converting between them requires knowing what the substance is. For water they happen to be close, which is why the confusion survives — for flour or oil they are not.
A US cup is a defined volume, not a cup
236.6 ml, and it measures volume even when the recipe is listing flour. That is the root of most baking failures across the divide: a cup of flour is anywhere between 120g and 150g depending on how it was packed, while 130g is 130g. Where a recipe gives weights, use them. Converting cups to grams has to be done per ingredient, which is why a single conversion factor does not exist.
Temperature is the one with an offset
Every other conversion here is a multiplication. Temperature is not, because the scales do not share a zero — so you cannot convert a temperature difference the same way you convert a temperature. A rise of 10°C is a rise of 18°F, not 50°F.
Worth knowing: −40 is the same in both, and 16°C is about 61°F while 61°C is about 142°F — the digits swapping at 16/61 is a coincidence that has misled more than one person into thinking there is a trick.
Why two systems still exist
Metric won on merit: everything scales by ten, the units are defined against physical constants rather than a king’s body, and there is one of each. The imperial units persist because changing them is expensive in ways that are not obvious — road signs, machine tooling, pipe fittings, timber sizes, and an enormous quantity of technical documentation that would all have to move together.
The UK sits in the middle and stayed there: road distances in miles, beer in pints, milk in pints, but fuel in litres, food in grams and weather in Celsius. Anyone finding that inconsistent is right.
The unit that has caused an actual disaster
In 1999 the Mars Climate Orbiter was lost because one team supplied thrust figures in pound-force seconds and the receiving software expected newton-seconds. The spacecraft came in about 170 km too low and broke up. It cost roughly $327 million, and the fix would have been a single multiplication.
The lesson is not that imperial is bad. It is that a number without its unit attached is not information, and every conversion is a place for an assumption to go unstated.
Converting
The unit converter handles everything here, and there are direct pages for the conversions people look up most — pounds to kilograms, kilograms to pounds, centimetres to inches, Celsius to Fahrenheit — each showing the formula rather than only the answer, so you can check the arithmetic instead of trusting it.
For a specific dimension there are dedicated converters for length, weight, volume, area, temperature and speed.