A US gallon and a UK gallon are not the same thing. The British one is about 20% bigger — 4.55 litres versus 3.79. So a car that does "40 miles per gallon" in London and "40 miles per gallon" in Los Angeles is not doing the same thing, and a recipe that says "one gallon of stock" means noticeably more liquid in a British cookbook than an American one. That single mismatch is a good warning for the whole subject: most metric-to-imperial conversions are simple, but a few hide a trap, and the trick is knowing which is which.
The good news is that the conversions you hit in daily life are ones you can do in your head with a rough multiplier. You almost never need six decimal places to figure out whether it's cold outside or how far the next town is.
The five conversions you actually use
Here are the everyday ones, with the mental shortcut and a worked number for each.
Kilometres to miles — multiply by 0.62. A kilometre is a bit under two-thirds of a mile. A 5K run is about 3.1 miles; a 10K is 6.2. A speed limit of 100 km/h is 62 mph. If you want it faster, multiply by 6 and drop a digit: 80 km → 8 × 6 = 48, so about 48 miles (the exact answer is 49.7, close enough for a road trip).
Kilograms to pounds — multiply by 2.2. A 70 kg person weighs 154 lb. A 20 kg suitcase is 44 lb. The common mistake is rounding to "times 2," which comes out about 10% short — fine for guessing, bad for anything that matters (more on that below).
Centimetres to inches — divide by 2.54. This one is exact, not approximate: an inch is defined as 2.54 cm. Someone who is 180 cm tall is 180 ÷ 2.54 = 70.9 inches, which is 5 feet 10.9 inches — call it 5'11". For a quick guess, divide by 2.5 (multiply by 0.4): 180 × 0.4 = 72, in the right neighbourhood.
Litres to gallons — multiply by 0.26 (US). A 2-litre bottle is about half a US gallon. A 50-litre tank holds roughly 13 US gallons. Remember the opening warning: for UK gallons, multiply by 0.22 instead, because the UK gallon is larger.
That covers distance, weight, height, and volume. For the odd one-off — square metres to square feet, millilitres to fluid ounces, kilometres per litre to miles per gallon — a browser tool like the Unit Converter does it without you memorising a fifth or sixth multiplier.
Temperature deserves its own section
Temperature is the conversion people freeze on, because it's the only one with two steps instead of one.
The formula is: °F = °C × 1.8 + 32.
Worked out: 20°C × 1.8 = 36, plus 32 = 68°F. A warm spring day. Boiling water is 100°C × 1.8 = 180, + 32 = 212°F. Freezing is 0 × 1.8 = 0, + 32 = 32°F. Human body temperature, 37°C, comes out to 37 × 1.8 = 66.6, + 32 = 98.6°F — the number on every American thermometer.
Going the other way, subtract 32 first, then divide by 1.8. A 98.6°F reading: 98.6 − 32 = 66.6, ÷ 1.8 = 37°C.
If multiplying by 1.8 in your head is annoying, use this: double the Celsius, knock off 10%, add 32. That's not a rough hack — it's the exact formula rewritten, because ×1.8 equals ×2 minus 10%. So 30°C → 60 − 6 = 54, + 32 = 86°F. Exactly right. The lazier "double and add 30" (30 → 90) is close for mild weather but drifts a few degrees at the extremes, so drop the extra 10% when you want it exact.
Why America never switched
People assume the US simply refused metric. The real story is that it half-switched and stalled. Congress passed the Metric Conversion Act in 1975, declaring metric the "preferred" system — but made adoption voluntary. Without a mandate, the cost of retooling factories, reprinting road signs, and reprogramming every recipe fell on private business, and most didn't bother. Science, medicine, and the military went metric decades ago; grocery stores and weather forecasts never did.
There's a subtler point that explains the gallon problem. The US doesn't actually use imperial units. It uses "US customary" units, which descend from older English measures fixed before Britain standardised the imperial system in 1824. The two families drifted apart, which is why a US gallon (3.79 L) and a UK imperial gallon (4.55 L) differ, and why US and UK fluid ounces and pints don't match either. If you're converting a British recipe on an American measuring jug, that gap is a real source of error.
When "close enough" isn't
Rough multipliers are perfect for travel, weather, and weighing your luggage. They are dangerous in exactly two places.
Baking. Volume is a lousy way to measure flour — a cup can vary by 20 to 30% depending on whether you scoop or spoon it. That's why serious baking recipes list grams. When you convert a European recipe's "250 g flour" into cups, you're stacking two errors: the conversion and the scooping variance. Weigh it instead. For the common kitchen swaps, a purpose-built Cups to Grams Converter beats a generic multiplier because a cup of flour and a cup of sugar weigh different amounts.
Medicine. Drug doses are often calculated per kilogram of body weight. Convert a 154 lb patient using "÷ 2" and you get 77 kg; the correct 2.2 divisor gives 70 kg — a 10% error that carries straight into the dose. For anything medical, use the precise factor or a calculator, never the mental shortcut.
The rule: estimate when the answer only needs to be roughly right, and use exact figures the moment a wrong number costs money, ruins a cake, or affects a body.
When this isn't the right approach
Memorising multipliers stops paying off fast once you leave the common handful. Nobody should be doing pascals-to-psi or joules-to-BTU in their head, and the mental tricks above quietly assume US units — they'll mislead you on a UK recipe or an Australian fuel figure. Compound conversions (kilometres per litre into miles per gallon) chain two multipliers and two rounding errors, so they're worth typing out rather than estimating.
A conversion tool earns its place there. The Unit Converter runs entirely in your browser — nothing is uploaded and there's no sign-up — so it's genuinely faster than searching, and it won't confuse US and imperial gallons the way a half-remembered factor will.
The one thing worth committing to memory today: 0.62 for kilometres, 2.2 for kilograms, 2.54 for inches, and "double, minus 10%, plus 32" for temperature. Those four cover roughly every conversion you'll meet in a normal week. Everything beyond that, look up — precision is cheap when a tool does it, and expensive when you guess.