Metric to Imperial, and the Units That Share a Name

Every metric to imperial conversion runs on six exact numbers: an inch is 2.54 cm, a foot is 0.3048 m, a mile is 1.609344 km, a pound is 0.45359237 kg, a US gallon is 3.785411784 L and an imperial gallon is 4.54609 L. None of those are rounded measurements. They are definitions, and every other conversion you will need — ounces, yards, acres, pints, miles per hour — is derived from them.

Which makes the arithmetic the easy part. What actually goes wrong is the unit: there are two gallons in daily use, two fluid ounces, two kinds of ounce by weight and three things called a ton, and the number on the page never tells you which one someone meant.

Which conversion factors do you actually need?

Rounded to four significant figures, which is more precision than anything short of a legal document needs.

Two reference points are worth more than the table: 180 cm is 5 ft 10.9 in, and 70 kg is 154 lb, or 11 stone. Heights and weights cluster near those, and scaling up or down from a number you already recognize is faster than multiplying from scratch.

How do you convert in your head, and how wrong will you be?

Each trades a known amount of accuracy for speed. For weather, walking distances and luggage limits the error is invisible, but it is worth knowing which way it leans.

• Kilometers to miles: multiply by 0.6. That reads 3.4% low. If you want better, use consecutive Fibonacci numbers — 5 km is about 3 mi, 8 km about 5 mi, 13 km about 8 mi. The 8-to-5 pair is only 0.6% off.
• Kilograms to pounds: double it and add 10%. That is multiplying by 2.2 against a true 2.2046, so you land 0.2% low. Nothing you weigh will notice.
• Meters to feet: triple it and add 10%. Multiplying by 3.3 against a true 3.281 runs 0.6% high — about 2 cm over a 3 m room.
• Centimeters to inches: divide by 2.5, then take off 1.5%. Straight division by 2.5 runs 1.6% high, which is a whole inch on a person's height — 180 cm comes out as 72 in against a real 70.9 in. The 1.5% correction closes almost all of that.
• Liters to US gallons: divide by 4, then add 6%. Straight division by 4 is 5.4% low, and that correction very nearly closes the gap.

Those all lean on adding a percentage to a number you just worked out, the piece of mental arithmetic that goes rusty fastest. If that step is the slow one, the trick that makes percentages easy to do in your head is what makes the rest usable.

When the answer goes on a shipping label, a spec sheet or an invoice, stop estimating. The unit converter here shows the amount in every unit of the category at once, which is faster than converting one pair at a time and makes a wrong-unit mistake obvious — the imperial gallon sitting three rows below the US one, both labelled.

Why are the factors exact instead of approximate?

Because imperial stopped being its own system in 1959. The international yard and pound agreement, signed by the standards bodies of the United States, the United Kingdom, Canada, Australia, New Zealand and South Africa, fixed the yard at 0.9144 m and the pound at 0.45359237 kg. Inch, foot, mile and ounce are all arithmetic on those two. An inch is not approximately 2.54 cm. It is 2.54 cm, the way a dozen is twelve.

Which means nothing accumulates. There is no measurement error hiding in the factor, only the rounding you do yourself — so round once, at the end, and the answer is as good as the number you started with.

One leftover to know about: the US survey foot, about 2 parts per million longer than the international foot. That is 2 mm over a kilometer, which is nothing until you are running a boundary across a county. It was retired at the end of 2022 and now turns up only in older survey records.

Which units share a name but not a size?

This is where conversions actually go wrong, and no amount of decimal places saves you.

Gallons, pints and fluid ounces

A US gallon is 3.785 L. An imperial gallon is 4.546 L, about 20% bigger, and nothing in a number quoted in gallons tells you which one it was. The split is historical rather than deliberate: the United States kept the old English wine gallon of 231 cubic inches, and Britain replaced its own assortment of gallons with a single imperial gallon in 1824.

Fluid ounces run the other way, which is what makes the pint confusing. The imperial fluid ounce is the smaller of the two, 28.41 mL against the US 29.57 mL. The pint is where they separate: 20 imperial ounces against 16 US ones, so 568 mL against 473 mL. The practical version of that: a British recipe asking for 10 fl oz of milk wants 284 mL, and measuring it in US ounces gives you 296 mL. Close enough for a stew. Not close enough for a batter.

Ounces and tons

There are two ounces by weight. Food, post and body weight use the avoirdupois ounce, 28.3495 g. Gold, silver and platinum are quoted in troy ounces of 31.1035 g, about 9.7% heavier. The converter carries the avoirdupois one only, so a precious metal quantity run through it comes out roughly a tenth light, and nothing in the labels warns you — both are just called an ounce.

Ton is worse, because there are three. A metric ton is 1,000 kg. A US short ton is 2,000 lb, or 907.2 kg. An imperial long ton is 2,240 lb, or 1,016 kg. The converter covers the metric ton and the US short ton; the long ton is not in it, so if a British shipping document from the last century quotes tons, do that one by hand.

Cups

There is no single cup. US customary is 236.6 mL. The cup printed on US nutrition labels is exactly 240 mL. An Australian metric cup is 250 mL, and older British recipes use 284 mL, which is half an imperial pint. The converter has the US customary cup and none of the others, so an Australian recipe converted there comes out about 5% short.

The deeper problem is that a cup measures volume and flour is sold by weight. Scooped or spooned, a cup of flour lands somewhere between roughly 120 and 150 g, and no converter can tell which one the recipe author had in mind. Weigh the ingredient. Converting a vague measurement to three decimal places does not make it less vague.

Which conversions are not simple multiplication?

Two, and both catch people who reach for a factor.

Temperature. Celsius and Fahrenheit disagree about where zero sits as well as about how big a degree is, so the conversion takes an offset and not just a factor: multiply by 1.8, then add 32. That is also why it lives in a separate temperature converter rather than in the unit list. The mental shortcuts, and the point where each one stops being close enough, are in converting Celsius to Fahrenheit.

Fuel economy. Miles per gallon and liters per 100 km are reciprocals, not multiples: doubling the mpg halves the liters figure. And because there are two gallons, the same "30 mpg" is 7.8 L/100 km in the United States and 9.4 L/100 km in Britain. A factor table cannot express either of those, which is why a general converter should not pretend to.

How much precision do you actually need?

For cooking, travel, body weight and furniture, three significant figures is generous. For a customs form or a machining tolerance, use the exact factors above and round only at the end — rounding at each step of a two-hop conversion is how you end up millimeters out on a part.

And for anything with legal or engineering weight, a browser calculator is not the final word. Arithmetic in JavaScript carries about 15 to 17 significant digits, so the last digit of a long result can be off by one. That is nothing in a kitchen or a garage, and not something to cite in a certification document.

Last updated September 19, 2026