Metric and Imperial Conversions: A Practical Cheat Sheet

By Undercover Nerd Project · Published November 12, 2026 · 3 min read

The exact conversion factors for length, weight and volume, easy mental shortcuts, and the US versus UK differences that cause most mistakes.

Most of the world uses the metric system, a few countries still rely on imperial or US customary units, and many people, Canadians included, move between the two every day: kilometres on the road signs, pounds on the bathroom scale, millilitres on the bottle and cups in the recipe. This cheat sheet gives the exact factors, quick estimates you can do in your head, and the traps that cause most conversion mistakes.

Length

Since 1959 the inch has been defined as exactly 25.4 millimetres, and every other imperial length follows from it. These factors are exact:

In your head: multiply kilometres by 0.6 to get miles (the exact factor is 0.621), and miles by 1.6 to get kilometres. An inch is about 2.5 cm, so 10 cm is roughly 4 inches.

Worked example: a marathon is 42.195 km. Dividing by 1.609344 gives 26.22 miles. A person who is 6 ft tall is 6 × 0.3048 = 1.8288 m, or about 183 cm.

Weight and mass

In your head: to turn kilograms into pounds, double the number and add ten percent: 70 kg is 140 + 14 = 154 lb (the exact value is 154.3). To go the other way, halve the pounds and take off ten percent of the result: 180 lb is 90 − 9 = 81 kg (exactly 81.6).

Watch out for the ton. A metric tonne is 1,000 kg. A US short ton is 2,000 lb (about 907 kg). A UK long ton is 2,240 lb (about 1,016 kg). Three different "tons" within about ten percent of each other make for expensive mistakes.

Volume

In your head: a litre is a little more than a US quart (0.946 L), and four litres is slightly more than a US gallon.

The US and UK differences that trip people up

• Gallons. An imperial gallon is about 20 percent larger than a US gallon (4.546 L versus 3.785 L). A car's fuel economy quoted in "miles per gallon" can be a different number depending on which gallon was used.
• Pints and fluid ounces. A US pint is 16 fluid ounces. An imperial pint is 20, and its fluid ounces are slightly smaller. A pint of beer is not the same volume in a London pub and a US bar.
• Cups. The US cup is 236.6 mL, but several countries use a 250 mL metric cup in recipes. For baking, use weight when the recipe offers it.
• Ounces. A fluid ounce measures volume and an ounce measures weight. Eight fluid ounces of water weighs about 8.3 ounces, not 8.

Cooking: weight versus volume

A cup of flour, a cup of sugar and a cup of honey all have the same volume and very different weights. That is why recipes that give grams are more reliable. When you convert a recipe, convert like with like: volumes to volumes and weights to weights. Do not turn a cup into grams unless you know the ingredient's density.

Rounding sensibly

Round to the precision that your purpose needs. A recipe rarely needs better than the nearest 5 mL. A shipping label needs a tenth of a kilogram. A machine part might need a hundredth of a millimetre. When you convert in several steps, keep extra digits until the end and round only the final answer, otherwise small errors build up.

Related units worth knowing

• Area. 1 hectare is 10,000 m² and about 2.47 acres. 1 acre is about 4,047 m².
• Speed. 100 km/h is about 62 mph. 60 mph is about 97 km/h.
• Temperature does not convert by a single multiplier. See the temperature conversion guide.

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