Converters Calculators

Which converter should you use?

Exact conversion factors

Since the 1959 international yard and pound agreement, the common imperial-to-metric factors are defined values, not measurements — they are exact by definition, and everything else derives from them.

The trap: US and imperial units share names but not sizes

A pint in London and a pint in Chicago are different volumes. So are gallons, fluid ounces and cups. A recipe or fuel figure converted with the wrong system is out by up to 20%.

Note the reversal: the imperial gallon is bigger, but the imperial fluid ounce is smaller. That is because the imperial gallon holds 160 fluid ounces and the US gallon holds 128.

Kilograms to stones and pounds

Body weight in the UK and Ireland is spoken in stones and pounds, and almost nowhere else. One stone is 14 pounds.

Stones = kg ÷ 6.35029318

Temperature does not scale — it shifts

Every other conversion on this page is a multiplication. Temperature is not, because the scales do not share a zero point. Doubling 10°C does not give 20°C worth of heat, and “twice as warm” has no meaning outside Kelvin.

°F = °C × 9 ÷ 5 + 32

°C = (°F − 32) × 5 ÷ 9

K = °C + 273.15

The two scales cross at exactly −40°, which is the same temperature in Celsius and Fahrenheit — a useful sanity check that a conversion has been done the right way round.

Weight and volume are not interchangeable

A cup of flour and a cup of water occupy the same space and weigh very different amounts — roughly 120 g against 240 g. Recipes written by volume therefore do not convert to grams with a single factor; each ingredient has its own density. The cooking converter handles the common ones by ingredient rather than applying one number to everything, and the volume calculator converts volume to weight when you supply the density.

Height in feet and inches, and in centimetres

Height is the conversion people look up most, usually to fill in a form written for the other system. One inch is exactly 2.54 cm, so a height in feet and inches converts as (feet × 12 + inches) × 2.54.

Going the other way, divide the centimetres by 2.54 for total inches, then take the whole feet and keep the remainder: 175 cm is 68.9 inches, which is 5 feet 8.9 inches. Rounding the inches before dividing is what produces the off-by-an-inch answers you sometimes see. Both directions are handled by the length converter, and BMI needs the metric form — the BMI calculator accepts either.

The formulas behind this category, and the published sources they come from, are listed on the methodology page.