Recipes in cups when your kitchen scale reads grams. A spec sheet in inches when everything else you own is in centimetres. A US job post listing a salary in pounds when you think in kilograms. Unit conversion is one of those small, constant annoyances — and most of the time, the actual maths is simple enough that a calculator shouldn't be necessary. Here's a quick reference for the conversions people look up most, plus a free tool that handles all of them at once.
Length
- 1 inch = 2.54 centimetres (exact — this is the legal definition of the inch)
- 1 foot = 0.3048 metres
- 1 yard = 0.9144 metres
- 1 mile = 1.609344 kilometres
Weight
- 1 pound = 0.45359237 kilograms (also an exact, legally defined value)
- 1 ounce = 28.349523125 grams
- 1 stone = 6.35029318 kilograms
- 1 US short ton = 907.18474 kilograms
Temperature
Temperature doesn't convert with a simple multiplication the way length and weight do, because the Celsius and Fahrenheit scales start at different zero points. The formulas:
- °F to °C: subtract 32, then multiply by 5/9
- °C to °F: multiply by 9/5, then add 32
- °C to Kelvin: add 273.15 (Kelvin has no negative numbers, so this one has no multiplication at all)
Volume
- 1 US cup = 236.588 millilitres
- 1 US gallon = 3.78541 litres
- 1 UK (imperial) gallon = 4.54609 litres — not the same as a US gallon, which trips people up in recipes and fuel-economy figures
- 1 tablespoon = 14.7868 millilitres
Why these numbers are worth knowing, even with a converter handy
Memorizing exact factors isn't necessary, but a rough sense of scale helps you catch an obviously wrong answer — if you convert 10 pounds and get back 453 kilograms instead of roughly 4.5, you'll know something was entered wrong before it causes a real mistake.
For anything beyond a rough mental estimate, Brisqnex's free Unit Converter covers length, weight, temperature, area, volume, speed, time and data units in one tool, with no signup and nothing to install. If the numbers you're converting are Roman numerals rather than units of measurement, the Roman Numeral Converter and Number Base Converter handle those instead.
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