Reading a recipe in cups when you cook in grams (or vice versa). Translating a flight distance in nautical miles into kilometres. Converting an outdoor-temperature forecast from °C to °F before travel. Sizing a piece of furniture: 72 inches wide → will it fit through a 1.9 m doorway? Reading scientific paper measurements in SI when you think in imperial.

Temperature is not a ratio. 0°C is not "no temperature" — it's a reference point. Doubling Celsius doesn't double the heat. The conversion uses the additive offsets (273.15 to/from Kelvin, 32 between C and F) which is why the tool uses functions for temperature, not multipliers. US vs UK fluid ounces are different. 1 US fl oz = 29.57 mL, 1 UK fl oz = 28.41 mL. Always check which standard a recipe uses. "Tonne" vs "ton". Metric tonne = 1000 kg. US short ton = 907 kg. UK long ton = 1016 kg. The tool's "t" is the metric tonne. Mass vs weight. Strictly, kg is mass and pounds are mass too (despite the colloquial "I weigh 70 kg"). The tool treats them as a mass-to-mass conversion. For force (newtons, pound-force), you need a different category. Round at the end, not the middle. Don't convert m → ft, round, then ft → in — accumulate errors. Go straight to the target unit. Stones & pounds. A British weight: 1 stone = 14 lb. The tool has stone (st) but you'll need to do the lb portion separately for "11 st 4 lb"-style entries.

The Mars Climate Orbiter cost $327M to a unit-conversion bug. 1999: Lockheed Martin sent thrust data in pound-seconds; NASA's navigation software expected newton-seconds. Result: orbit insertion at 57 km altitude instead of the planned 226 km, atmospheric burn-up. The lesson institutionalised in aerospace afterwards: all interfaces carry units in their type signatures, and conversions happen explicitly at boundaries. Same lesson applies to everyday spreadsheets and code. Significant-figures preservation is its own discipline. Converting 100 km/h to mph and reporting "62.1371192237 mph" is wrong — the source had 3 significant figures (could be anywhere 99.5-100.5), so the result should be 62.1 mph. Over-precision in unit conversion is a tell that the converter doesn't understand measurement uncertainty. This tool shows generous precision so you can pick the right number of digits; you should then round to the input's precision. Cooking recipes are a special case where exactness is a trap. "Convert 1 cup of flour to grams" — the answer is 120g, 125g, 128g, or 145g depending on whether the flour is sifted, spooned, scooped, or packed. Recipe conversions need a step-by-step approach (weigh flour to known density, account for ingredient temperature for liquids near phase boundaries) more than they need 6-decimal arithmetic. Volume is rarely a clean conversion across systems. A US fluid ounce is 29.57 mL; a UK (Imperial) fluid ounce is 28.41 mL — close but not identical. A US pint is 473 mL; a UK pint is 568 mL — different by ~20%. Recipes specifying "1 pint" without country context routinely fail. When converting volume, identify the source system before reaching for the math. Temperature isn't a ratio — be careful with deltas. "20°C is twice as hot as 10°C" is wrong in any meaningful physical sense — both are arbitrary points on a scale with a non-zero origin. Temperature differences (Δ) convert cleanly (10 K change = 10°C change = 18°F change), but absolute temperatures don't. Most physics formulas using temperature actually need Kelvin (absolute scale, origin at 0).

Unit Converter

Convert between metric and imperial units of length, weight, temperature, volume, and area.

What is this for?

Most of the world is metric, the US is imperial, the UK is half-and-half, recipes are in cups when they should be in grams, and somewhere in the middle a kid's school project asks for "5 yards in centimetres". This tool runs the conversions for length, weight, temperature, volume, area, time, and speed using high-precision definitions — and spreads the result across every unit in the category at once, so you don't have to convert twice.

When to use it

Reading a recipe in cups when you cook in grams (or vice versa).
Translating a flight distance in nautical miles into kilometres.
Converting an outdoor-temperature forecast from °C to °F before travel.
Sizing a piece of furniture: 72 inches wide → will it fit through a 1.9 m doorway?
Reading scientific paper measurements in SI when you think in imperial.

What's accurate, and what isn't

Length, weight, temperature, area, speed use the SI definitions and the international yard-and-pound agreement (1959), so they're precise to the precision your input has.
Volume can be fiddly: a US "gallon" (3.785 L) and a UK "imperial gallon" (4.546 L) are different. The tool labels which is which.
Cup / tablespoon / teaspoon default to US measure here. UK and Australian cups are slightly different (250 mL in AU, 240 mL in US).
"Month" and "year" in the time category use averages (30.44 days / 365.25 days). Don't use this for legal or accounting calculations where exact months matter — use a date calculator instead.

Common gotchas

Temperature is not a ratio. 0°C is not "no temperature" — it's a reference point. Doubling Celsius doesn't double the heat. The conversion uses the additive offsets (273.15 to/from Kelvin, 32 between C and F) which is why the tool uses functions for temperature, not multipliers.
US vs UK fluid ounces are different. 1 US fl oz = 29.57 mL, 1 UK fl oz = 28.41 mL. Always check which standard a recipe uses.
"Tonne" vs "ton". Metric tonne = 1000 kg. US short ton = 907 kg. UK long ton = 1016 kg. The tool's "t" is the metric tonne.
Mass vs weight. Strictly, kg is mass and pounds are mass too (despite the colloquial "I weigh 70 kg"). The tool treats them as a mass-to-mass conversion. For force (newtons, pound-force), you need a different category.
Round at the end, not the middle. Don't convert m → ft, round, then ft → in — accumulate errors. Go straight to the target unit.
Stones & pounds. A British weight: 1 stone = 14 lb. The tool has stone (st) but you'll need to do the lb portion separately for "11 st 4 lb"-style entries.

Expert notes

The Mars Climate Orbiter cost $327M to a unit-conversion bug. 1999: Lockheed Martin sent thrust data in pound-seconds; NASA's navigation software expected newton-seconds. Result: orbit insertion at 57 km altitude instead of the planned 226 km, atmospheric burn-up. The lesson institutionalised in aerospace afterwards: all interfaces carry units in their type signatures, and conversions happen explicitly at boundaries. Same lesson applies to everyday spreadsheets and code.
Significant-figures preservation is its own discipline. Converting 100 km/h to mph and reporting "62.1371192237 mph" is wrong — the source had 3 significant figures (could be anywhere 99.5-100.5), so the result should be 62.1 mph. Over-precision in unit conversion is a tell that the converter doesn't understand measurement uncertainty. This tool shows generous precision so you can pick the right number of digits; you should then round to the input's precision.
Cooking recipes are a special case where exactness is a trap. "Convert 1 cup of flour to grams" — the answer is 120g, 125g, 128g, or 145g depending on whether the flour is sifted, spooned, scooped, or packed. Recipe conversions need a step-by-step approach (weigh flour to known density, account for ingredient temperature for liquids near phase boundaries) more than they need 6-decimal arithmetic.
Volume is rarely a clean conversion across systems. A US fluid ounce is 29.57 mL; a UK (Imperial) fluid ounce is 28.41 mL — close but not identical. A US pint is 473 mL; a UK pint is 568 mL — different by ~20%. Recipes specifying "1 pint" without country context routinely fail. When converting volume, identify the source system before reaching for the math.
Temperature isn't a ratio — be careful with deltas. "20°C is twice as hot as 10°C" is wrong in any meaningful physical sense — both are arbitrary points on a scale with a non-zero origin. Temperature differences (Δ) convert cleanly (10 K change = 10°C change = 18°F change), but absolute temperatures don't. Most physics formulas using temperature actually need Kelvin (absolute scale, origin at 0).