Unit converter
Pick one of eleven categories — length, weight, temperature, volume, area, speed, energy, power, pressure, data or time — type a number, and every unit in that category is converted at once. That is deliberately more than you asked for. The second question is usually another unit in the same family, and the arithmetic is already done.
How to convert units
The factors here are the exact legal definitions and not rounded approximations, which matters more than it sounds for anything that compounds. The inch has been exactly 25.4 millimetres since the international agreement of 1959, and the pound exactly 0.45359237 kilograms, so a chain of conversions through several units does not accumulate error along the way.
Temperature is the one that needs an offset
It is the only category here whose scales do not share a zero point. Celsius to Fahrenheit is a multiplication by 9/5 followed by an addition of 32, and a converter built on factors alone gets it silently and badly wrong. The two scales cross at exactly −40, the single temperature at which both read the same number. A temperature difference is a separate case again and takes the factor with no offset, so a rise of 10 °C is a rise of 18 °F. Kelvin and Rankine both count from absolute zero, the first in Celsius-sized degrees and the second in Fahrenheit-sized ones; kelvin carries no degree symbol and no capital, having been redefined that way in 1967.
Every other category has its own trap
Each one is a separate row here, so the choice stays with you instead of being made on your behalf.
Data carries two whole systems. The gigabyte a drive manufacturer sells is 10⁹ bytes; the gibibyte Windows counts while still writing "GB" is 2³⁰, about 7.4 per cent more. Binary sizes exist because memory addressing is binary, which makes powers of two the natural quantities, and "kilo" was borrowed loosely for 1,024 until the habit stuck. macOS has used the SI meaning since 10.6, so the same drive honestly reports two different sizes on the two systems.
Power carries two horsepowers. Mechanical horsepower was defined as 550 ft·lbf/s and works out at about 745.7 W; metric horsepower was defined as 75 kgf·m/s, comes to about 735.5 W and is sold as PS. Neither convention gave way, so a PS figure is roughly 1.4 per cent smaller than the same number written as bhp. Energy carries the BTU, which has several definitions; the international table BTU of about 1,055 joules is the one used here. An air conditioner labelled 12,000 BTU means BTU per hour, a power rating of about 3.5 kW, or one ton of cooling.
Volume has two gallons about twenty per cent apart, and a cup that means three different things: a US cup is about 237 ml, a metric cup exactly 250 and a Japanese cup 200, and a recipe rarely says which it meant. The rows here are the US customary ones — for the cup and for the spoons alike — because a recipe written in cups is almost always American; for a metric or Japanese cup, work from millilitres. The fluid ounce is the trap inside that trap, because it differs in the opposite direction to the gallon: an imperial fluid ounce is slightly smaller than a US one, about 28.4 ml against 29.6, and yet an imperial pint is the larger of the two, because it holds twenty of them against the US sixteen. Speed carries the knot, one nautical mile per hour, about 1.15 mph or 1.85 km/h; it survives because a nautical mile is one minute of latitude, so a speed in knots maps straight onto a chart. Pressure carries mmHg, a survival of the mercury manometer that medicine never gave up, and the gap between gauge and absolute readings. Area is where the arithmetic itself surprises people: the factor is squared, so a foot of 0.3048 m becomes a square foot of 0.0929 m². The acre stays awkward for a different reason, having begun as the area a yoke of oxen could plough in a day before settling at 4,840 square yards, which fits nothing metric.
What people use it for
- Converting Celsius to Fahrenheit for a recipe or a forecast
- Turning litres into US or imperial gallons
- Reading a floor plan in square feet as square metres
- Checking a car’s kW figure against its horsepower
- Converting tyre pressure between bar and psi
- Reading a speed limit in km/h as mph
- Comparing an electricity bill in kWh against joules or BTU
- Working out why a 1 TB drive shows as 931 GB
Questions
Minus forty. It is the only point where the two scales cross, which makes it the one reading that needs no conversion at all.
Where an exact definition exists, yes. The inch is exactly 25.4 mm and the pound exactly 0.45359237 kg by international agreement.
Its scales do not share a zero. Celsius to Fahrenheit needs an offset as well as a factor, unlike every other category here.
Multiply by 9/5 and add 32. Reverse it by subtracting 32 first, then multiplying by 5/9. A difference converts by the factor alone, so a rise of 10°C is a rise of 18°F.
No, and both are listed as separate rows. A US gallon is 3.785 litres and an imperial gallon 4.546, about twenty percent larger, which is why US and UK mpg figures are not comparable.
A US cup is about 237 ml, a metric cup exactly 250, and a Japanese cup 200. Recipes rarely say which. The cup row here is the US one, since that is what a recipe written in cups almost always means; for the other two, work from millilitres. A litre, for reference, is exactly one cubic decimetre, so a cubic metre is exactly 1,000 litres.
US customary throughout: a teaspoon of about 4.93 ml, a tablespoon of about 14.79 and a cup of about 237. An Australian tablespoon is 20 ml, which takes four teaspoons rather than three — the tablespoon-to-teaspoon page covers that one.
No, slightly smaller: about 28.4 ml against the US 29.6. An imperial pint is still the larger of the two, because it holds twenty fluid ounces against the US sixteen. The gallon differs in the other direction, which is what makes this pair so easy to get backwards.
A gigabyte is 10⁹ bytes; a gibibyte is 2³⁰, about 7.4 percent more. The IEC introduced the second set of names in 1998 to end the confusion.
The manufacturer counts a terabyte as 10¹² bytes; Windows divides by 1,024 three times while still writing "GB". Both describe the same drive.
No, a byte is eight bits. A 100 Mbps connection tops out around 12.5 MB/s.
Mechanical horsepower is about 745.7 W and metric horsepower, sold as PS, about 735.5 W. Both are listed, and they differ by roughly 1.4 percent.
A kilowatt is a rate; a kilowatt hour is an amount. A 2 kW heater running three hours uses 6 kWh, which is 21.6 million joules.
No. The Calorie on a food label is a kilocalorie: a thousand small calories, about 4,184 joules. That is why European labels prefer kJ.
About 14.504, so a tyre at 2.2 bar is about 32 psi. A bar is exactly 100,000 pascals.
Gauge reads zero in open air; absolute counts from vacuum and is about 14.7 psi higher at sea level. A tyre gauge shows gauge pressure.
About 62.1 mph; the mental version is divide by eight and multiply by five, accurate to within one percent. A knot is one nautical mile per hour, about 1.15 mph or 1.85 km/h, and it has its own row.
About 2.471. A hectare is exactly 10,000 square metres; the acre is 4,840 square yards and fits nothing metric neatly.
Because area is two-dimensional, so the factor is squared. A foot is 0.3048 m and a square foot 0.0929 m².
365.25; the Julian year, which is the convention in unit conversion. Calendar years are a different question.