Unit Converter: Metric and Imperial, Done Right
Convert length, weight, volume, temperature, area, speed, digital storage, and time — and see your number in every unit of the category at once. Each factor follows the international definition, and the ones that are exact by definition are labelled as exact, so you always know whether a result is precise or rounded.
1 cm = 0.393701 in — this conversion is exact by definition, so the only rounding is in the display.
Conversion factors follow the international definitions (BIPM/NIST). Factors marked exact are exact by definition rather than rounded. Everything runs in your browser; nothing you type is sent anywhere.
TL;DR
Pick a category, type a number, and read the answer — plus the same number in every other unit. Three things trip people up, and this page explains all of them. One: most conversions between metric and imperial are exact, not approximate — an inch is 2.54 cm by definition, and a pound is 0.45359237 kg. Two: US customary and imperial are two different systems, and their volume units are about 20% apart — the single biggest cause of wrecked recipes and mismatched fuel-economy figures. Three: temperature converts with an offset, not a ratio, because Celsius and Fahrenheit start counting from different places.
Which conversions are exact, and which are approximations
This is the detail almost every converter site leaves out. Since the 1959 international yard and pound agreement, the imperial and US customary units have been defined in terms of metric ones. Nobody measures an inch against a metre any more — an inch is declared to be 2.54 centimetres, full stop. So when you convert 12 inches to 30.48 cm, that is not a close approximation; it is the definition, and the only rounding that ever happens is in how many digits fit on your screen.
Every factor below is exact. Write out all the digits and the arithmetic is perfect.
| Unit | Is exactly | Where it comes from |
|---|---|---|
| 1 inch | 2.54 cm | 1959 yard & pound agreement |
| 1 foot | 0.3048 m | 12 inches |
| 1 yard | 0.9144 m | 36 inches |
| 1 mile | 1,609.344 m | 5,280 feet |
| 1 nautical mile | 1,852 m | Defined outright in 1929 |
| 1 pound | 0.45359237 kg | 1959 yard & pound agreement |
| 1 ounce | 28.349523125 g | One-sixteenth of a pound |
| 1 stone | 6.35029318 kg | 14 pounds |
| 1 US gallon | 3.785411784 L | 231 cubic inches |
| 1 imperial gallon | 4.54609 L | Defined outright in 1985 |
| 1 acre | 4,046.8564224 m² | 4,840 square yards |
| 1 mph | 0.44704 m/s | One mile per hour |
| 1 knot | 0.5144̅ m/s | 1,852 m per hour (recurring decimal) |
What is notexact? Anything tied to the physical world rather than a committee. A calendar year is not 365 days — it is about 365.2425 — so the converter flags the year as an approximation while every other time unit is exact. Temperature conversions are exact as formulas but produce endless decimals in one direction (a third of a degree can never be written out in full). And a US “cup” depends on which cup you mean, which is the next section’s problem.
US customary and imperial are not the same system
People say “imperial” when they mean “not metric,” but the United States and the United Kingdom use two genuinely different sets of units. For length and weight they happen to agree — a US foot and a UK foot are the same foot, and a pound is a pound. For volume they do not agree at all, and the gap is large enough to ruin a recipe or make a car look 20% more economical than it is.
The oddity is that the imperial fluid ounce is smaller than the US one, yet every larger imperial measure is bigger — because an imperial pint holds 20 fluid ounces while a US pint holds only 16.
| Unit | US customary | Imperial (UK) | Imperial vs US |
|---|---|---|---|
| Fluid ounce | 29.5735 ml | 28.4131 ml | 3.9% smaller |
| Pint | 473.18 ml | 568.26 ml | 20.1% larger |
| Quart | 946.35 ml | 1,136.52 ml | 20.1% larger |
| Gallon | 3.785411784 L | 4.54609 L | 20.1% larger |
| Fl oz in a pint | 16 | 20 | 4 more |
Two places this bites in real life. Fuel economy: a car rated 30 MPG in the US is quoted as 36 MPG in Britain — the car has not changed, only the size of the gallon, so never compare a US MPG figure with a UK one directly. Cooking: a British recipe calling for a pint of milk wants 568 ml, and pouring a US pint gives you 95 ml too little. That is why this converter lists US and imperial fluid ounces, pints, and gallons as separate units rather than pretending there is one of each.
Temperature is the odd one out
Every other category on this page works by multiplication: metres times a number gives feet, and zero of one thing is zero of the other. Temperature breaks that rule, because Celsius and Fahrenheit do not share a zero. Celsius starts at the freezing point of water; Fahrenheit puts freezing at 32. So you need an offset as well as a ratio:
°C = (°F − 32) × 5/9
K = °C + 273.15
The 9/5 exists because a Celsius degree is a bigger step than a Fahrenheit one — 180 Fahrenheit degrees span the same range as 100 Celsius degrees between freezing and boiling. The +32 shifts the zero point. Miss the offset and everything is wrong; use it and the two scales line up perfectly: 0°C = 32°F, 37°C = 98.6°F, 100°C = 212°F.
There is exactly one temperature where the two scales agree: −40. Minus forty Celsius is minus forty Fahrenheit — the crossover point where the rising ratio and the falling offset cancel out. It is a handy sanity check: if a converter does not return −40 for −40, it is broken.
Kelvin is the tidy one. It uses the same size degree as Celsius but starts at absolute zero, so converting is pure addition: 0 K is −273.15°C, and there is nothing colder. One thing to watch: a temperature differenceconverts differently from a temperature. A rise of 10°C is a rise of 18°F, not 50°F — for differences you apply the ratio only, and drop the offset.
Why your “1 TB” drive shows up as 931 GB
Nobody stole your storage. The drive manufacturer and your operating system are counting in different bases, and both are being honest about it.
- Drive makers use decimal (SI) units. A kilobyte is 1,000 bytes, a megabyte 1,000², a gigabyte 1,000³, a terabyte 1,000⁴. This matches the ordinary meanings of kilo, mega, and giga everywhere else in science.
- Memory and many operating systems use binary units. Computers address memory in powers of two, so 1,024 bytes is a kibibyte (KiB), 1,024² a mebibyte (MiB), 1,024³ a gibibyte (GiB).
- The labels drifted. Windows still writes “GB” on a figure it calculated in GiB, which is where the confusion comes from. macOS and most Linux distributions switched to decimal GB years ago, which is why the same drive can read differently on two machines.
The arithmetic: a 1 TB drive holds 1,000,000,000,000 bytes. Divide that by 1,024³ and you get 931.32 GiB— the number Windows displays, labelled “GB.” A 500 GB drive lands at 465.66 GiB, and a 2 TB drive at 1.82 TiB. The gap grows with each prefix: about 2.4% at kilo, 4.9% at mega, 7.4% at giga, and 10% at tera.
This converter lists both families side by side — kB/MB/GB/TB and KiB/MiB/GiB/TiB — so you can put in the number on the box and see exactly what your file manager will report. Type 1 TB and look at the TiB and GiB rows.
The dozen conversions worth keeping in your head
For a precise answer, use the tool. For a mental estimate at a market stall or on a road sign, these rough rules get you within a few percent — and the third column tells you which direction the error goes.
| Conversion | Mental shortcut | Precise value |
|---|---|---|
| km to miles | Multiply by 0.6 | × 0.621371 |
| Miles to km | Add 60% | × 1.609344 (exact) |
| kg to pounds | Double it, add 10% | × 2.204623 |
| Pounds to kg | Halve it, take off 10% | × 0.45359237 (exact) |
| cm to inches | Divide by 2.5 | ÷ 2.54 (exact) |
| Metres to feet | Multiply by 3.3 | × 3.280840 |
| °C to °F | Double it, add 30 | × 9/5 + 32 |
| °F to °C | Take off 30, halve it | (− 32) × 5/9 |
| Litres to US gallons | Divide by 4 | × 0.264172 |
| Litres to imperial gallons | Divide by 4.5 | × 0.219969 |
| m² to ft² | Multiply by 10.8 | × 10.763910 |
| Hectares to acres | Multiply by 2.5 | × 2.471054 |
The “double it and add 30” trick for Celsius is deliberately crude — it is spot on at 10°C, about 2°F high at 20°C, and 10°F high at 40°C. Fine for deciding on a jacket, useless for an oven.
Where conversions actually go wrong
In practice, mistakes almost never come from a bad factor. They come from converting the wrong thing:
- Confusing weight with volume. A fluid ounce measures space; an ounce measures mass. They are only interchangeable for water, and even then only roughly. A cup of flour and a cup of honey weigh wildly different amounts, which is why serious baking recipes are written in grams.
- Squaring the wrong number. Area factors are the length factor squared, not the length factor. A metre is 3.28 feet, but a square metre is 10.76 square feet, because 3.28 × 3.28 ≈ 10.76. Doing area conversions with the length factor is a classic and expensive flooring mistake.
- Assuming a cup is a cup. The US legal cup used on nutrition labels is 240 ml, the older US customary cup is about 236.6 ml, and the metric cup used in Australia and New Zealand is 250 ml. This tool uses the 240 ml legal cup and says so.
- Mixing up short tons and long tons. A US short ton is 2,000 lb (907.18 kg), a UK long ton is 2,240 lb (1,016.05 kg), and a metric tonne is 1,000 kg. Three different weights sharing one word.
- Rounding too early. Chain three conversions with two decimal places each and the error compounds. Convert once, straight from source unit to target unit, and round only at the end — which is what this tool does internally.
Frequently asked questions
Is 1 inch exactly 2.54 cm?
Yes, exactly — not approximately. Under the 1959 international yard and pound agreement, the inch is defined as 2.54 centimetres, so the conversion is a definition rather than a measurement. The same is true of the foot (0.3048 m), the yard (0.9144 m), and the mile (1,609.344 m). Any rounding you see in a converted answer comes from the display, not the factor.
What is the difference between a US gallon and an imperial gallon?
A US gallon is 3.785411784 litres and an imperial gallon is 4.54609 litres, so the imperial gallon is about 20% larger. That is why a car rated 30 MPG in the United States is quoted as roughly 36 MPG in Britain for identical fuel use. Never compare US and UK fuel-economy figures without converting first, and always check which gallon a recipe or spec sheet means.
How do I convert Celsius to Fahrenheit in my head?
The exact formula is °F = °C × 9/5 + 32. For a quick estimate, double the Celsius number and add 30 — that is accurate at 10°C and drifts a few degrees high as it gets warmer, which is fine for weather and useless for cooking. Going the other way, subtract 30 and halve. The one temperature that needs no conversion is −40, where both scales read the same.
Why does my 1 TB hard drive show as 931 GB?
Because two different definitions are in play. The manufacturer sells you 1,000,000,000,000 bytes, using the decimal terabyte. Windows divides that by 1,024 three times, giving 931.32 gibibytes, then labels the result "GB" anyway. No storage is missing — it is a labelling mismatch. macOS and most Linux systems report in decimal GB and will show the drive as roughly 1 TB.
Is a UK pint the same as a US pint?
No. A US pint is 16 US fluid ounces, about 473 ml, while an imperial pint is 20 imperial fluid ounces, about 568 ml — roughly 20% more. Confusingly, the imperial fluid ounce is the smaller of the two ounces at 28.41 ml against 29.57 ml; the imperial pint is still bigger because it contains four more of them.
How many pounds is 1 kilogram?
One kilogram is about 2.204623 pounds. The exact definition runs the other way: one pound is exactly 0.45359237 kilograms, so kilograms to pounds is a division that produces a non-terminating decimal. For mental arithmetic, double the kilograms and add 10% — 70 kg becomes roughly 154 lb, which is almost exactly right.