Unit converter
Length, mass, volume, temperature, area, speed and data — with every unit shown side by side.
Runs entirely in your browser. Nothing you enter is uploaded.
Written and maintained by ReadoutKit / Takashi Yamazaki
How this works
One method, every unit
Each category here has a single base unit — the metre for length, the kilogram for mass, the litre for volume. Every other unit is stored as a multiplier against that base, so converting between any two units is two operations: divide into the base, multiply out of it.
result = value × factor(from) / factor(to)
This matters more than it sounds. A conversion table for nine length units would need 72 entries, every one a chance to introduce an error. Storing nine factors instead means all 72 conversions are derived from the same nine numbers and are guaranteed to be mutually consistent.
Temperature is the exception
Temperature scales do not share a zero, so a factor alone is not enough. Each unit also carries an offset:
°C = (°F − 32) × 5/9°C = K − 273.15
There is a consequence worth remembering: you cannot convert a temperature difference with these formulas. A rise of 10 °C is a rise of 18 °F, not 50 °F. The offset applies to a point on the scale, not to an interval along it.
The two scales cross at −40, where Celsius and Fahrenheit read the same number. It is a genuinely useful check: if a converter does not return −40 for −40, something is wrong with it.
Units that share a name but not a size
Several units come in more than one flavour. The differences are not rounding noise — they are large enough to spoil a recipe, misprice a shipment, or make a fuel-economy figure meaningless. Every number below is computed by the same converter above, from the defining factors rather than from a lookup table.
| Unit | Size（ml） | vs US cup |
|---|---|---|
| Cup (US) | 236.59 | — |
| Cup (metric) | 250.00 | +5.7% |
| Cup (Japan) | 200.00 | -15.5% |
| Fluid ounce (US) | 29.57 | -87.5% |
| Fluid ounce (UK) | 28.41 | -88.0% |
Cooking is where this bites most often. An American recipe asking for a cup of liquid means 236.59 ml, while the metric measuring jug on your counter pours 250 ml — 13.4 ml more per cup, or nearly a tablespoon extra in a four-cup batch. A Japanese cup is 200 ml, so the same recipe made with a Japanese measure comes out 36.6 ml short per cup instead.
The fluid ounce runs the other way: the UK one is smaller than the US one, even though the imperial gallon is larger. That is not a contradiction, it is a different division — an imperial gallon is 160 imperial fluid ounces, a US gallon is 128 US fluid ounces.
| Unit | Size（L） | vs US gallon |
|---|---|---|
| Gallon (US) | 3.785 | — |
| Gallon (UK) | 4.546 | +20.1% |
A 20% gap is enough to make fuel economy figures incomparable: 40 miles per imperial gallon is 33.3 miles per US gallon for exactly the same car. The two gallons descend from different standards — the US one from the old English wine gallon of 231 cubic inches, the imperial one redefined in 1824 as the volume of ten pounds of water — and neither was going to yield.
Mass has the same problem, under a shorter word.
| Unit | Size（kg） | vs tonne |
|---|---|---|
| Tonne (metric) | 1,000.00 | — |
| Short ton (US) | 907.18 | -9.3% |
| Long ton (UK) | 1,016.05 | +1.6% |
A short ton is 2,000 lb, a long ton is 2,240 lb, and a tonne is 1,000 kg. Read “50 tons” as the wrong one and you are out by roughly 4.6 tonnes — about the weight of two cars. Where a unit is ambiguous this converter names the variant explicitly rather than choosing one silently.
Decimal and binary data sizes
Data size is the one category where two competing systems are both in daily use. The decimal prefixes (KB, MB, GB) step by 1,000. The binary prefixes (KiB, MiB, GiB) step by 1,024, which matches how memory is actually addressed. Both are listed here.
| Prefix | Decimal (bytes) | Binary (bytes) | Gap |
|---|---|---|---|
| kilo | 1,000 | 1,024 | +2.4% |
| mega | 1,000,000 | 1,048,576 | +4.9% |
| giga | 1,000,000,000 | 1,073,741,824 | +7.4% |
| tera | 1,000,000,000,000 | 1,099,511,627,776 | +10.0% |
The gap compounds: 2.4% at kilo, 4.9% at mega, 7.4% at giga, 10.0% at tera. That last row is the whole explanation for the most common complaint about storage. A drive sold as 1 TB really does hold a trillion bytes; the operating system divides by 1,024 three times and reports about 931 GB. Nothing is missing, and no space was taken by formatting — the two numbers are the same quantity under two definitions.
The effect scales with the drive. A 4 TB disk shows as roughly 3.64 TiB, so the apparent shortfall grows from 69 GB to 364 GB as you buy larger. Network speeds add a second trap on top of this one: they are quoted in bits, not bytes, so a 100 Mbps connection tops out around 12.5 MB/s before any overhead.
Reading the full list
Below the result, the same value appears in every unit of the current category at once. This is usually faster than converting one pair at a time, particularly when you are checking whether a figure is plausible: seeing 180 cm alongside 5.9 ft and 70.9 in tells you immediately whether you typed the right number.
Common questions
- Why is a US gallon different from a UK gallon?
- They descend from different historical standards. The US gallon was fixed to the old English wine gallon of 231 cubic inches; the imperial gallon was redefined in 1824 as the volume of ten pounds of water. The imperial gallon is about 20% larger. Both are listed separately here, because quietly picking one is how recipes and fuel figures go wrong.
- What is the difference between GB and GiB?
- A gigabyte is 1,000³ bytes; a gibibyte is 1,024³. Storage manufacturers use the decimal definition, while most operating systems report the binary one. That is the entire explanation for why a "1 TB" drive shows up as about 931 GB: it really does hold a trillion bytes, but the computer is dividing by 1,024 three times instead of 1,000.
- Why does temperature need a different kind of conversion?
- Because the scales do not share a zero point. Length, mass and volume conversions are pure multiplication — half of two metres is one metre in every unit. Temperature needs an offset as well as a factor, which is why 0 °C is 32 °F rather than 0 °F, and why you cannot convert a temperature difference the same way you convert a temperature.
- How exact are these figures?
- The factors are the exact defined values, not rounded approximations. An inch is exactly 25.4 mm and a pound is exactly 0.45359237 kg, by international agreement since 1959. The calculation carries full precision internally; only the display is rounded.
- Which cup does a recipe mean?
- A US cup is 236.59 ml, a metric cup is 250 ml, and a Japanese cup is 200 ml. An American recipe measured with a metric jug gets 13.4 ml too much per cup — close to a tablespoon over four cups, which is enough to matter in baking. All three are listed separately here rather than folded into one "cup", because choosing one silently is exactly how the error gets made.
- How much is a ton?
- It depends which ton. A metric tonne is 1,000 kg, a US short ton is 2,000 lb (907.18 kg), and a UK long ton is 2,240 lb (1,016.05 kg). The spread between the smallest and largest is about 12%, so reading "50 tons" as the wrong variant misses by roughly 4.6 tonnes. Freight quotes and equipment specifications are where this usually surfaces.
- What is a tsubo?
- A Japanese unit of area still used in property listings, equal to two tatami mats. It is defined as exactly 400/121 square metres, about 3.31 m². Floor areas in Japan are frequently quoted in both tsubo and square metres, which makes converting between them a common daily task.
Sources
The formulas and rates used by this tool come from the documents below. Rules change; when a figure here disagrees with the source, the source is right.
Figures checked against the sources below on September 1, 2026.
- The International System of Units (SI Brochure), 9th editionBureau International des Poids et Mesures
- NIST Special Publication 811 — Guide for the Use of the International System of UnitsNational Institute of Standards and Technology