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ConverterUnit Converter
Length, mass, volume, area, speed and temperature, all in one converter. It reads from the same factor table the rest of the site uses, so a value converted here matches the quick-answer bar on the homepage exactly.
Result
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Exact by definition, or exact by measurement
Not every conversion factor has the same kind of certainty behind it. Some are fixed by international agreement and will never change; others were originally measured against a physical object or a natural phenomenon and only look precise because they have been rounded to a lot of decimal places.
| Unit | Exact value | Status |
|---|---|---|
| 1 inch | 25.4 mm | Exact, by definition |
| 1 yard | 0.9144 m | Exact, by definition |
| 1 mile | 1,609.344 m | Exact, by definition |
| 1 pound (avoirdupois) | 0.45359237 kg | Exact, by definition |
| 1 nautical mile | 1,852 m | Exact since 1929 |
Before 1959, the US and UK each defined the inch slightly differently against their own national standard bars, and the two versions of the yard drifted apart by a couple of parts per million. The 1959 agreement between the US, UK, Australia, Canada, New Zealand and South Africa fixed the inch at exactly 25.4 mm and derived everything else — foot, yard, mile, pound, ounce, stone — from that single number and the exact pound figure above. Nothing in that table is rounded; it is the definition.
Compare that with the nautical mile. It was never tied to a metal bar — it was originally meant to be one minute of arc along a meridian, which is not a fixed distance because the Earth is not a perfect sphere. Rather than pick one of the many measured versions, the 1929 International Hydrographic Conference simply legislated a round number, 1,852 metres, and everyone adopted it. It is exact today, but for a different reason than the inch is: it is a chosen constant, not a physical measurement refined into agreement.
A related oddity: the US kept using its own slightly longer "US survey foot" (about 0.3048006 m, versus the international foot's exact 0.3048 m) for land surveying until 2022 — a gap of roughly two parts per million that only matters over very long distances. This converter uses the international foot throughout, which is what every other context means by "foot".
The gallon and the pint are not one unit — they are two
"Gallon" and "pint" mean a different volume depending on which side of the Atlantic wrote the recipe, the fuel-economy figure or the pub menu. Both are legitimate units; they are not rounding of each other.
| Unit | US value | Imperial (UK) value | UK is larger by |
|---|---|---|---|
| Fluid ounce | 29.5735 ml | 28.4131 ml | US oz is 4.1% bigger |
| Pint | 473.176 ml | 568.261 ml | 20.1% |
| Quart | 946.353 ml | 1,136.523 ml | 20.1% |
| Gallon | 3.785412 L | 4.546090 L | 20.1% |
The fluid ounce runs the other way — the US ounce is the bigger one — because the US kept the old English wine gallon (divided into 128 of its own ounces) while the UK moved to a single Imperial system in 1824 with a gallon defined as the volume of 10 pounds of water, divided into 160 ounces. A US gallon is a little over four-fifths of an Imperial gallon (3.785412 ÷ 4.54609 ≈ 0.8327), which is why a car rated at 40 miles per US gallon would show a different, larger-sounding number if the same fuel economy were quoted in Imperial gallons. This converter lists both explicitly in the volume category, rather than guessing which one you mean from "gal".
Temperature does not scale like the others
Every other family here is a straight multiplication — double the metres, double the millimetres. Temperature is not, because Celsius and Fahrenheit have different zero points as well as different step sizes. Converting requires an offset as well as a ratio: °F = °C × 9/5 + 32, and Kelvin simply shifts Celsius by 273.15 with no multiplication at all, since a kelvin and a degree Celsius are the same size — Kelvin only moves the zero point to absolute zero.
Questions people ask
Why do UK and US recipes disagree on cup and tablespoon sizes?
A "cup" is not a standard unit at all in the way a litre is — it is a kitchen convenience that different countries have defined differently (US cup 236.59 ml, and Australia and the UK use 250 ml when they use cups at all). This converter uses the US customary cup, since that is the value it is most consistently defined against; for UK recipes, work in millilitres instead.
Which mile does the speed converter use?
The statute mile — 1,609.344 metres exactly, from the same 1959 agreement as the other imperial units. The nautical mile (1,852 m) is listed separately in the length category because knots are built from it, not from the statute mile.
Why does swapping direction not always give back my original number?
It does, to the precision shown. If you convert 10 km to miles and swap, you get back 10 km — the display just rounds to six decimal places, so a value that does not terminate cleanly (like most metric-to-imperial conversions) can look one digit different if you round twice.
Does this converter send anything to a server?
No. Every factor above is stored in the page's own script and the arithmetic runs in your browser tab.