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Unit converter

Convert length, weight, temperature, speed, area, volume and time between the units in everyday use, with the exact definition shown beside each answer.

What you are converting

Distances and dimensions, metric and imperial.

Where two units share a name and are not the same size, both are listed and both say which they are. A US gallon and an imperial gallon are a fifth apart.

Converted

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Pick a category, enter a figure and choose the two units.

Seven categories, and the units people actually use

There are hundreds of units a converter could carry. Carrying them all makes the tool slower to use, because the one you want is buried among twenty you will never need. So each category here holds what is in daily use and stops: length, weight, temperature, speed, area, volume and time.

The exception is where two units share a name and are not the same size. Then both are listed and both say which they are, because guessing on the visitor’s behalf is the one thing a converter must not do.

The conversions that catch people out

Gallons and pints. A US gallon is about 3.79 litres and an imperial gallon is about 4.55, a difference of a fifth. The same split runs through pints (473 mL against 568 mL) and fluid ounces. A recipe, a fuel figure or a beer measure can be either, and nothing in the number tells you which.

Tons. Three of them. A US short ton is 2,000 lb, a UK long ton is 2,240 lb, and a metric tonne is 1,000 kg. They sit within 12% of each other, which is close enough to look like a rounding difference and far enough to matter on an invoice.

Ounces. A fluid ounce measures volume and an ounce measures weight. They share a name because a fluid ounce of water weighs about an ounce, which is a coincidence of water rather than a rule.

Why the imperial numbers are exact

The international yard and pound agreement of 1959 defined the inch as exactly 25.4 millimetres and the pound as exactly 0.45359237 kilograms. Everything else in the imperial system is a whole number multiple of one of those, so the conversions are definitions rather than measurements.

That is why this tool prints 1.609344 kilometres for a mile rather than 1.61. Every digit is real. Where a unit has a definition worth seeing, it appears under the answer.

Two things this tool will not do

It will not convert months or years. A month is 28 to 31 days and a year is 365 or 366, so a fixed factor for either would be wrong most of the time without saying so. Time stops at weeks. A gap between two real dates is a calendar question rather than a multiplication.

It will not convert a temperature below absolute zero. Nothing is colder than -273.15°C, so a figure below it is a typo rather than a reading, and answering it would confirm the mistake instead of catching it.

Questions

Which units does this cover?
Seven categories: length, weight, temperature, speed, area, volume and time. Each holds the units in everyday use rather than every unit that exists. A converter carrying furlongs and hogsheads is slower to use for the conversion you actually came for, so the lists are kept short on purpose.
Why is a US gallon different from a UK gallon?
They were standardised separately and never reconciled. A US gallon is exactly 231 cubic inches, about 3.785 litres. An imperial gallon is exactly 4.54609 litres, about a fifth larger. The same split runs through pints and fluid ounces, so a US pint is 473 mL and an imperial pint is 568 mL. Every one of them is labelled here, because an unlabelled 'gallon' is wrong for roughly half the people who type it.
Why can I not convert months or years?
Because they are not fixed lengths. A month is 28 to 31 days and a year is 365 or 366, so any single factor would be wrong most of the time and quietly so. Time here stops at weeks. For a gap between two real dates, use the date difference calculator, which works on the calendar rather than on a multiplier.
Are these conversions exact?
Nearly all of them, yes. Since the international yard and pound agreement of 1959 an inch has been exactly 25.4 mm and a pound exactly 0.45359237 kg, and the rest of the imperial units are exact multiples of those. So 1.609344 kilometres in a mile is a definition rather than a rounded measurement. Where a figure is defined exactly, this tool says so beside the answer.
Why does temperature behave differently?
Because the temperature scales do not share a zero. Metres and feet both start at nothing, so converting is one multiplication and twice the number really is twice as long. Celsius starts at water freezing and Fahrenheit about 32 degrees below that, so each scale needs its own formula, and there is no such thing as 'how many degrees Fahrenheit in a degree Celsius'. This tool leaves that line out rather than printing a number that looks like an answer.
What happens to my figure when I change the category?
The figure stays and the units change. Millimetres are not a unit of weight, so the two unit choices are replaced with the pair that category opens on. Nothing you typed is thrown away.
Why are there no cups?
Because there is no single answer. A US customary cup is 236.6 mL, the cup used on US nutrition labels is 240 mL, and an Australian cup is 250 mL. Converting one without knowing which kitchen the recipe came from would be guessing, so the volume category offers millilitres, fluid ounces and the rest instead.
Does anything I type here get sent anywhere?
No. The conversion runs in your browser, in the page you are already looking at. Nothing is uploaded, stored or logged, and closing the tab clears it. You can disconnect from the internet and the tool will keep working.

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