Unit Converter
Convert between more than seventy units across eight categories. Type in either box — whichever you touch last drives the other — and expand the table to see every unit at once.
1 m = 3.280839895 ft
Converted on your device. Nothing you type is sent anywhere.
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How to use the converter
- Pick a category — length, weight, temperature, area, volume, speed, time or data.
- Choose your units in the two dropdowns.
- Type in either box. The one you touch last drives the other.
- Use the swap button to reverse the direction without retyping.
- Expand the table to see your value in every unit of that category at once.
The constants are exact, not approximate
Most unit conversions are not measurements. They are definitions, agreed internationally, and this converter uses them at full precision:
| Unit | Exact value |
|---|---|
| 1 inch | 0.0254 metres |
| 1 foot | 0.3048 metres |
| 1 mile | 1609.344 metres |
| 1 nautical mile | 1852 metres |
| 1 pound | 0.45359237 kilograms |
| 1 US gallon | 3.785411784 litres |
| 1 imperial gallon | 4.54609 litres |
| 1 acre | 4046.8564224 square metres |
The inch has been exactly 0.0254 m since the international yard and pound agreement of 1959, which finally aligned the US and Commonwealth definitions that had differed slightly until then.
Because these are definitions, the only error in a conversion here is floating-point rounding in the last few significant digits — orders of magnitude below anything that matters in practice.
Results are shown to ten significant figures and drop trailing zeros, so 1.5 stays 1.5 rather than becoming 1.500000000. Very large and very small results fall back to exponential notation instead of printing a wall of zeros.
Temperature is the one that breaks naive converters
Every other category here works by multiplication. Metres to feet is one number. Kilograms to pounds is one number. Reverse it by dividing.
Temperature does not work that way, and the difference is not cosmetic.
Celsius and Fahrenheit have different zero points. 0°C is the freezing point of water; 0°F is roughly the freezing point of a brine solution, chosen in the 1720s for reasons that made sense to Fahrenheit at the time. So converting between them needs a scale and an offset:
°F = °C × 9/5 + 32
A converter built on a table of multipliers — which is the obvious way to build one — will convert 20°C to 36°F instead of 68°F. That answer is wrong but not obviously absurd, which is exactly what makes the bug survive.
Two consequences worth knowing:
−40 is the same in both scales. It is the single point where the two lines cross. A quick way to check any temperature converter.
Kelvin and Rankine start at absolute zero. 0 K is −273.15°C, the coldest temperature that can exist. Rankine is the Fahrenheit-sized equivalent, so 0°R is also absolute zero and 0 K equals 0°R exactly. Enter something below absolute zero here and the tool says so, because the arithmetic will happily continue producing numbers for a temperature that cannot exist.
US and imperial are not the same, and it matters most in the kitchen
Both systems descend from English units, and both kept the same names while changing some of the sizes. That is a worse outcome than using different names would have been.
| Unit | US | Imperial | Difference |
|---|---|---|---|
| Fluid ounce | 29.57 ml | 28.41 ml | 4% |
| Pint | 473 ml | 568 ml | 20% |
| Gallon | 3.785 L | 4.546 L | 20% |
A recipe calling for "a pint of stock" means noticeably different amounts on either side of the Atlantic. For a stew it does not matter; for bread or a custard it does.
Weight has a similar trap. The ton is three different things: a US short ton (2000 lb, about 907 kg), an imperial long ton (2240 lb, about 1016 kg), and a metric tonne (exactly 1000 kg), which sits between them. All three are listed separately here.
Length is the well-behaved exception — a US mile and an imperial mile really are the same length.
Kilobytes, kibibytes, and the missing disk space
The Data category exists mostly to explain one thing.
SI prefixes mean powers of 1000. A kilobyte is 1000 bytes, a megabyte 1,000,000, a terabyte 1,000,000,000,000. That is what "kilo" means everywhere else in measurement.
Computers are organised in powers of 2. 1024 is close to 1000, so early on the prefixes were borrowed loosely: a "kilobyte" meant 1024 bytes.
The two definitions drift further apart at every step — 2.4% at kilo, 4.9% at mega, 7.4% at giga, 10% at tera.
This is why a "1 TB" drive shows as about 931 GB in Windows. Nothing is missing and nobody is cheating. The manufacturer counted 1,000,000,000,000 bytes, which is what a terabyte means. Windows divided by 1024 three times, got 931.32, and labelled it GB when the correct name is GiB — gibibyte.
The binary prefixes (KiB, MiB, GiB, TiB) were standardised in 1998 precisely to end this confusion. macOS and Linux mostly use the SI meaning now; Windows still reports in 1024s with SI labels. Both meanings are in the converter so you can see the gap for yourself.
Where the odd units come from
Nautical mile. One minute of latitude — one sixtieth of a degree — which is why it is the natural unit for navigation. Defined as exactly 1852 metres. A knot is one nautical mile per hour.
Acre. Historically the area a yoke of oxen could plough in a day: one furlong by one chain, or 4840 square yards. The awkward number is the fossil of a very practical origin.
Stone. 14 pounds, still the standard way body weight is discussed in the UK and Ireland. Rare almost everywhere else.
Mach. A ratio rather than a fixed speed — how fast something is moving relative to the speed of sound in the surrounding air. Sound is slower in colder air, so Mach 1 at cruising altitude is slower than at sea level. The figure here uses the sea-level standard of 340.29 m/s.
Limits and requirements
Conversions within a category only. There is no meaningful answer to "how many kilograms in a metre".
Months and years are averages. 30.44 days and 365.25 days. For exact date arithmetic use a date calculator.
Mach is a sea-level reference, not a live calculation against current conditions.
Ten significant figures. More would be false precision given typical inputs.
Browser support. Any modern browser, desktop or mobile. No installation, no account.
Privacy: how this tool handles your input
Every conversion runs in your browser. Nothing is transmitted, logged or stored, and closing the tab discards it.
For a unit converter the privacy argument is weaker than for a file tool — nobody is leaking a contract by converting kilometres to miles. But the practical benefit is real and immediate: the tool works offline. Load the page once and it keeps converting on a plane, in a basement, on a train through a tunnel, or anywhere the signal drops. There is no request to fail because there is no request.
It is also instant. A conversion that makes a round trip to a server takes a hundred milliseconds or more before the answer appears; one that happens in the page appears as you type. For a tool people use dozens of times in a row, that difference is the whole experience.
You can confirm it: open developer tools, watch the Network tab while you convert, and see that nothing goes out. Or disconnect entirely once the page has loaded.
Frequently asked questions
Why can I type in both boxes?
Because conversion goes both ways and it is annoying to have to decide in advance which direction you want. Whichever box you type in last is the one driving, and the other updates. The swap button flips the units without you having to re-enter anything, which is the fastest way to check a conversion in reverse.
How accurate are the conversions?
The constants are the exact defined values wherever one exists. An inch is exactly 0.0254 metres by international agreement, a pound is exactly 0.45359237 kilograms, a US gallon is exactly 231 cubic inches, and a nautical mile is exactly 1852 metres. Those are definitions rather than measurements, so the only error in a result is floating-point rounding in the last few digits — far below anything you would notice.
Why is temperature handled differently from everything else?
Because it is the only category here where the scales do not share a zero. Converting metres to feet is one multiplication. Converting Celsius to Fahrenheit needs a multiplication and an offset, because 0°C is 32°F rather than 0°F. A converter built on a simple table of multipliers gets every temperature wrong, and gets it wrong plausibly enough that people do not notice. This one handles the offset properly, which is why −40°C really does come out as −40°F.
Which gallon is this — US or imperial?
Both are listed separately, because they are genuinely different sizes and mixing them up is the classic recipe disaster. A US gallon is 3.785 litres; an imperial gallon is 4.546 litres, about 20% larger. The same applies to pints and fluid ounces: a US pint is 473 ml, an imperial pint is 568 ml. Every volume unit here is labelled US or imperial so there is no guessing.
Why does my 1 TB hard drive show as 931 GB?
Because the manufacturer and the operating system are using different definitions of the prefix. The drive maker counts a terabyte as 1,000,000,000,000 bytes, which is what the SI prefix means. Windows reports in units of 1024, so it divides by 1024 three times and shows 931 — but labels the result GB. The correct name for the 1024-based unit is gibibyte (GiB). Both are in the Data category here, so you can see the gap directly.
How long is a month?
In this converter, an average one: 30.44 days, which is a year divided by twelve. There is no other sensible answer, since real months are 28 to 31 days. The year is 365.25 days, the length that averages in leap years. If you need an exact number of days between two particular dates, use a date difference calculator rather than a unit conversion — the answer depends on which dates, not on a ratio.
What is Mach based on?
The speed of sound at sea level at 15°C, which is about 340.29 metres per second. It is worth knowing that Mach is not a fixed speed: sound travels more slowly in colder air, so Mach 1 at altitude is meaningfully slower than at sea level. The figure here is the standard sea-level reference, which is what the number usually means in general use.
Is anything I type sent anywhere?
No. Every conversion is arithmetic your browser does instantly, so there is no server involved. Nothing is transmitted, logged or stored, and closing the tab discards it. That is less about secrecy here than about the tool simply working — it keeps converting with the network disconnected, because there was never anything to send.