Length, weight, temperature, area, volume, speed, time, digital storage, pressure and energy. Runs entirely in your browser, so it keeps working with the connection off.
Quick answer: Pick a category, choose the units either side, and type a number — the result updates as you type. Conversions use the exact defining values where one exists, so an inch is exactly 25.4 mm and a pound exactly 0.45359237 kg.
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Why your hard drive looks smaller than the box said
This is the single most common unit confusion in computing, and nothing is actually wrong. Storage manufacturers use decimal units: a terabyte is 1,000,000,000,000 bytes. Windows reports binary units but labels them with decimal names, so it divides by 1,073,741,824 and calls the answer GB when it is really GiB.
The result is that a 1 TB drive correctly shows as 931 GB. Convert 1 TB to gibibytes above and you get the same 931.32. The bytes are all there; only the yardstick changed. This converter lists both sets explicitly — kB against KiB, MB against MiB — so you can see which one a number is using.
Exact definitions, not approximations
Many conversions people assume are approximate are exact by international agreement. The inch has been defined as precisely 25.4 millimetres since 1959. The pound is exactly 0.45359237 kilograms. The thermochemical calorie is exactly 4.184 joules. Where a defining value exists, it is used here rather than a rounded decimal.
The honest exceptions are in Time. A month has no fixed length, and a year is 365 or 366 days depending on where you are in the leap cycle, so those entries use 30 and 365 days and should be treated as conventions rather than facts.
Frequently Asked Questions
Because two different definitions of a gigabyte are in play. Drive manufacturers use the decimal one where 1 TB is 1,000,000,000,000 bytes, while Windows reports the binary one where a gibibyte is 1,073,741,824 bytes but is labelled GB. The same bytes, divided differently: 1 TB decimal is 931.32 GiB. Nothing is missing and nothing is faulty.
Most are exact by definition. An inch is defined as exactly 25.4 millimetres, a pound as exactly 0.45359237 kilograms, and a calorie as exactly 4.184 joules, so those conversions carry no error beyond what a computer's decimal arithmetic introduces. The month and year entries under Time are the deliberate exceptions, using 30 and 365 days because calendar months and leap years have no single correct length.
They were standardised separately. A US gallon is 3.785411784 litres and an Imperial gallon is 4.54609 litres, about 20% larger, and the pints and fluid ounces inside them differ to match. Recipes and fuel figures are the usual place this bites, so it is worth checking which one a source means.
Once the page has loaded, yes. All the maths runs in your browser with no network calls, so nothing is sent anywhere and no connection is needed to convert.
Because temperature scales have different zero points, not just different step sizes. Doubling a length in metres doubles it in feet, but doubling 10°C does not double 50°F. Celsius, Fahrenheit and Kelvin each need an offset applied as well as a ratio, which is why they are converted separately.
Results are computed at full double precision and shown to a sensible number of significant figures. For engineering or scientific work, check the last digit against the defining standard rather than trusting any converter's rounding, including this one.