About Unit Converter
Unit Converter handles the eight unit families you actually look up — length, weight, volume, area, temperature, time, digital data and energy — with the precision a calculator app skips. Type a value, pick the source and target units, and the result updates live as you type. No round-trip to a server, no ads injected mid-calculation. It's the kind of tool you keep open in a tab for the day: switching between metric and imperial for a recipe, sizing a shipping carton, or converting kilowatt-hours on an electricity bill into joules.
- No uploads
- Browser-only
- Works offline
- 100% free
How it works
- 1
Pick a category
Choose Length, Weight, Volume, Area, Temperature, Time, Data or Energy. Each category exposes its own set of units.
- 2
Type the source value
Enter the number and the source unit (e.g. kilograms, kelvin, gigabytes). Decimals and scientific notation are accepted.
- 3
Read the result
The target value updates on every keystroke. Swap the source and target with a single click, or copy the converted number to the clipboard.
Metric, imperial, and the conversions that trip people up
Most unit conversion is a clean multiply-by-a-factor: 1 inch is exactly 2.54 cm, 1 mile is exactly 1.609344 km, 1 pound is exactly 0.45359237 kg. These are defined values, not approximations, which is why a good converter gives the same answer every time. The errors creep in not from the arithmetic but from picking the wrong unit — and several unit names are dangerously overloaded.
The classic traps: the US gallon (3.785 L) is not the imperial gallon (4.546 L), so a "miles per gallon" figure means different things on each side of the Atlantic. The US fluid ounce and the UK fluid ounce differ. A "ton" can be a metric tonne (1000 kg), a US short ton (907 kg) or a UK long ton (1016 kg). Ounces of weight (mass) are a different quantity entirely from fluid ounces (volume) — they only share a name. Always confirm which variant you mean before you trust the number.
Temperature is affine, not a simple scale
Length, mass and volume convert by multiplication because they share a zero — zero inches is zero centimetres. Temperature scales do not. The Celsius and Fahrenheit zeros sit at different physical points, so converting between them needs both a multiply and an add: Fahrenheit = Celsius times 9/5 plus 32. That offset (the plus 32) is what makes temperature an affine conversion rather than a linear one.
The consequence catches people out constantly: you cannot convert a temperature difference the same way you convert a temperature. A change of 10 Celsius is a change of 18 Fahrenheit (you scale by 9/5 but you do not add 32, because the offsets cancel for a difference). Convert the two endpoint temperatures and subtract — don't run a delta through the point-temperature formula, or you'll be 32 degrees wrong.
Significant figures and rounding
- A conversion can't add precision that wasn't in your input. "About 5 miles" is not 8.04672 km — it's roughly 8 km. Reporting more digits than your source justifies is false precision.
- Round once, at the end. Converting through an intermediate unit and rounding at each step accumulates error; keep full precision until the final figure.
- For exact-by-definition factors (inch, mile, pound) the conversion is precise; for measured quantities, match the significant figures of your least-precise input.
- Watch unit-squared and unit-cubed: 1 square metre is 10,000 square centimetres (not 100), and 1 cubic metre is 1,000,000 cubic centimetres — the factor is squared or cubed along with the unit.
Frequently asked questions about Unit Converter
How accurate are the conversions?
All ratios are stored as full-precision floating-point constants — for example, 1 inch is exactly 0.0254 metres by the international yard agreement of 1959. Temperature uses the standard Celsius / Fahrenheit / Kelvin equations, not lookup tables. Display is rounded to a sensible number of decimals but the underlying maths is full double-precision.
Does it support both binary and decimal data units?
Yes. The Data category includes both decimal units (KB = 1000 bytes, MB = 1,000,000 bytes — used by storage manufacturers and network speeds) and binary units (KiB = 1024 bytes, MiB = 1,048,576 bytes — used by RAM and most operating systems). Pick whichever your spec actually means.
Can I convert between fuel economy units like mpg and L/100 km?
Not yet — the current build covers eight standard families. Fuel economy, currency, and pressure are on the roadmap. For currency you'll always need a live exchange rate, which would require a network call, so we'd add it as an opt-in feature.
Privacy, offline use, browser support, and pricing questions are answered on the site-wide FAQ.