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Enter one value and see it expressed in every unit of the category simultaneously — no repeated back-and-forth between pairs.
Unit Converter
Convert length, weight, temperature, volume, area, speed, data and time.
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Overview
About the Unit Converter
Free unit converter for length, mass, temperature, volume, area, speed, digital storage, time and pressure. Instant conversion across hundreds of units.
Unit conversion is the most frequently performed calculation in science, engineering, cooking and international commerce, and the most frequently botched. Almost all errors come from one of three causes: a wrong factor, a mishandled offset, or an output quoted to absurd precision.
Multiplicative units
Most units convert by a single fixed factor, because they measure the same quantity with different-sized rulers. The modern definitions are exact:
- 1 inch = 25.4 mm (defined 1959, not measured)
- 1 pound = 0.45359237 kg (exact by international agreement)
- 1 nautical mile = 1852 m (exact)
- 1 US gallon = 3.785411784 litres (exact, derived from the inch)
- 1 imperial gallon = 4.54609 litres (exact, defined 1985)
Note that a US gallon and an imperial gallon differ by about 20%. Recipes and fuel economy figures cross the Atlantic regularly, and this single discrepancy explains a great deal of confusion.
Affine units: temperature
Temperature scales do not share a zero. The conversion is output = (input − offset₁) × factor + offset₂, and getting the order wrong produces a plausible but incorrect number.
°C → °F: F = C × 9/5 + 32
°F → °C: C = (F − 32) × 5/9
°C → K: K = C + 273.15
K → °C: C = K − 273.15The one useful anchor: −40 is the same number on both the Celsius and Fahrenheit scales.
Compound units
Area and volume conversions square and cube the linear factor, which is where intuition fails most often. One square metre is not 3.28 square feet — it is 3.28² ≈ 10.76. One cubic metre is about 35.3 cubic feet. A factor of three in length becomes a factor of twenty-seven in volume.
Speeds expressed as distance per time convert the numerator and denominator separately: 1 mph = 1.609344 km/h exactly, and 1 knot = 1.852 km/h exactly.
Data units, and the two competing systems
The IEC introduced the binary prefixes in 1998 to end the ambiguity, and the ambiguity persists anyway. Drive manufacturers use decimal; memory is addressed in binary; operating systems report a mixture. Knowing both readings is the only reliable strategy.
Precision discipline
A conversion cannot create information. If you measured 12 feet, converting to 3.6576 metres reports four more digits than you actually know. Round to the input's precision — usually two or three significant figures — and reserve the full float for intermediate calculation steps only.
Step by step
How to use the Unit Converter
Choose a category — length, mass, temperature, volume, area, speed, data or time.
Enter a value; every other unit in the category updates instantly.
Pick the units you care about, or read the full conversion table.
Use the swap button to reverse the direction.
Copy any single result, or the whole table.
Why use it
Benefits and common use cases
What this tool is good for, and what it deliberately does not try to do.
Correct temperature handling
Temperature is not a multiplication. Celsius, Fahrenheit and Kelvin use affine conversions with different zero points, and the tool applies the proper formula rather than a factor.
Binary and decimal data units
Distinguishes KB (1000 bytes) from KiB (1024 bytes), which is the source of the perennial 'my 1 TB drive is only 931 GB' confusion.
Precision control
Set significant figures or decimal places so the output matches the precision your calculation actually warrants.
Questions
Frequently asked questions
Short, honest answers about quality, limits and privacy.
Why does temperature conversion work differently?
Because temperature scales have arbitrary zero points. Length and mass conversions are pure multiplications; temperature conversions are affine — a scale plus an offset. °F to °C is (F − 32) × 5/9, and the offset must be applied before the scaling or the answer is wrong.
What is the difference between KB and KiB?
KB is 1000 bytes (decimal, SI) and KiB is 1024 bytes (binary, IEC). Storage manufacturers use decimal, operating systems historically reported binary while labelling it decimal — which is why a 1 TB drive shows as about 931 GiB. Nothing is missing; two definitions of the same prefix are in use.
How accurate are the conversions?
They use exact defined factors wherever one exists. Since 1959 the international inch is exactly 25.4 mm, the pound is exactly 0.45359237 kg, and the nautical mile is exactly 1852 m. Where a conversion is inherently approximate — such as between metric and imperial volumes in cooking — the tool notes it.
How many significant figures should I keep?
Never more than your input had. Converting a measurement of '5 miles' into '8.04672 kilometres' implies a precision that was not there. If the input is given to two significant figures, the output should be too: 8.0 km.
Are cooking conversions exact?
No, and this matters in practice. A US cup is 236.6 ml, a metric cup is 250 ml, and a Japanese cup is 200 ml. Tablespoons vary between 15 ml and 20 ml depending on country. For baking, weigh ingredients in grams instead of converting volumes.