Convert between metric and imperial units across length, weight, temperature, volume, area, speed and data size. Conversions use the exact modern definitions rather than rounded approximations, so a round-trip returns the value you started with.
What "exact" means here
Since 1959 the imperial units have been defined in terms of metric ones, exactly. An inch is precisely 25.4 mm, a pound is precisely 0.45359237 kg, and a mile is precisely 1609.344 m — these are definitions, not measurements.
That is why converting 12.345 kg to pounds and back gives 12.345 kg again. Tools using rounded factors like 2.2 pounds per kilogram drift on every conversion.
Temperature is not a ratio
Every other conversion here is multiplication by a constant, because the units share a zero point: zero metres is zero feet.
Temperature scales do not. Zero Celsius is 32 Fahrenheit, so converting needs both a scale factor and an offset. This is why 20°C is not "twice as warm" as 10°C, and why doubling a Celsius figure is meaningless.
Kelvin is the exception that proves it — it starts at absolute zero, so it is the only one where ratios mean anything. Converting to a negative Kelvin value is rejected here, because there is no such temperature.
Why your 1 TB drive shows as 931 GB
Two different systems share the same prefixes. Drive manufacturers use decimal: 1 TB is 1,000,000,000,000 bytes. Operating systems have traditionally counted in binary: 1 TiB is 1,099,511,627,776 bytes.
The drive is not smaller than advertised and nothing is missing — the same number of bytes is being divided by a different figure. This converter offers both, with the binary units labelled KiB, MiB, GiB and TiB, and the decimal ones kB, MB, GB and TB.
Cups, pints and gallons differ by country
The volume units here are US liquid measures. Imperial measures of the same name are different: an imperial pint is 568 ml against the US 473 ml, and an imperial gallon is about 20% larger than a US gallon.
This matters most in recipes. A recipe in US cups measured with an imperial cup gives roughly 25% too much of everything — enough to ruin baking, where ratios are unforgiving.
FAQ
How many pounds are in a kilogram?
Approximately 2.20462. A kilogram is defined as exactly 0.45359237 pounds inverted, so this converter uses the full precision rather than the rounded 2.2 that accumulates error.
Why does 100°C convert to 212°F rather than something rounder?
The scales were defined independently — Celsius on the freezing and boiling points of water, Fahrenheit on a different pair of reference points entirely. The conversion is a factor of 9/5 plus an offset of 32, which lands on 212. The two scales agree at exactly one temperature: −40.
Which cup is used for the volume conversions?
The US legal cup, at 236.588 ml. An imperial cup is 284 ml and a metric cup is 250 ml, so check which your recipe means before measuring.
Is anything sent to a server?
No. All conversions are arithmetic performed in your browser, with nothing transmitted or stored.