Free Unit Converter — Length, Weight, Temperature & More
Convert between metric and imperial units instantly. Pick a category, type a value, done.
Where these definitions actually come from
The meter dates back to the 1790s, originally defined as one ten-millionth of the distance from the equator to the North Pole — an attempt to base a unit of measurement on something universal rather than a king's forearm or foot. Modern science has since redefined it in terms of the speed of light for far greater precision, but the practical length hasn't meaningfully changed since. The inch, by contrast, was formally pinned to the metric system only in 1959, when the US, UK, and other countries agreed by international treaty that one inch equals exactly 2.54 centimeters — which is why every length conversion here involving inches is a clean, exact ratio rather than an approximation.
Why a nautical mile isn't a regular mile
The Speed category's "knots" unit reflects a genuinely different definition of distance: a nautical mile is defined as one minute of latitude — roughly 1/21,600th of the Earth's circumference — which makes it about 15% longer than a standard statute mile. It's still used in aviation and maritime navigation because it ties directly to position on a map, unlike the statute mile.
Frequently Asked Questions
How many centimeters are in an inch?
Exactly 2.54 cm per inch — the definition was fixed internationally in 1959. That makes a foot 30.48 cm and a yard 0.9144 m exactly.
Are these conversions exact?
Length, weight, volume, and area conversions use the exact legal definitions. Results display up to 8 significant digits; tiny rounding in the last digit is normal for repeating decimals.
Does the volume category use US or UK measures?
US customary: a US gallon is 3.785 liters, a US fluid ounce 29.574 ml. UK (imperial) pints and gallons are about 20% larger — worth checking when following British recipes.
How does temperature conversion work?
°F = °C × 9/5 + 32, and Kelvin is Celsius + 273.15. Temperature is the one category where units don't scale proportionally, so the converter handles those formulas specially.
Why is a nautical mile different from a regular mile?
A nautical mile is defined as one minute of latitude on the Earth's surface, which makes it about 15% longer than a statute (land) mile. It's used in aviation and maritime navigation specifically because it ties directly to position and distance on a map or globe.
Where does the modern definition of the meter come from?
It was originally set in the 1790s as one ten-millionth of the distance from the equator to the North Pole. Today it's defined with far greater precision using the speed of light, though the practical length is essentially unchanged from the original.