How to use it
Pick what you are measuring, choose the two units, and type a number in either box. The other box answers straight away, and the list underneath shows the same amount in every other unit of that kind, so you rarely need to change the second unit at all. The swap button turns the question around, and the address keeps the question, so a conversion can be bookmarked or sent on.
Where the numbers come from
Each unit is stored as its exact definition wherever one exists: an inch is 2.54 centimetres and a pound is 0.45359237 kilograms by international agreement, not by approximation. A whole-number answer is shown in full, every digit, so a tebibyte is exactly 1,099,511,627,776 bytes. Anything else is rounded to twelve significant figures, more than any ruler or kitchen scale can tell apart. Temperature is the one kind that is not a simple multiplication, so the converter shows the formula it used.
Every unit it knows
Length: millimetres, centimetres, metres, kilometres, inches, feet, yards, miles, nautical miles. Weight: milligrams, grams, kilograms, tonnes, ounces, pounds, stone, us tons, imperial tons. Temperature: celsius, fahrenheit, kelvin. Volume: millilitres, litres, cubic metres, teaspoons (us), tablespoons (us), fluid ounces (us), cups (us), pints (us), quarts (us), gallons (us), pints (uk), gallons (uk), cubic inches, cubic feet. Area: square centimetres, square metres, hectares, square kilometres, square inches, square feet, square yards, acres, square miles. Speed: metres per second, kilometres per hour, miles per hour, knots, feet per second. Time: milliseconds, seconds, minutes, hours, days, weeks, months (average), years (average). Data: bits, kilobits, megabits, gigabits, bytes, kilobytes, megabytes, gigabytes, terabytes, petabytes, exabytes, zettabytes, yottabytes, kibibytes, mebibytes, gibibytes, tebibytes, pebibytes, exbibytes, zebibytes, yobibytes. Energy: joules, kilojoules, calories, kilocalories (food calories), watt-hours, kilowatt-hours, british thermal units. Pressure: pascals, kilopascals, bar, pounds per square inch, atmospheres, millimetres of mercury.
Questions
Is there a page for just one kind of unit?
Yes, one for each: the Length, Weight and Temperature converters and the rest, and pages for single pairs such as cm to inches with a table of common values. They are this same converter, opened on that kind.
Is a US gallon the same as a UK gallon?
No. A US gallon is 3.785 litres and an imperial (UK) gallon is 4.546 litres, about 20% more. The same goes for pints. Both are listed separately here.
Why is my 1 TB drive smaller than 1 TB?
Drive makers count a terabyte as 1012 bytes, while many operating systems count in powers of 1024 and call the result TB. 1 TB is about 0.909 TiB, which is the number your computer shows. Both kinds of unit are here.