Unit converter

Length, weight and area all share a zero. Zero metres is zero feet, zero kilograms is zero pounds, and that is what lets a single multiplication do the whole job. Convert to a base unit, divide by the factor of the unit you want, done.

Temperature scales do not share a zero. Water freezes at 0 Celsius and at 32 Fahrenheit, and absolute zero sits at 0 Kelvin, which is minus 273.15 Celsius. So converting needs an offset as well as a factor, and the two steps cannot be collapsed into one number:

The anchors worth checking any converter against: 0C is exactly 32F and exactly 273.15K. 100C is 212F and 373.15K. Absolute zero, 0K, is minus 273.15C and minus 459.67F. And minus 40 is the one reading where Celsius and Fahrenheit agree, which is a genuinely useful test, because a converter that has mixed up the order of the offset and the factor will get that case wrong while looking fine at zero.

The imperial to metric conversions are defined exactly, not measured, so there is no approximation to argue with:

Area units follow from squaring the length ones, which is why a square foot is 0.09290304 square metres and not 0.3048. It is also why area conversions feel so much larger than people expect: doubling a length quadruples the area.

A few results that fall out of these definitions and make good sanity checks: 1 metre is 3.2808399 feet, 1 mile is exactly 1.609344 km, 16 ounces make a pound, 14 pounds make a stone, a square yard is exactly 9 square feet, an acre is exactly 43,560 square feet, and a square mile is exactly 640 acres.

The arithmetic here keeps full precision throughout and rounds only when the number is printed, to eight significant figures. Any rounding you can see is in the display, not in the sum.

How do I convert Celsius to Fahrenheit?

Multiply by 9, divide by 5, then add 32. So 20C is 68F and 100C is 212F. Going the other way, subtract 32 first, then multiply by 5 and divide by 9.

Why is temperature different from the other conversions?

Length, weight and area scales all start at the same zero, so converting is a single multiplication. Temperature scales have different zero points, so Celsius to Fahrenheit needs an offset of 32 as well as a factor, and Kelvin sits 273.15 below Celsius. That offset is why you cannot convert a temperature by scaling alone.

What is 0 Celsius in Fahrenheit and Kelvin?

0C is exactly 32F and exactly 273.15K. Another useful anchor is minus 40, which is the one temperature where Celsius and Fahrenheit read the same number.

How many feet are in a metre?

One metre is about 3.2808399 feet, because a foot is defined as exactly 0.3048 metres. The tool uses the exact definition and then shows the result to eight significant figures rather than rounding early.

Are the conversion factors exact?

The imperial to metric factors used here are the internationally agreed definitions, which are exact: an inch is exactly 25.4 millimetres, a pound is exactly 0.45359237 kilograms, and the area units follow from squaring the length ones. Any rounding you see is in the display, not in the arithmetic.

Does this convert weight or mass?

Mass, which is what kilograms and pounds measure in everyday use. Converting a kilogram to a pound assumes standard gravity, which is the assumption behind every bathroom scale, so for ordinary purposes the answer is the one you want.