A unit converter changes the name of a quantity without changing the quantity. One centimetre is a length. Calling it 0.3937007874 inches does not make the object shorter. The page multiplies by a defined factor and divides by another. It does not look the factor up on a server, and it does not round to two decimal places and then convert again. The box shows enough digits to check the definition, then drops trailing noise. The two conversions people ask for by name have their own pages: cm to inches and kg to lbs. Both use the factors below. The dropdowns on this page are the same tool with the rest of the list opened.
Length is anchored on the metre. A millimetre is 0.001 m, a centimetre is 0.01 m, a kilometre is 1000 m. The inch is exactly 0.0254 m, which is why 2.54 cm is exactly one inch and one centimetre is exactly 1/2.54 inches. A foot is 12 inches, 0.3048 m. A yard is 3 feet, 0.9144 m. A mile is 1609.344 m, which is 5280 feet. If a tailor's tape and this page disagree by a millimetre, the tape moved. If they disagree by a factor of ten, the field was millimetres when you meant centimetres. Mass is anchored on the kilogram. A gram is 0.001 kg, a milligram is 0.000001 kg, a tonne is 1000 kg. The avoirdupois pound is exactly 0.45359237 kg, so one kilogram is 1/0.45359237 pounds, about 2.2046226218, not the 2.2 that a headline uses. An ounce is 1/16 of that pound. The BMI calculator uses these same two definitions before it divides weight by height squared, so a figure you convert here will match a figure you type there.
Temperature is the exception that is not a factor. Zero Celsius is not zero Fahrenheit, and doubling the Celsius number does not double the Fahrenheit number. The page converts through Celsius: Fahrenheit is Celsius times 9/5 plus 32, and kelvin is Celsius plus 273.15. So 0 °C is 32 °F and 273.15 K, 100 °C is 212 °F, and 0 K is −273.15 °C. A body temperature of 37 °C is 98.6 °F. If you only needed that one fact, you still use the same control: choose temperature, type 37, convert from Celsius to Fahrenheit. There is no separate "body temperature" mode, because it would be the same arithmetic with a costume.
Volume on this page is labelled US because the names are shared with other systems and the sizes are not. A US gallon is 3.785411784 litres. A US pint is one eighth of that. A US cup is 236.5882365 ml. A US fluid ounce is 29.5735295625 ml. A US tablespoon is 14.78676478125 ml and a teaspoon is one third of that. An imperial pint, still poured in some Malaysian coffee shops when someone says "pint" and means the British one, is about 568 ml and is not in this list. If the recipe says cup and was written in America, use this cup. If it was written in a Malaysian kitchen and the cup was a teacup, measure the teacup. The page will not guess which cup a blog meant. Speed uses metres per second as the base. Divide kilometres per hour by 3.6 to get metres per second. A mile per hour is 0.44704 m/s. A knot is one nautical mile per hour, 1852 metres per hour. Area uses the square metre. A hectare is 10,000 m². An acre is 4046.8564224 m². A square foot is 0.09290304 m².
Data size is where converters quietly lie by using one name for two sizes. Decimal kilobytes, megabytes, and gigabytes on this page are powers of 1000, which is what drive makers and the SI prefixes mean. Kibibytes, mebibytes, and gibibytes are powers of 1024, which is what a lot of operating systems mean when they write KB anyway. 1 GiB is 1,073,741,824 bytes, not 1,000,000,000. The labels in the dropdown say which one you picked. If a phone says a photo is 3.2 MB and a file window says something else, they may be dividing by different thousands. Convert both to bytes before you argue. The page does not read your disk. You type the number.
Nothing is uploaded. The result is the amount times a factor, or the temperature formula, in this tab. Close the tab and it is gone. There is no currency conversion here, because a currency needs a rate that changes and this page refuses to invent one. There is no "stones" row; multiply pounds by 14 if a British form asked. Check one known value after you change category: 2.54 cm must be 1 inch, 1 kg must be about 2.2046 lb, and 0 °C must be 32 °F. If those three hold, the factor table you are using is the one described on this page.