Converting between systems
Most of the world uses metric; the United States largely does not; and the United Kingdom uses an inconsistent mixture of both. That leaves a great many people converting units routinely — following recipes, reading specifications, checking shipping weights, or interpreting measurements written for another audience.
This converter covers length, weight, volume, area, speed, digital storage and temperature, using exact conversion factors rather than rounded approximations wherever an exact value is defined.
Why temperature is different
Every other category on this page converts by simple multiplication, because the units share a common zero point. Zero metres is zero feet; zero grams is zero ounces. Temperature scales do not share a zero, so conversion requires both a multiplication and an offset.
This is why you cannot convert a temperature difference the same way you convert a temperature. A rise of ten degrees Celsius is a rise of eighteen degrees Fahrenheit, not fifty. The formula that converts a reading is not the formula that converts an interval.
Kelvin differs again. It shares the size of the Celsius degree but starts at absolute zero, the point at which molecular motion ceases, so conversion between them is a straight offset of 273.15 with no multiplication.
Volume needs care
Fluid ounces, pints and gallons are not the same on both sides of the Atlantic. An imperial pint is roughly twenty percent larger than a US pint, and an imperial gallon is about twenty percent larger than a US gallon. This converter lists them separately for exactly that reason.
Cooking measurements are a particular trap because a cup is a measure of volume, while most recipe ingredients outside the United States are given by weight. Converting a cup of flour to grams requires knowing the density of flour, which varies with how it is packed. Where precision matters, a kitchen scale is more reliable than any conversion.
Digital storage
The storage conversions here use 1024 bytes to the kilobyte, which is how operating systems report file sizes. Drive manufacturers use 1000 instead, which is why a drive sold as one terabyte shows as roughly 931 gigabytes when you plug it in. Neither is wrong; they are different conventions, and the strictly correct names for the 1024-based units are kibibyte, mebibyte and gibibyte.
Why exact factors matter
An inch is defined as exactly 0.0254 metres, and a pound as exactly 0.45359237 kilograms. Using rounded factors introduces error that compounds when converting large values or chaining conversions. This tool uses the defined values.
Frequently Asked Questions
Why is a US gallon different from an imperial gallon?
They derive from different historical standards. An imperial gallon is about 4.546 litres against roughly 3.785 for the US gallon, a difference of around twenty percent.
How do I convert a temperature difference?
Differently from a temperature reading. A change of one degree Celsius equals a change of 1.8 degrees Fahrenheit, with no offset applied.
Why does my hard drive show less space than advertised?
Manufacturers count a terabyte as a trillion bytes, while operating systems typically use 1024-based units. The same drive is described accurately by both, using different conventions.
Can I convert cups to grams?
Not reliably, because cups measure volume and grams measure weight. The conversion depends on the density of the specific ingredient.
Are the conversion factors exact?
Yes where an exact definition exists. An inch is defined as exactly 0.0254 metres and a pound as exactly 0.45359237 kilograms, and those defined values are used here.