Unit Converter

How to use Unit Converter

1. Choose a measurement category.
2. Enter a value and pick the units to convert from and to.
3. Read the result, along with the conversion factor and a table of common values.

The conversions people look up most

Several of these factors are exact by definition rather than measured. The inch has been defined as exactly 25.4 mm since the 1959 international yard and pound agreement, which makes the mile exactly 1,609.344 m. Conversions built on those are exact; ones involving the pound-to-kilogram relationship are exact to the definition of 0.45359237 kg.

Why do US and UK measurements differ?

Because they are two different systems that share most of their names, and volume is where they diverge badly.

A US fluid ounce is 29.5735 ml. A UK imperial fluid ounce is 28.4131 ml — about 4% smaller. But a US pint is 16 US fl oz (473 ml) while a UK pint is 20 imperial fl oz (568 ml), so the UK pint is 20% larger despite the UK ounce being smaller.

The gallon gap is wider still: a US gallon is 3.785 L, an imperial gallon is 4.546 L. Fuel economy figures are not comparable between the two without conversion, which is why the same car is quoted at different mpg in the US and the UK.

Length and mass are identical between the two — an inch, a foot, a mile and a pound are the same on both sides of the Atlantic. It is only volume and the ton that differ, which is precisely why recipe conversions go wrong so often.

How does temperature conversion differ from the rest?

Every other conversion here is a multiplication, because the scales share a zero point. Temperature scales do not.

Celsius to Fahrenheit is °F = °C × 9/5 + 32, and the +32 is the offset between the two zero points. Fahrenheit to Celsius reverses it: °C = (°F − 32) × 5/9.

Kelvin shares Celsius’s degree size but starts at absolute zero, so K = °C + 273.15 with no scaling.

The consequence is that temperature differences convert differently from temperature values. A rise of 10 °C is a rise of 18 °F, not 50 °F. Getting this wrong is a classic error in weather reporting and in engineering specifications.

The mental shortcut for everyday use: double the Celsius and add 30. It is accurate within 3 degrees between 0 and 30 °C, which covers almost all weather.

Which countries use which system?

Three countries have not adopted the metric system as their primary standard: the United States, Liberia and Myanmar. Everywhere else is officially metric.

The UK is the interesting case — officially metric since 1965, but road distances remain in miles, speed in mph, beer in pints, and body weight in stones and pounds. Fuel is sold in liters and consumption is quoted in miles per gallon, which requires a conversion nobody performs.

Canada is metric with heavy US customary influence in construction and cooking — lumber is sold in feet and inches, oven temperatures are often quoted in Fahrenheit, and body weight is commonly given in pounds.

For anyone writing content or building products for these markets, the practical rule is to show both. A recipe for a US and UK audience needs grams and cups; a product page needs cm and inches.

Frequently asked questions