📏 Unit Converter

Metric, imperial, and why both survive

Almost every country uses the metric system for official purposes. The United States is the main exception in daily life, and the United Kingdom runs a hybrid — road signs in miles, shop scales in kilograms, beer in pints, milk in litres.

The practical result is that conversion errors are common in international contexts, and occasionally expensive. A Mars orbiter was lost in 1999 because one team supplied thrust data in pound-seconds while another expected newton-seconds.

Conversions worth memorising

Note the last two rows. A US gallon and an imperial gallon are not the same volume, and neither are US and imperial pints or fluid ounces. Recipes crossing the Atlantic are a frequent casualty.

Temperature works differently

Length and mass conversions are pure multiplication, because both scales start at zero. Temperature scales do not, so conversion requires an offset as well.

°F = °C × 9 ÷ 5 + 32  ·  °C = (°F − 32) × 5 ÷ 9

For rough mental conversion, doubling Celsius and adding 30 gets close enough for weather. The two scales meet at −40, which is the same temperature in both. Kelvin shares Celsius degree size but starts at absolute zero, so K = °C + 273.15.

Common questions

Why is 2.54 described as exact?

Because the inch is legally defined as exactly 2.54 centimetres. Since 1959 the imperial units have been defined in terms of metric ones, so these conversions carry no rounding error at all.

Is a metric ton the same as a US ton?

No. A metric tonne is 1,000 kg, a US short ton is 907 kg, and a UK long ton is 1,016 kg. The difference is around ten percent.

How do I convert areas and volumes?

Square the linear factor for area and cube it for volume. One metre is 3.281 feet, so one square metre is 10.76 square feet and one cubic metre is 35.3 cubic feet.

Why do recipes cause so many conversion problems?

Because they mix volume and weight. A US cup is a volume, but a cup of flour and a cup of sugar weigh very different amounts. Converting a volume-based recipe to grams requires knowing the ingredient density, which is why baking recipes written by weight are far more reliable across regions.

Are nautical miles different from statute miles?

Yes. A nautical mile is 1,852 metres — about 15 percent longer than a statute mile — and is defined as one minute of latitude. Aviation and shipping use it, along with knots, meaning nautical miles per hour.