Exact factors, not rounded ones
Since 1959 the inch has been defined as exactly 25.4 millimetres and the pound as exactly 0.45359237 kilograms, so every imperial length, area and weight unit converts without approximation. The US gallon is 231 cubic inches and the oil barrel is 42 US gallons. This converter uses those definitions directly and only rounds the displayed answer to ten significant figures, which hides computer artefacts such as 0.30000000000000004.
Square feet and square metres for property
Apartment and office sizes are often advertised in square feet, especially in the UAE, Qatar and other markets with many international buyers, while title deeds and floor plans may use square metres. One square metre is 10.7639 square feet, so a 1,200 sq ft flat is about 111.5 m². The area group also covers acres and hectares for land, and square miles and square kilometres for larger regions.
US and UK measures are not the same
A US gallon holds 3.785 litres but a UK (imperial) gallon holds 4.546 litres, about 20% more, which matters when you read fuel economy figures from British sources. The cup, spoon, pint and quart options here are US customary measures, the ones American recipes use; British and Australian recipes may use slightly different metric cups. For crude oil, one barrel is 42 US gallons, or about 159 litres.
Temperature works differently
Temperature scales do not share a zero point, so a single multiplier cannot convert them. Fahrenheit equals Celsius times 9/5 plus 32, and kelvin equals Celsius plus 273.15. That is why -40 is the same in Celsius and Fahrenheit, and why a summer afternoon of 45 °C is 113 °F. The converter warns you if a value is below absolute zero, the coldest temperature possible.
Speed, knots and other tools
Road speeds are given in kilometres per hour in most of the world and in miles per hour in the US and UK; 100 km/h is about 62 mph. Ships and aircraft use knots, one nautical mile per hour, or 1.852 km/h. There is no sign-up and no limit on how many conversions you make, and the page keeps working offline once loaded. For gold weights and parcel sizes, these tools go further: