Measurement FAQ
Is the inch exactly 2.54 centimetres?
Yes. Since the international yard and pound agreement of 1959 the inch has been defined as exactly 25.4 millimetres, the foot as exactly 0.3048 metres and the pound as exactly 0.45359237 kilograms. These are definitions, not measurements, so they will never change.
What changed in the 2019 SI redefinition?
From 20 May 2019 all SI units are defined by fixing seven constants of nature. The kilogram, ampere, kelvin and mole were redefined through the Planck constant, the elementary charge, the Boltzmann constant and the Avogadro constant, and the International Prototype Kilogram stopped being the standard of mass.
Why are US and UK gallons different?
The United States kept the old English wine gallon of 231 cubic inches, 3.785411784 litres, while Britain created the larger imperial gallon in 1824, now exactly 4.54609 litres. The difference carries through pints, quarts and fuel economy figures.
Which countries do not use the metric system?
The United States is the main country where customary units dominate everyday life, though it uses metric units in science, medicine and much of industry. Liberia and Myanmar are often listed too, although both have taken steps towards metrication. The United Kingdom is officially metric but keeps miles, pints and stones in daily use.
What is the difference between mass and weight?
Mass, measured in kilograms, is the amount of matter in an object and is the same everywhere. Weight is the force gravity exerts on that mass, measured in newtons. A kilogram weighs about 9.8 newtons on Earth and about 1.6 newtons on the Moon, but it is still one kilogram.
Is a nautical mile the same as a mile?
No. The international nautical mile is exactly 1852 metres, about 1.151 statute miles, and approximates one minute of latitude. The statute mile is exactly 1609.344 metres. Speeds in nautical miles per hour are knots.