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About the units

• Milligrams (mg) – A milligram is one thousandth of a gram, one millionth of a kilogram, written mg and never “microkilogram”, since prefixes attach to the gram. It is the measure for medicine doses and for vitamins on food labels, and for the drink-drive limit: 80 milligrams of alcohol in 100 millilitres of blood in England and Wales, 50 in Scotland. The trap is the microgram, a thousand times smaller. Vitamin D is counted in micrograms, which American labels write “mcg”.
• Grams (g) – A gram is one thousandth of a kilogram. The EU units directive states that the multiples and submultiples of the unit of mass are formed on the word “gram”, not on kilogram, and the gram is a legal unit throughout the EU. It is the measure on food labels, in recipes and in the post. A millilitre of water weighs about a gram. An ounce is 28.349523125 grams. The letter g also stands for the acceleration of gravity, which is a different quantity entirely.
• Decagrams (dag) – A decagram is 10 grams, since the SI prefix deca means 10¹. As a decimal multiple of the unit of mass it is a legal unit throughout the EU, though it is rare in British and American usage. Ten decagrams make 100 grams, about one serving of ham or cheese, which is the scale the unit is built for. The symbol is “dag” and the prefix is written “da”, never “D”. Do not confuse deca, meaning ten, with deci, meaning a tenth.
• Hectograms (hg) – A hectogram is 100 grams, since the SI prefix hecto means 10². It is among the metric units of the Weights and Measures Act 1985 and so legal for UK trade, though far more at home in Scandinavia. In Swedish the shortened “hekto” is an everyday word for 100 grams, yet Swedish rules require unit prices per kilogram, not per hectogram: 20 kronor a hekto is 200 a kilo. The symbol is hg in lower case, because Hg is mercury, as in mmHg.
• Kilograms (kg) – The kilogram is the SI base unit of mass, fixed since 2019 by a defined value for the Planck constant, 6.62607015 × 10⁻³⁴ J s. Before that it was a platinum-iridium cylinder in Paris, sanctioned in 1889 and dropped as the definition in 2018. It is a legal unit across the EU, and US consumer packaging must carry a metric quantity too. A litre of water weighs almost exactly a kilogram. Mass is not weight, and the kilogram is the only base unit whose name carries a prefix.
• Tonnes (t) – A tonne is 1,000 kilograms. It is not an SI unit, but the International Committee for Weights and Measures adopted it in 1879, and the SI Brochure calls it a historical name for the megagram. It is in the EU units directive and the metric part of the Weights and Measures Act 1985. A category B licence covers petrol and diesel vehicles up to 3,500 kg, or 3.5 tonnes. The trap is the name: the imperial ton is 2,240 pounds, 1,016 kilograms, and the US short ton 907 kilograms.
• Ounces (oz) – An ounce is one sixteenth of a pound, exactly 28.349523125 grams. It is used in American recipes and on American packaging, while in British and other European trade it may only be a supplementary indication. Three different things are called an ounce: this avoirdupois ounce, the troy ounce of 31.1034768 grams used for precious metals and permitted in the EU for exactly that trade, and the fluid ounce, which is a volume. The imperial fluid ounce is 28.41 millilitres, the US one 29.57.
• Pounds (lb) – A pound has been exactly 0.45359237 kilograms since 1 July 1959, settled jointly by six countries and written into both US and British law. It divides into 16 ounces. It is the general unit for weight and body weight in the US, where adult men average 199 pounds, while in the EU and the UK it may only be a supplementary indication. Do not confuse the pound as a mass with pound-force, nor with the German Pfund, which in everyday use is 500 grams.
• Stone (st) – A stone is 14 pounds, exactly 6.35029318 kilograms. It survives in British and Irish everyday speech for body weight: an adult of 70 kilograms weighs eleven stone. It is not a legal trade unit in the UK or in the EU, where it may appear only as a supplementary indication. Body weight is written in mixed base, “11 st 4 lb”, with 14 pounds to the stone. Americans do not use the stone at all and count straight through in pounds.