Milligram (mg)

The milligram is a unit of mass equal to one thousandth of a gram, or one millionth of a kilogram. Its symbol is mg. It is the smallest mass most people encounter by name, and the one printed on nearly every medicine box and nutrition label.

Pharmacy is built on it. Tablet strengths are quoted in milligrams almost universally, from a 500 mg paracetamol tablet to a 5 mg dose of a cardiac drug, and dosing by body weight is written in milligrams per kilogram. The unit suits the job because the therapeutic range of most oral drugs falls between one and a thousand milligrams, so doses can be stated as whole numbers without prefixes or decimals.

Nutrition labels use it for the minerals and for anything present in quantities too small for grams. Sodium, calcium, potassium, magnesium, iron and vitamin C are all listed in milligrams, and so is cholesterol. Caffeine content is given the same way: a strong cup of coffee holds roughly 100 mg, and regulators in several countries require energy drinks above 150 mg per litre to carry a warning.

In the laboratory the milligram is the comfortable working range of an analytical balance, which typically reads to a tenth of a milligram and settles within seconds. Below that, weighing becomes an exercise in controlling draughts, static electricity and the operator’s own body heat, which is why sub-milligram quantities are usually prepared by dissolving a larger weighed mass and taking an aliquot rather than by weighing directly.

Law and public health borrow it whenever a limit has to be enforceable. Drink-driving is defined in most of Europe as 50 milligrams of alcohol per 100 millilitres of blood, and in the United Kingdom as 80; food regulations cap sodium, additives and contaminants in milligrams per kilogram; and vehicle emission standards are written in milligrams per kilometre for particulates and oxides of nitrogen. In each case the number has to be small enough to matter biologically and large enough to be measured reliably in an ordinary testing laboratory, and the milligram sits at exactly that point. A cup of coffee carries about 100 milligrams of caffeine, which is a useful mental anchor for the whole scale.

One milligram equals 0.001 grams, one thousand micrograms, and one millionth of a kilogram.

Milligram conversion table