| Reference | Grams (g) | Milligrams (mg) |
|---|---|---|
| A sheet of A4 paper (80 gsm) | 5 g | 5000 mg |
| One litre of water | 1000 g | 1000000 mg |
| Average adult human | 70000 g | 70000000 mg |
| A small car | 1400000 g | 1.4 × 109 mg |
The gram is a unit of mass equal to one thousandth of a kilogram. Its symbol is g. Although the kilogram is the SI base unit, the gram is the one that carries the prefixes, so milligram and microgram are built on the gram rather than on the kilogram.
That awkwardness is historical. The original metric system of 1795 defined the gramme as the mass of one cubic centimetre of water at the temperature of melting ice. It was intended as the base unit, but a one-gram standard is too light to be a practical reference: a speck of dust or a fingerprint changes it measurably. The thousandfold-larger kilogram was made the standard artefact instead, and the SI inherited the arrangement, leaving the base unit as the only one carrying a prefix in its own name.
Cooking outside the English-speaking world runs on grams, and increasingly inside it as well. Weighing flour rather than measuring it by volume removes the largest source of error in baking, because a cup of flour can vary by twenty per cent depending on how it was packed. Professional bakers work entirely in grams and express recipes as percentages of flour weight.
Nutrition panels report protein, fat, carbohydrate, sugar and fibre in grams, so the unit is the everyday currency of dietary arithmetic. Postal services price letters and small parcels in grams, and precious metals, though traded in troy ounces, are sold to the public in gram bars.
It is also the unit the SI prefixes attach to, which is why the system has a base unit that already carries a prefix. Smaller and larger masses are named from the gram and never from the kilogram: a thousandth of a gram is a milligram, not a microkilogram, and a million grams is a megagram, better known as the tonne. The rule spares the language a set of double prefixes that would otherwise be unavoidable, but it leaves the kilogram in the odd position of being the only base unit whose name contains one. Postal tariffs show the gram at work in ordinary life: letter rates step at 20, 50 and 100 grams, and a sheet of office paper weighs about five.
One gram equals 0.001 kilograms, one thousand milligrams, or about 0.035 ounces.
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.