| Reference | Stones (st) | Milligrams (mg) |
|---|---|---|
| A sheet of A4 paper (80 gsm) | 0.000787365 st | 5000 mg |
| One litre of water | 0.157473 st | 1000000 mg |
| Average adult human | 11.0231 st | 70000000 mg |
| A small car | 220.462 st | 1.4 × 109 mg |
The stone is a unit of mass equal to 14 pounds, or about 6.35029 kilograms. Its symbol is st. It is a British unit with an unusual property: it is almost entirely confined to one purpose, human body weight, and almost entirely confined to Britain and Ireland.
Before standardisation the stone was not a fixed quantity at all. It varied by commodity and by region, from eight pounds for meat to twenty-four for certain wools, and a stone of one good was simply not a stone of another. The Weights and Measures Act of 1835 fixed it at fourteen pounds for all purposes, ending a genuine source of commercial dispute.
Its survival is a study in how measurement habits actually change. Britain converted trade to metric units over decades, and shops now weigh in kilograms, but personal weight remained stubbornly in stones and pounds. A person who buys potatoes by the kilogram will still say they weigh eleven stone four. Medical practice sits awkwardly between the two, with charts in kilograms and patients who think in stones, so most clinical software displays both.
The convention is to give a whole number of stones followed by the remaining pounds, never a decimal, so twelve stone six means 174 pounds. This makes mental conversion to metric awkward and is one reason the unit has no traction outside its home. Ireland uses it similarly, and Australia and New Zealand abandoned it in the 1970s.
Sport keeps the unit visible where medicine has quietly dropped it. British boxing reports fighters at, say, twelve stone seven, horse racing states the weight a jockey must carry in stones and pounds, and rowing crews are selected against a stone-based limit. Clinical practice has moved the other way: hospital notes, drug dosing and growth charts are all metric, because a dose calculated per kilogram cannot safely pass through a conversion. The result is that many British patients hear their weight in kilograms from a nurse and repeat it in stones to a friend an hour later, translating in their heads at roughly six and a third kilograms to the stone.
One stone equals 14 pounds, about 6.35029 kilograms, or 224 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.