Conversion from 4 Milligrams to Pounds

Information about the 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.

Information about the Pound (lb)

The pound is a unit of mass equal to exactly 0.45359237 kilograms. Its symbol is lb, from the Latin libra, and the plural is sometimes written lbs. It is the principal unit of mass in the United States and remains widely used in the United Kingdom for body weight and food.

That exact value dates from the International Yard and Pound Agreement of 1959, in which the United States, the United Kingdom, Canada, Australia, New Zealand and South Africa agreed a single definition for both units. Before it, the American and British pounds differed slightly, which was tolerable in commerce and unacceptable in engineering as aircraft and munitions were built to shared drawings. Defining the pound in terms of the kilogram, rather than the other way round, quietly made the imperial system a derivative of the metric one.

The abbreviation lb is a reminder of the Roman libra pondo, a pound by weight, from which English took the word pound and the symbol from different halves of the phrase. The same libra gave the pound sterling its name and its old symbol, which is why a currency and a weight share a word.

Everyday American life is dense with the unit. Body weight, groceries sold loose, meat, flour and sugar, freight, and gym equipment are all in pounds. Weight-class sports set limits in pounds, and infant birth weights are given in pounds and ounces. In the United Kingdom the pound survives socially rather than commercially: shops sell in kilograms while people still describe their own weight in stones and pounds.

A second pound causes endless trouble in engineering. The pound-force is the weight of one pound of mass in standard gravity, about 4.448 newtons, and it is written with the same abbreviation in most American practice. Since force equals mass times acceleration, using the two interchangeably requires a conversion constant, and generations of textbooks have handled that either by introducing the slug — the mass that one pound-force accelerates at one foot per second squared, about 14.59 kilograms — or by carrying a factor of 32.174 through every calculation. Neither is elegant, and the confusion is the strongest practical argument the metric system has ever had in its favour.

One pound equals 0.45359237 kilograms exactly, 16 ounces, or 7000 grains.