| Reference | Grams (g) | Pounds (lb) |
|---|---|---|
| A sheet of A4 paper (80 gsm) | 5 g | 0.0110231 lb |
| One litre of water | 1000 g | 2.20462 lb |
| Average adult human | 70000 g | 154.324 lb |
| A small car | 1400000 g | 3086.47 lb |
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 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.