Gram (g)

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.

Gram conversion table