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A gram per litre or liter (g/L) is a measurement of concentration used to measure the how many grams of a certain substance are present in one litre of liquid.

It is not an SI unit because it contains the non-SI unit "litre". The SI unit of concentration is kilogram per cubic metre, which is numerically equivalent (1 g/L = 1 kg/m³).

SI prefixes are often applied so that there are units like milligrams per litre (mg/L). When measuring concentration in water, parts per million is an older expression of mg/L, since one liter of water under standard conditions weighs one kilogram or one million milligrams.

Milligram per litre is often used in medicine and is also used among prescriptions. For example you may be given a solution which involves one substance and another substance and one of the substances involves adding water then it would state: "10 mg/L water and ..."