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A gram per litre or gram per liter (g/L or g/l) is a unit of measurement of mass concentration that shows how many grams of a certain substance are present in one litre of a usually liquid or gaseous mixture. It is not an SI unit - the SI unit of mass concentration is kilogram per cubic metre, which is equivalent (1 g/L = 1 kg/m3).

Metric prefixes are often applied, giving units like milligrams per litre (mg/L) or micrograms per decilitre (µg/dL). When measuring concentration in water, parts per million is an older expression of mg/L, since one litre of water under standard conditions weighs one kilogram.

Milligrams per litre are often used in medicine and in medical prescriptions. For example, a description of a solution that involves two substances, where one of the substances involves adding water, would state: "10 mg/L water and substance". (i.e. 10mg of substance for every L of water) Blood sugar concentration is sometimes measured in milligrams per decilitre (mg/dL).