A gram per liter or litre (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 because it contains the non-SI unit litre. The SI unit of mass concentration is kilogram per cubic meter, which is numerically equivalent (1 g/L = 1 kg/m³).
SI-style prefixes are often applied, giving units like milligrams per litre (mg/L) or micrograms per decilitre (μg/dL or mcg/dL). 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.
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"