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.
Metric 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 and equivalent of mg/L; one litre of water under standard conditions (when water is at its densest at 4 degrees celsius and at standard pressure) 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. 10 mg of substance for every L of water). Blood sugar concentration is sometimes measured in milligrams per decilitre (mg/dL).