milligram per litre
One milligram of dissolved substance in one litre of solution. It is the working unit of water chemistry, environmental limits and most laboratory reporting — a chlorine residual, a hardness figure, a nitrite reserve and a drinking-water standard are all written in it.
Watch out: In dilute water 1 mg/L and 1 ppm are numerically equal, and that equality is a property of water's density rather than a conversion. The solution has to weigh a kilogram per litre for it to hold. In seawater, in a strong brine, in a glycol loop or in whole blood it does not, and the error runs with the density — several percent in seawater, and far more in a concentrated solution. This site keeps ppm in a separate unit type for exactly that reason: they are numerically equal in one common case and are not the same quantity.
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It has no separate history: it is the milligram over the litre, and it won because those are the sizes the quantities actually come in. A gram per litre would put most useful results in the third decimal place, and a kilogram per cubic metre — the SI form, and numerically identical to a gram per litre — is a unit almost nobody writing a water report has ever used.
About the milligram per litre
A milligram per litre is one part of solute in a million parts of water by mass, provided the water weighs what water usually weighs. That proviso is the entire subtlety of the unit, and it is why mg/L and ppm are not the same thing even though a water report will use them interchangeably and be right to.
The arithmetic: a litre of pure water at room temperature masses very nearly 1000 g, so a milligram of solute in it is , one part per million. Change the solvent and the identity breaks. Seawater runs about 1.025 kg/L, so 1 mg/L is 0.976 ppm — a 2.4% error, which sounds small until it is applied to a discharge limit. In a 50% glycol loop at roughly 1.07 kg/L it is worse, and in whole blood at about 1.06 kg/L a clinical result quoted the wrong way is a real difference.
The conversions that matter day to day are all powers of ten, which is the unit's other virtue: , , and exactly — the last one being genuinely useful, because process engineers size dosing systems in grams per cubic metre and lab reports come back in milligrams per litre, and they are the same number.
The one conversion this unit deliberately cannot do is to millimoles per litre. That crossing needs a molar mass, and a molar mass needs to know what the substance is. A unit picker that offered it would have to guess, so instead it is a calculation with the substance named — which is the honest arrangement, and the reason molarity is a separate type on this site.
One milligram per litre in every mass concentration unit
| 1 | milligram per litre (mg/L)this unit | |
| 1,000 | microgram per litre (µg/L) | |
| 0.001 | gram per litre (g/L) | |
| 1 | microgram per millilitre (µg/mL) | |
| 0.1 | milligram per decilitre (mg/dL) | |
| 0.0001 | gram per decilitre (g/dL) | |
| 0.001 | kilogram per cubic metre (kg/m³) | |
| 1,000 | nanogram per millilitre (ng/mL) |