In water, 1 ppm is 1 mg/L — just rewrite the number
A litre of water weighs a kilogram (1,000,000 mg). One milligram dissolved in it is one part per million, and also one milligram per litre. For drinking water and dilute aqueous solutions the two units are interchangeable, so a lab result of 3.2 mg/L is the same as 3.2 ppm.
The density caveat, and when it bites
ppm is a mass ratio (mg per kg); mg/L is mass per volume. They only coincide because water's density is 1 kg/L. In a denser or lighter liquid, multiply ppm by that density.
| Solution | Density (kg/L) | 1000 ppm → mg/L |
|---|---|---|
| Fresh water | 1.000 | 1000 |
| Seawater | 1.025 | 1025 |
| Vegetable oil | 0.92 | 920 |
| Ethanol | 0.789 | 789 |
| Sulfuric acid | 1.839 | 1839 |
Ignoring density for a non-water solution can throw the result off by tens of percent. For seawater the error is only about 2.5%; for concentrated acid it is nearly double.
Common water-quality limits, in mg/L
- Fluoride. US EPA sets a maximum of 4.0 mg/L; the WHO guideline is 1.5 mg/L.
- Nitrate. EPA limit is 10 mg/L (as nitrogen).
- Lead. Action level is 0.015 mg/L, which is 15 ppb — a thousandth of a ppm, not a ppm.
- Total dissolved solids. Soft water runs 50–150 mg/L; very hard water tops 500 mg/L.
Common questions
Is 1 ppm the same as 1 mg/L?
In water, yes. One litre of water weighs one kilogram, so 1 mg of substance in that litre is 1 mg per kilogram, which is 1 part per million. The equality holds for any solution whose density is close to 1 kg/L.
When does ppm stop equalling mg/L?
When the solution is not water-like. ppm is a mass ratio and mg/L is mass per volume, so they only match when 1 litre weighs 1 kg. For denser or lighter liquids you multiply ppm by the density to get mg/L.
How do I convert ppb to mg/L?
One part per billion is 0.001 mg/L in water. So 15 ppb of lead is 0.015 mg/L. Do not confuse ppb with ppm; they differ by a factor of 1,000.
