PPM to mg/L Converter
Convert ppm to mg/L and back, allowing for the density of the solution.
The same concentration, every way
| Unit | Value |
|---|---|
| Percent (by mass) | 0.025 % |
| Parts per thousand | 0.25 ppt |
| Parts per million | 250 ppm |
| Parts per billion | 250,000 ppb |
| mg/L | 250 mg/L |
| µg/L | 250,000 µg/L |
| g/L | 0.25 g/L |
| mg/kg | 250 mg/kg |
They are not the same kind of unit
ppm is a mass fraction — milligrams of solute per kilogram of solution. mg/L is a mass per unit volume. Density connects them:
mg/L = ppm × density in g/ml
For dilute water at 1.00 g/ml the two are numerically identical, because one litre weighs one kilogram. That coincidence is why lab reports treat them as interchangeable — and why the habit quietly breaks for seawater (1.025 g/ml), brines, solvents and anything hot.
The family of ratios
1% = 10,000 ppm1 ppt (per thousand) = 1,000 ppm1 ppm = 1,000 ppb1 ppm = 1 mg/kg, exactly, by definition
Gases work differently
For air, ppm almost always means parts per million by volume, not by mass. Converting that to mg/m³ needs the gas's molar mass along with temperature and pressure, so the density figure here does not apply — this calculator is for liquids.
Drinking-water limits are usually quoted in mg/L. For fresh water you can read those as ppm without adjustment.