ppm / ppb converter
Convert a trace concentration between every common unit at once — mass fractions and mass per volume alike — with the solution density on screen instead of assumed.
The formula
mg/L = ppm × ρ
- ppm
- mass fraction: parts of analyte per million parts of sample, by mass
- ρ
- density of the solution in g/mL — 1.000 for dilute aqueous samples at room temperature
Worked example
A 5 ppm lead standard in water, and in brine
The mass-fraction ladder is exact by definition: 1% = 10 ppth = 10,000 ppm = 10,000,000 ppb. Only the step to a volumetric unit needs a density.
Parts per million is a ratio of masses, so 5 ppm means 5 mg of lead per kilogram of solution. In water (ρ 1.000 g/mL) a kilogram occupies a litre, so the result is 5 mg/L — the familiar identity.
Prepare the same 5 ppm in a brine of ρ 1.170 g/mL and a litre now weighs 1.170 kg, so it carries 5 × 1.170 = 5.85 mg/L. The mass fraction never changed; the volumetric concentration did. Reporting 5 mg/L there would understate the true concentration by 0.85 mg/L — a recovery of 85.5%, or 14.5% low.
Common pitfalls
ppm is not automatically mg/L
The identity holds only when the solution density is 1.000 g/mL. For brines, acids, organic solvents and slurries it does not, and the error scales directly with the density. Enter the real density — the field is there for exactly this.
"ppt" is ambiguous in the wild
On this page ppt means parts per trillion and parts per thousand is written ppth. Published methods use ppt for both, so check which one a limit refers to before comparing a result against it.
mg/kg is ppm, by definition
Solid-matrix results — soils, feeds, tablets — are reported in mg/kg, which is the same quantity as ppm. No density is involved, because both sides are masses. The same holds for µg/kg and ppb.
Apply the dilution factor before comparing to a limit
An instrument reads the diluted solution, not the sample. Multiply by the dilution factor first; only then is the number comparable to a specification.