Parts per million (ppm) expresses a very small concentration as a mass ratio scaled up by a million: ppm = (mass of solute ÷ mass of solution) × 10⁶. The convenient working unit is milligrams of solute per kilogram of solution, because a milligram is a millionth of a kilogram, so 1 mg/kg is exactly 1 ppm. For dilute water solutions, where a litre weighs about a kilogram, that also makes 1 ppm ≈ 1 mg/L — the form used throughout water testing. Scale by a thousand for ppb, or divide by ten thousand for percent.
Reviewed: June 19, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: the SI definition of a parts-per-million mass fraction.
The ppm equations
Because a milligram is one-millionth of a kilogram, the factor of a million is absorbed when you work in mg per kg — so the ratio reads straight off as ppm. Solving the other way, the solute mass for a target ppm is solute = ppm × solution mass, and the solution mass needed is solute ÷ ppm. The percent and ppb figures are just the same number on different scales.
Worked example — a trace concentration
Scenario: A sample contains 5 mg of a dissolved substance in 2 kg of solution. Express the concentration in ppm, ppb and percent.
The solution is 2.5 ppm. If the solvent were water, that is also about 2.5 mg/L. Keep the same 5 mg of solute but spread it through 5 kg and the concentration falls to 1 ppm — more solution, more dilute. To push the value to a round 1 ppm in 2 kg instead, you would need only 2 mg of solute (1 ppm × 2 kg).
Frequently Asked Questions
ppm = (solute ÷ solution) × 10⁶, or simply mg/kg. 5 mg in 2 kg = 2.5 ppm.
In dilute water, yes — 1 L water ≈ 1 kg, so 1 mg/L ≈ 1 ppm. Differs for other solvents.
Divide by 10,000. 10,000 ppm = 1%. So 2.5 ppm = 0.00025%.
ppm = 10⁶, ppb = 10⁹. 1 ppm = 1000 ppb. ppb is for very dilute traces.
ppm needs no molar mass — just two masses. Molarity needs moles and volume, better for reactions.