ppm ↔ mg/m³ ↔ mg/L converter
air: C(mg/m³) = ppm × M(g/mol) / Vm(L/mol)
“ppm” does not mean the same thing in air and in water: in a gas it is a volume fraction (hence molar), in a solution a mass fraction. The tool treats them separately, air: C(mg/m³) = ppm × M / Vm. With the molar volume computed at your temperature and pressure; solution: C(mg/L) = ppm × ρ, with the density asked for instead of assumed to be 1. The familiar “÷ 24.45” of hygiene tables is this formula frozen at 25 °C and 1 atm, and the tool shows it.
57.244
- Calculation
- C = ppm × M / Vm = 50 × 28.01 ÷ 24.465 = 57.244 mg/m³
Molar volume 24.465 L/mol at the entered conditions: that is what carries the temperature and pressure dependence. The “÷ 24.45” of industrial-hygiene tables is this same formula frozen at 25 °C and 1 atm, inexact elsewhere.
Scientific dossier
What the tool computes, what it assumes, where it stops being valid, and where its data comes from.
Method & formulasair: C(mg/m³) = ppm × M(g/mol) / Vm(L/mol)
air: C(mg/m³) = ppm × M(g/mol) / Vm(L/mol)
molar volume: Vm = R·T / P
solution: C(mg/L) = ppm(mg/kg) × ρ(kg/L)
R = 8.31446261815324 J·mol⁻¹·K⁻¹ (exact since the 2019 SI)
In a gas, a volume fraction is also a mole fraction (Avogadro’s law): converting ppm to mass per volume therefore means multiplying by the molar mass and dividing by the volume one mole occupies. Hence the temperature and pressure dependence, carried entirely by Vm. In a solution, ppm is a ratio of masses: 1 ppm = 1 mg/kg by definition, and only the density bridges to mg/L. The two calculations share neither their parameters nor their physical meaning.
- ppm by volume (ppmv)
- · one volume of pollutant gas per million volumes of air, at the same T and P. It is the unit of occupational exposure limits and air-quality measurements.
- ppm by mass (mg/kg)
- · one milligram of solute per kilogram of solution (not of solvent). Exact and parameter-free: it is the step to mg/L that needs one.
- Molar volume (Vm)
- · 24.465 L/mol at 25 °C and 1 atm; 22.414 L/mol at 0 °C. It is the only quantity in the air calculation that depends on conditions, and the reason a frozen factor cannot be universal.
Validity domainThe air calculation assumes an ideal gas mixture, excellent for dilute pollutants at ambient conditions, less so at very high pressures.
The air calculation assumes an ideal gas mixture, excellent for dilute pollutants at ambient conditions, less so at very high pressures. Mass ppm in a gas (used in some industrial contexts) is a third convention, different again: treat it as a solution, using the gas density. No exposure limit value is provided or implied: this tool converts, it does not judge regulatory compliance, which depends on the texts in force in your country.
Common pitfall: “1 ppm = 1 mg/L” and “÷ 24.45”Two widespread shortcuts, true only under their conditions.
Two widespread shortcuts, true only under their conditions. “1 ppm = 1 mg/L” assumes a density of 1 kg/L: off by 2.5% in seawater, far more in brine or an organic solvent. “ppm ÷ 24.45” assumes 25 °C and 1 atm: at 0 °C the molar volume is 22.41 L/mol, a 9% difference in the result. And altitude shifts the pressure again. The tool asks for the conditions instead of assuming them, and displays the resulting Vm so the gap is visible.