PPM vs mg/m³
Occupational exposure limits may be published in ppm (volume) or mg/m³ (mass). For ideal gases and vapours:
mg/m³ = (ppm × MW) ÷ Vm · ppm = (mg/m³ × Vm) ÷ MW
where MW is molecular weight (g/mol) and Vm is molar volume — commonly 24.45 L at 25 °C / 1 atm, or 24.05 L at 20 °C.
PPM to mg/m³ Calculator
Gas units · ppm ↔ mg/m³
Convert workplace gas/vapour concentrations using molecular weight and molar volume (24.45 L @ 25 °C or 24.05 L @ 20 °C).
Formula: mg/m³ = (ppm × MW) ÷ Vm · ppm = (mg/m³ × Vm) ÷ MW
- Compare results to OELs published in the same unit and conditions.
- Dusts and fibres stay in mg/m³ or fibres/ml — do not convert via ppm.
- Use SDS molecular weights for mixtures only when a method allows it.
Frequently asked questions
Which temperature should I use?
Match the convention in the limit you are comparing (often 25 °C for ACGIH-style, sometimes 20 °C in Europe). State Vm in your notes.
Does this work for aerosols?
No. Particulates are already mass-based (mg/m³). PPM conversion is for gases and vapours.
Where do I find MW?
Safety data sheet or chemistry reference — e.g. CO = 28.01, H₂S = 34.08, toluene ≈ 92.14.
Pressure other than 1 atm?
Adjust molar volume for site pressure and temperature if your method requires it — this tool uses standard Vm choices.
