Four scales, one cloud of gas
The same gas concentration gets expressed four ways depending on who's asking. ppm and %vol are the same molecular-count scale at different magnification (1%vol = 10,000 ppm). mg/m³ converts molecules to mass via the molecular weight — the language of occupational exposure limits. And %LEL rescales everything against flammability — the language of safety systems:
Worked example
A methane detector reads 10,000 ppm (MW 16.04, LEL 5.0 %vol):
- %vol = 10,000 ÷ 10⁴ = 1.0 %vol
- %LEL = 1.0 ÷ 5.0 × 100 = 20 %LEL — typically a high-alarm level
- mg/m³ = 10,000 × 16.04 ÷ 24.45 = 6,560 mg/m³
Common gases quick table (25 °C)
| Gas | MW | LEL %vol | 1 ppm in mg/m³ | 100% LEL in ppm |
|---|---|---|---|---|
| Methane | 16.04 | 5.0 | 0.656 | 50 000 |
| Propane | 44.10 | 2.1 | 1.804 | 21 000 |
| Hydrogen | 2.016 | 4.0 | 0.082 | 40 000 |
| CO | 28.01 | 12.5 | 1.146 | 125 000 |
| H₂S | 34.08 | 4.0 | 1.394 | 40 000 |
| Ammonia | 17.03 | 15.0 | 0.697 | 150 000 |
Field notes
- LEL values vary slightly between references (IEC vs NFPA vs older tables) — a catalytic detector calibrated to one convention can read several %LEL differently against another. Use your site's accepted values.
- The temperature basis matters at the percent level: mg/m³ at 0 °C vs 25 °C differs by 9%. Regulations state their reference — match it.
- Toxic vs flammable are different scales entirely: H₂S is deadly at 100 ppm (0.25 %LEL) — a flammability detector is not a toxicity monitor.
- ppm here is by volume (ppmv), the gas-phase convention. Liquid-phase ppm is by mass — a different quantity.
Frequently asked questions
How do I convert ppm to mg/m³?
mg/m³ = ppm × MW ÷ Vm, where MW is the molecular weight (g/mol) and Vm the molar volume — 24.45 L/mol at 25 °C and 1 atm (24.06 at 20 °C, 22.41 at 0 °C). For CO (MW 28.01) at 25 °C, 1 ppm = 1.145 mg/m³.
What does % LEL mean on a gas detector?
Percent of the Lower Explosive Limit — how close the atmosphere is to the minimum flammable concentration. 100% LEL of methane equals 5% by volume (50,000 ppm). Detectors typically alarm at 10-20% LEL, far below flammability.
How many ppm is 1% of volume?
1% by volume = 10,000 ppm. The scales are the same quantity: ppm = %vol × 10⁴.
Why do occupational limits use mg/m³ but detectors read ppm?
Toxicology doses scale with mass inhaled, so exposure limits are set in mg/m³; sensors respond to molecular concentration, naturally read in ppm. The molecular weight bridges the two — which is why the same ppm reading of two gases represents very different mass exposures.
Provided for reference and education. Gas detection settings must follow the applicable safety standards and site procedures. See our disclaimer.