The conversion, and why molecular weight is unavoidable
ppm counts molecules; mg/m³ weighs them. Moving between the two needs the molecular weight of the specific gas, which is why a generic converter cannot answer "what is 100 mg/m³ in ppm" without being told what the gas is. Benzene and hydrogen differ by a factor of 39 at the same ppm.
Conversion factors by gas (25 °C)
| Gas | MW | 1 ppm = | 1 mg/m³ = | 100% LEL |
|---|---|---|---|---|
| Methane (CH₄) | 16.04 | 0.656 mg/m³ | 1.524 ppm | 50,000 ppm |
| Propane (C₃H₈) | 44.1 | 1.804 mg/m³ | 0.554 ppm | 21,000 ppm |
| Butane (C₄H₁₀) | 58.12 | 2.377 mg/m³ | 0.421 ppm | 18,000 ppm |
| Ethylene (C₂H₄) | 28.05 | 1.147 mg/m³ | 0.872 ppm | 27,000 ppm |
| Hydrogen (H₂) | 2.016 | 0.082 mg/m³ | 12.128 ppm | 40,000 ppm |
| Carbon monoxide (CO) | 28.01 | 1.146 mg/m³ | 0.873 ppm | 125,000 ppm |
| Hydrogen sulphide (H₂S) | 34.08 | 1.394 mg/m³ | 0.717 ppm | 40,000 ppm |
| Ammonia (NH₃) | 17.03 | 0.697 mg/m³ | 1.436 ppm | 150,000 ppm |
| Methanol (CH₃OH) | 32.04 | 1.310 mg/m³ | 0.763 ppm | 60,000 ppm |
| Ethanol (C₂H₅OH) | 46.07 | 1.884 mg/m³ | 0.531 ppm | 33,000 ppm |
| Acetone (C₃H₆O) | 58.08 | 2.375 mg/m³ | 0.421 ppm | 25,000 ppm |
| Benzene (C₆H₆) | 78.11 | 3.195 mg/m³ | 0.313 ppm | 12,000 ppm |
| Toluene (C₇H₈) | 92.14 | 3.769 mg/m³ | 0.265 ppm | 11,000 ppm |
| Formaldehyde (CH₂O) | 30.03 | 1.228 mg/m³ | 0.814 ppm | 70,000 ppm |
The LEL scale is a different question
Percent LEL rescales concentration against flammability, not toxicity, and the two have nothing to do with each other:
Methane's LEL is 5% volume, so 100% LEL is 50,000 ppm and a typical 20% LEL alarm fires at 10,000 ppm. Hydrogen sulphide, by contrast, is lethal around 100 ppm — which is 0.25% LEL, a reading a flammability detector would treat as nothing at all. A gas detector calibrated for explosion risk is not a toxicity monitor, and confusing the two is a recurring cause of serious incidents.
Worked examples
- 1,608 mg/m³ of ammonia → ppm. 1,608 × 24.45 ÷ 17.03 = 2,309 ppm at 25 °C.
- 133 mg/m³ of methanol → ppm. 133 × 24.45 ÷ 32.04 = 101.5 ppm.
- 20% LEL of methane → ppm. 20% of 5% volume = 1% volume = 10,000 ppm = 6,560 mg/m³.
- 1,000 ppm of ammonia → mol/m³. 1 m³ of gas holds 40.9 mol at 25 °C, so 1,000 ppm is 0.0409 mol/m³.
Field notes
- State the temperature basis. Converting at 25 °C when the regulation specifies 0 °C overstates ppm by about 9%. Occupational limits, emissions rules and datasheets do not all agree — match whichever applies.
- mg/L equals g/m³. They are the same number in different clothing, which trips people up when a lab reports in mg/L and the analyser reads mg/m³ — those differ by 1,000.
- LEL values vary slightly between references (IEC, NFPA and older tables disagree at the margin). Use your site's accepted figure rather than this table where a trip setting depends on it.
- In liquids, ppm means by mass — a different quantity with a different conversion. Everything here is gas-phase ppm by volume.
- Analyser output still travels as 4-20 mA — check the range with the 4-20 mA converter, and see the excess air calculator for flue gas work.
Frequently asked questions
How do I convert ppm to mg/m³?
mg/m³ = ppm × molecular weight ÷ molar volume. At 25 °C and 1 atm the molar volume is 24.45 L/mol, so 1 ppm of ammonia (MW 17.03) is 0.697 mg/m³. The molecular weight is essential — the same ppm value gives a different mg/m³ for every gas.
How do I convert mg/m³ to ppm?
ppm = mg/m³ × molar volume ÷ molecular weight. For example 1,608 mg/m³ of ammonia is 1,608 × 24.45 ÷ 17.03 = 2,309 ppm at 25 °C.
How do I convert %LEL to ppm?
Multiply the percent LEL by the gas LEL in percent volume, then by 100 to reach ppm. Methane has an LEL of 5% volume, so 20% LEL is 1% volume, which is 10,000 ppm.
Does temperature affect the ppm to mg/m³ conversion?
Yes. The molar volume changes with temperature: 22.41 L/mol at 0 °C, 24.06 at 20 °C and 24.45 at 25 °C. Using the 25 °C value for a 0 °C basis introduces about 9% error, so always match the reference the regulation or datasheet states.
What is the difference between ppm and ppmv?
For gases they are the same thing — parts per million by volume. The v is sometimes added for clarity because in liquids ppm conventionally means by mass, which is a different quantity entirely.
Provided for reference and education. Gas detection settings and exposure assessments must follow the applicable standards and site procedures. See our disclaimer.