Gas Converter
ppm ↔ mg/Nm³
Reference conditions
The conversion applies the ideal gas law. The molar volume of any gas depends on temperature and pressure, so a mass concentration in mg/m³ has no meaning until the reference state is stated.
Vm = R × T / p
M is the molecular weight in g/mol, Vm the molar volume in m³/mol at the selected reference state, R = 8.314463 Pa·m³/(mol·K), T the reference temperature in kelvin and p the reference pressure in pascal. At 0 °C and 101.325 kPa, Vm = 0.022414 m³/mol.
Which reference state applies
| Reference | Used for |
|---|---|
| 0 °C, 101.325 kPa | Normal cubic metre (Nm³). Stack and process emission limits under the EU Industrial Emissions Directive and the EN monitoring standards. This is the default here. |
| 20 °C, 101.3 kPa | Occupational exposure limits in the EU chemical agents directives and the Swedish hygieniska gränsvärden. |
| 25 °C, 101.325 kPa | ACGIH threshold limit values and much of US occupational hygiene literature. |
The spread is not trivial. The same ppm value converts to a mass concentration about 6.8 % lower at 20 °C and about 8.4 % lower at 25 °C than at 0 °C, so quoting mg/m³ without the reference state invites a mismatch against a permit limit.
What this tool does not do
The result is on the same moisture basis as the input. Emission limits are normally set on dry gas and corrected to a stated oxygen reference level, typically 3, 6, 11 or 15 % O₂ depending on the plant type. Apply the dry-basis and O₂ corrections separately before comparing against a permit value. The ideal gas assumption holds well for trace concentrations at near-atmospheric pressure and introduces no meaningful error in this range.
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