ppm ↔ mg/m³ gas table: factors at 25, 20 and 0 °C

How many mg/m³ 1 ppm of CO, CO₂, H₂S, SO₂, NO₂, NH₃, Cl₂ and other gases is worth at 25, 20 and 0 °C and 1 atm, from the molar mass of each gas.

In gases, ppm is a volume fraction (mol per mol) and mg/m³ is mass per volume. The conversion depends on the gas molar mass and on the temperature and pressure at which the volume is measured; that is why the same limit changes with the reference condition.

Conversion factors at 101.325 kPa
Gas	Formula	Molar mass (g/mol)	1 ppm = mg/m³ at 25 °C	1 ppm = mg/m³ at 20 °C	1 ppm = mg/m³ at 0 °C	1 mg/m³ = ppm at 25 °C
Carbon monoxide	CO	28.010	1.145	1.164	1.250	0.8735
Carbon dioxide	CO₂	44.009	1.799	1.830	1.963	0.5559
Hydrogen sulfide	H₂S	34.081	1.393	1.417	1.521	0.7179
Sulfur dioxide	SO₂	64.066	2.619	2.663	2.858	0.3819
Nitrogen dioxide	NO₂	46.006	1.880	1.913	2.053	0.5318
Nitric oxide	NO	30.006	1.226	1.247	1.339	0.8154
Ammonia	NH₃	17.031	0.6961	0.7080	0.7598	1.437
Chlorine	Cl₂	70.906	2.898	2.948	3.163	0.3450
Hydrogen chloride	HCl	36.461	1.490	1.516	1.627	0.6710
Hydrogen cyanide	HCN	27.026	1.105	1.124	1.206	0.9053
Ozone	O₃	47.997	1.962	1.995	2.141	0.5097
Methane	CH₄	16.043	0.6557	0.6669	0.7158	1.525
Benzene	C₆H₆	78.114	3.193	3.247	3.485	0.3132
Toluene	C₇H₈	92.141	3.766	3.830	4.111	0.2655
Formaldehyde	CH₂O	30.026	1.227	1.248	1.340	0.8148
Ethanol	C₂H₆O	46.069	1.883	1.915	2.055	0.5311
Acetone	C₃H₆O	58.080	2.374	2.414	2.591	0.4212

25 °C: usual in occupational hygiene (ACGIH, NIOSH).
20 °C: usual in European exposure limits.
0 °C: normal condition, usual for emissions.

Pressure of 101.325 kPa and ideal gas behaviour, suitable for low concentrations. Always check the reference condition of the limit or the analyser.

Values calculated from the reference functions; they do not replace measurement or a calibration certificate.

See also