Air Pollutant ppm ⇄ mg/m³ Converter

Understanding the ppm to mg/m³ converter — concept and principle

Gas concentrations in air are reported either as a volume fraction (ppm or ppb) or as a mass concentration (mg/m³ or µg/m³). Air quality standards, workplace exposure limits and stack-emission permits often use different units, so you need to convert between them using the pollutant's molar mass and the temperature and pressure of the air.

ppm counts volume; mg/m³ weighs it — the molar volume links the two.

How it is calculated

mg/m³ = ppm × M × P ÷ (R × T), with M in g/mol, P in kPa, R = 8.314462618 J/(mol·K) and T in kelvin (°C + 273.15).
µg/m³ = mg/m³ × 1000.
R × T ÷ P is the molar volume of an ideal gas in litres per mole: about 24.47 L/mol at 25 °C and 101.325 kPa (the familiar rounded figure is 24.45), and about 22.41 L/mol at 0 °C.
To go the other way: ppm = mg/m³ × R × T ÷ (M × P).

The principle

For an ideal gas, equal volumes contain equal numbers of molecules at the same temperature and pressure (Avogadro's law). One ppm by volume therefore means one micromole of pollutant per mole of air. Multiplying moles by molar mass converts them to grams, and dividing by the molar volume converts "per mole of air" to "per cubic metre of air". Because warm or low-pressure air takes up more space per mole, the same ppm corresponds to fewer mg/m³ at high temperature or altitude. Ppm itself does not change with temperature, which is why it is convenient for gases.

Worked example

Inputs: 1 ppm of NO₂, M = 46.0055 g/mol, T = 25 °C, P = 101.325 kPa.
T = 25 + 273.15 = 298.15 K.
Molar volume = 8.314462618 × 298.15 ÷ 101.325 = 24.465 L/mol.
mg/m³ = 1 × 46.0055 ÷ 24.465 = 1.880 mg/m³.
µg/m³ = 1.880 × 1000 = 1,880 µg/m³.

Each point is the calculator re-run with only “ppm” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

ppm	mg/m³
0.25	0.4701077138
0.5	0.9402154277
0.75	1.410323142
1	1.880430855
1.25	2.350538569
1.5	2.820646283
2	3.760861711

How to read the result

Always check the reference conditions behind a limit value. Ambient air standards and occupational limits are commonly stated at 25 °C and 1 atm, while many stack emission limits refer to normal conditions of 0 °C and 101.3 kPa, sometimes also on a dry basis and corrected to a reference oxygen level. The same 1 ppm of NO₂ is 1.880 mg/m³ at 25 °C but about 2.05 mg/m³ at 0 °C. Typical molar masses: SO₂ 64.07, CO 28.01, O₃ 48.00, NH₃ 17.03 g/mol.

Common mistakes

Using 24.45 at conditions other than 25 °C and 1 atm.
Entering pressure in atm or hPa instead of kPa.
Mixing up ppm and ppb (a factor of 1,000).
Trying to convert particulate matter (PM2.5, PM10) to ppm; particles have no molar mass, so only mass units apply.

Keep in mind: The formula assumes ideal-gas behaviour, which is excellent for trace gases in air but not for concentrated mixtures or condensing vapours. Regulatory reporting must use the reference conditions defined in the applicable rule.

Frequently asked questions

Why is the result slightly different from ppm × M ÷ 24.45?

The calculator uses the exact molar volume at 25 °C (about 24.465 L/mol). The difference is under 0.1%.

How do I convert ppb to µg/m³?

Use the same formula: ppb × M ÷ molar volume gives µg/m³ directly.

Does humidity matter?

Ppm in ambient air is normally on a wet (as-is) basis. Stack measurements are often reported dry, and water vapour must then be removed from the total.

Sources: IUPAC, Quantities, Units and Symbols in Physical Chemistry (Green Book), 3rd ed., 2007 · CODATA 2018 recommended value of the molar gas constant R = 8.314462618 J/(mol·K)

Checked 8 Nov 2026. Standard textbook formulas in SI units, computed in your browser. For learning and estimates — real designs must follow your local codes and be checked by a qualified engineer.

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