Gas Concentration Converter (ppm ⇄ mg/m³)

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

This converter changes a gas concentration between parts per million by volume (ppm) and milligrams per cubic metre (mg/m³) at the temperature and pressure you choose. Use it when an exposure limit, a sensor reading and a lab report use different units, for example for carbon monoxide, ammonia or solvent vapours in workplace or indoor air.

How it is calculated

mg/m³ = ppm × M ÷ Vm and ppm = mg/m³ × Vm ÷ M

Vm = R × T ÷ P

• M is the molar mass of the gas in g/mol (28.01 for carbon monoxide).
• T is the absolute temperature in kelvin: °C + 273.15.
• P is the pressure in kPa.
• R = 8.314462618 J/(mol·K), so Vm comes out in litres per mole.

The principle

In an ideal gas, equal volumes at the same temperature and pressure contain the same number of molecules (Avogadro's law). So 1 ppm by volume also means 1 mole of the gas per million moles of air. One mole of any gas fills Vm litres, so 1 m³ (1,000 L) of air holds 1,000 ÷ Vm moles. At 1 ppm, one millionth of that is the gas, which weighs 1,000 ÷ Vm × 10⁻⁶ mol × M g/mol = M ÷ Vm milligrams. Warmer air or lower pressure spreads the same molecules over more volume, so the same ppm corresponds to fewer mg/m³.

Worked example

Default inputs: 50 ppm, M = 28.01 g/mol, 25 °C, 101.325 kPa.

1. T = 25 + 273.15 = 298.15 K.
2. Vm = 8.314462618 × 298.15 ÷ 101.325 = 24.4654037 L/mol.
3. mg/m³ = 50 × 28.01 ÷ 24.4654037 = 57.24409936 mg/m³.

So 50 ppm of carbon monoxide is about 57.2 mg/m³ at 25 °C and 1 atm.

How to read the result

Occupational hygiene usually uses 25 °C and 1 atm (Vm ≈ 24.45–24.47 L/mol), while some environmental and engineering standards use 0 °C (22.414 L/mol) or 20 °C. Convert at the reference conditions the limit or standard specifies, not at room conditions, so you compare like with like. For reference, the US OSHA permissible exposure limit for carbon monoxide is 50 ppm as an 8-hour average.

Common mistakes

• Using the molar mass of the wrong substance or of a mixture.
• Entering pressure in atm or hPa instead of kPa.
• Applying the formula to particles, mists or dust, which are measured only in mg/m³.
• Confusing ppm by volume (gases) with ppm by mass (liquids and solids).

Frequently asked questions

How many mg/m³ is 50 ppm of CO?

About 57.2 mg/m³ at 25 °C and 1 atm.

Why does temperature change the result?

Gas volume changes with temperature and pressure while mass does not, so the same mass occupies a different volume.

What is 24.45 in ppm conversions?

It is the molar volume in L/mol at 25 °C and 1 atm, rounded. This calculator uses the more precise 24.465.

Can I use it for ppb?

Yes. Enter the ppb value divided by 1,000 as ppm, or enter the ppb value as it is and read the result as µg/m³ instead of mg/m³.

Sources: CODATA 2018 recommended value of the molar gas constant R (NIST) · NIOSH Pocket Guide to Chemical Hazards, conversion convention at 25 °C and 1 atm · OSHA 29 CFR 1910.1000, Table Z-1 (carbon monoxide)