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Gas Concentration Converter

For an ideal gas, equal volumes hold equal numbers of molecules, so a volume-based unit (ppm = parts per million by volume) and a mass-per-volume unit (mg/m³) are linked by the gas's molar mass and its molar volume.

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How to use this calculator

Enter Values

Enter your values and press Calculate to see results.

How to use this calculator

  1. Enter the molar mass of the gas in g/mol (e.g. CO = 28.01, benzene = 78.11, NO₂ = 46.01). This is the one input the converter always needs.
  2. Fill EITHER the ppm field OR the mg/m³ field — never both. Leave the other blank and the tool converts in that direction.
  3. Optionally set the air temperature in °C (default 25 °C) so the molar volume matches your sampling conditions, then read the converted value and the collapsible working.

How it works

For an ideal gas, equal volumes hold equal numbers of molecules, so a volume-based unit (ppm = parts per million by volume) and a mass-per-volume unit (mg/m³) are linked by the gas's molar mass and its molar volume. The core formula is mg/m³ = ppm × M ÷ Vm, where M is the molar mass in g/mol and Vm is the molar volume in L/mol. Rearranged, ppm = mg/m³ × Vm ÷ M. At 25 °C and 1 atmosphere the molar volume is 24.45 L/mol, which is the value most occupational exposure standards assume.

Molar volume grows with absolute temperature, so the tool scales it as Vm = 24.45 × (T + 273.15) ÷ 298.15 when you supply a temperature. Everything is pure arithmetic — no lookups or network calls. Because ppm is a ratio it is temperature-independent, but the mg/m³ value it maps to shifts with temperature, which is why the same ppm reading gives a slightly different mg/m³ on a cold morning versus a warm afternoon.

Worked example

Carbon monoxide: 35 ppm to mg/m³. A workplace air sample reads 35 ppm of carbon monoxide (CO, molar mass 28.01 g/mol) at 25 °C. Enter 35 in the ppm field, 28.01 as the molar mass, and leave mg/m³ blank. The converter uses a molar volume of 24.45 L/mol: mg/m³ = 35 × 28.01 ÷ 24.45 = 40.0961 mg/m³ (about 40,096.11 µg/m³). This lets you compare the reading against a limit quoted in mg/m³.

Common mistakes

  • Filling in both ppm and mg/m³. The converter needs exactly one so it knows which way to convert — fill one and leave the other blank.
  • Using the wrong molar mass, or confusing it with molecular weight of a mixture. Use the molar mass of the single gas of interest in g/mol (CO = 28.01, not 28,010), otherwise the result is off by that factor.
  • Assuming ppm and mg/m³ are interchangeable numbers. They are only equal for a gas whose molar mass happens to equal the molar volume; for most gases the two values differ, so always convert before comparing to an exposure limit.

Frequently asked questions

Does ppm here mean by volume or by mass?

By volume (ppmv), which is how gas and vapour exposure limits are normally expressed. The mg/m³ side is a mass-per-volume figure, and the molar mass plus molar volume is what links the two.

Why does the answer change when I set a different temperature?

A gas's molar volume increases with absolute temperature (Vm = 24.45 × (T+273.15) ÷ 298.15 L/mol at 1 atm). The ppm value is a ratio and does not change, but the mg/m³ it corresponds to does. Most exposure standards are quoted at 25 °C, so leave the temperature at the default unless your sampling conditions differ.

Can I use this to decide if a workplace is safe?

No. This is a units converter for guidance only. Compliance with an occupational exposure limit depends on your jurisdiction's WHS/OSH regulations, correct sampling and calibration, and interpretation by a competent person or occupational hygienist. Use the converted figure as one input, not as a compliance decision.

What molar mass should I enter for common gases?

Carbon monoxide (CO) 28.01, carbon dioxide (CO₂) 44.01, nitrogen dioxide (NO₂) 46.01, sulphur dioxide (SO₂) 64.07, ammonia (NH₃) 17.03, hydrogen sulphide (H₂S) 34.08, benzene (C₆H₆) 78.11 g/mol. Enter the value for the specific gas you sampled.

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