Gas PPM to mg/m³ Converter
Convert gas concentrations between ppm (volume) and mg/m³ (mass) using the ideal gas law. Enter the concentration, the gas molar mass, temperature and pressure to get instant results in mg/m³, ppm and µg/m³.
Convert Between ppm and mg/m³
Gas PPM to mg/m³ Converter Formula
Where:
- \(C_{ppm}\) — concentration in parts per million by volume (ppm)
- \(C_{mg/m^3}\) — concentration in milligrams per cubic metre (mg/m³)
- \(M\) — molecular mass (molar mass) of the gas, in g/mol
- \(P\) — pressure in atmospheres (atm)
- \(T\) — absolute temperature in kelvin (K)
- \(R = 0.082057\) — ideal gas constant in L·atm/(mol·K)
These formulas convert between a volume-based concentration (ppm) and a mass-based concentration (mg/m³) using the ideal gas law. At 25 °C and 1 atm the factor \(R \cdot T\) equals 24.45, which is the basis of the familiar rule \(mg/m^3 = ppm \times M / 24.45\).
Gas PPM to mg/m³ – Calculation Example
Example: convert 100 ppm of carbon dioxide (CO₂) to mg/m³ at 25 °C and 1 atm. The molecular mass of CO₂ is 44.01 g/mol.
\[
T = 25 + 273.15 = 298.15\ \text{K}
\]
\[
C_{mg/m^3} = \frac{100 \times 44.01 \times 1}{0.082057 \times 298.15}
= \frac{4401}{24.4686} \approx 179.86
\]
Result: 100 ppm of CO₂ at 25 °C and 1 atm ≈ 179.86 mg/m³ (about 179,861 µg/m³).
How to use the calculator
- Enter the Gas concentration you know and choose its unit — ppm or mg/m³ — from the dropdown next to the value.
- Enter the Molecular mass of the gas in g/mol (e.g. 44.01 for CO₂, 28.97 for air, 64.07 for SO₂).
- Enter the Temperature of the gas and select °C, °F or K.
- Enter the Pressure and select atm, kPa, bar or mmHg.
- Read the results: the equivalent concentration is shown in mg/m³, ppm and µg/m³.
What ppm and mg/m³ mean
Gas concentrations are expressed in two fundamentally different ways. ppm (parts per million) is a volume-based unit: it tells you how many volumes of gas are present per million volumes of the air mixture. mg/m³ is a mass-based unit: it tells you how many milligrams of the substance are present in each cubic metre of air. Occupational exposure limits, workplace measurements and air-quality standards switch between the two systems, so a reliable conversion is often needed.
The conversion depends on the gas itself. One ppm of a heavy molecule such as sulfur dioxide (M = 64.07 g/mol) corresponds to more than twice as many mg/m³ as one ppm of a light molecule such as methane (M = 16.04 g/mol). That is why the calculator asks for the molecular mass — there is no single universal conversion factor.
Temperature and pressure also matter. Because the conversion is based on the ideal gas law, the volume occupied by a mole of gas shrinks at low temperatures or high pressures, so the same ppm value corresponds to a higher mg/m³ concentration. If you leave the default values (25 °C, 1 atm), the results match the standard 24.45 divisor used in most occupational hygiene references. To see how temperature and pressure shape the properties of air itself, try the Air Density calculator.
Assumptions and limitations
- Ideal gas behaviour. The calculation assumes the gas mixture follows the ideal gas law. This is accurate for trace concentrations at ordinary temperatures and pressures; deviations appear at very high pressures or low temperatures.
- ppm by volume. The calculator converts ppmv (volume ratio). Do not use it for ppm by weight (ppmw) of liquids or dissolved substances — for those, the PPM Converter (ppm to percent and mg/L) is the right tool.
- Dry gas. Humidity is not taken into account. High humidity slightly changes the actual molar volume of air.
- Reference conditions matter. When comparing with legal limits or published data, check whether they refer to 25 °C (as in OSHA and Australian practice) or 20 °C/101.3 kPa (common in Europe) and set the temperature accordingly.
- Accuracy of M. Use the correct molar mass of the pure gas or mixture; a wrong molecular mass shifts the result proportionally.
Quick reference: common gases at 25 °C and 1 atm
| Gas | Molecular mass (g/mol) | 1 ppm equals (mg/m³) | 1 mg/m³ equals (ppm) |
|---|---|---|---|
| Air | 28.97 | 1.18 | 0.845 |
| Carbon monoxide (CO) | 28.01 | 1.14 | 0.873 |
| Carbon dioxide (CO₂) | 44.01 | 1.80 | 0.556 |
| Sulfur dioxide (SO₂) | 64.07 | 2.62 | 0.382 |
| Nitrogen dioxide (NO₂) | 46.01 | 1.88 | 0.532 |
| Ozone (O₃) | 48.00 | 1.96 | 0.510 |
The values above hold only at 25 °C and 1 atm. At other conditions, always run the full conversion with the actual temperature and pressure — or explore other unit conversions such as the Density Converter or the Pressure Altitude calculator.
Frequently Asked Questions
How do I convert ppm to mg/m³ by hand?
Multiply the ppm value by the gas molar mass and divide by the molar volume: \(mg/m^3 = ppm \times M / 24.45\) at 25 °C and 1 atm. For example, 50 ppm of CO (M = 28.01) is \(50 \times 28.01 / 24.45 \approx 57\) mg/m³. At 20 °C use 24.0 instead of 24.45, and at other temperatures use \(24.45 \times T/298.15\) where T is in kelvin.
Why do some references use 24.45 and others 24.0 or 22.4?
These are molar volumes at different conditions. 24.45 L/mol corresponds to 25 °C and 1 atm (OSHA convention), 24.0 L/mol to 20 °C (common in European standards), and 22.4 L/mol to 0 °C (STP). Using the wrong divisor introduces an error of a few percent, so always check which reference temperature your legal limit or data source assumes and match it in the calculator.
What is a normal or safe ppm level for common gases?
It depends entirely on the gas. For CO₂, outdoor air is about 420 ppm and indoor levels below 1000 ppm are generally considered acceptable. For CO, OSHA limits workplace exposure to 50 ppm over 8 hours, while SO₂ and NO₂ have much lower limits of a few ppm. Exposure limits differ between countries and between occupational and ambient air standards, so always verify against the specific regulation that applies to your situation.
Can I use this calculator for liquids or dissolved substances?
No. This tool converts ppm by volume (ppmv) of gases in air, which relies on the ideal gas law. For dissolved substances in water, ppm usually means mg/L by weight, and 1 ppm equals 1 mg/L in dilute aqueous solutions — no temperature or molar mass is needed. Use a dedicated ppm-to-mg/L converter for those cases.
Does humidity affect the ppm to mg/m³ conversion?
Slightly, yes. This calculator assumes dry air. Water vapour displaces other gases, so in very humid air the actual molar volume changes by roughly 1–2% at ordinary conditions. For most workplace and ambient measurements this error is negligible, but for high-precision work at high temperature and humidity you may need a correction using the actual water vapour pressure.
What is the most common mistake when converting ppm to mg/m³?
Using a generic conversion factor without the correct molar mass. One ppm of a heavy gas like SO₂ (64.07 g/mol) equals 2.62 mg/m³, while one ppm of methane (16.04 g/mol) equals only 0.65 mg/m³ — a fourfold difference. Another frequent error is entering °C while the formula expects kelvin, or mixing up reference temperatures when comparing against legal exposure limits.
How do I find the molar mass of a gas mixture?
Average the molar masses of the components weighted by their volume fractions. For example, a mixture of 80% CO₂ and 20% methane has \(M = 0.8 \times 44.01 + 0.2 \times 16.04 = 38.4\) g/mol. For trace gases in air, always use the molar mass of the trace gas itself, not that of air (28.97 g/mol), because the conversion concerns the pollutant molecule, not the carrier gas.