PPM to mg/m³ Converter
| Gas | Familiar limit | Weighs |
|---|---|---|
| Carbon monoxide | 25 ppm | 28.6 mg/m³ |
| Carbon dioxide | 5,000 ppm | 9,000.0 mg/m³ |
| Hydrogen sulfide | 10 ppm | 13.9 mg/m³ |
| Ammonia | 25 ppm | 17.4 mg/m³ |
| Sulfur dioxide | 5 ppm | 13.1 mg/m³ |
| Chlorine | 1 ppm | 2.9 mg/m³ |
For gases, ppm and mg/m³ measure the same air two ways - ppm counts molecules per million, mg/m³ weighs the milligrams in a cubic meter - and the bridge is each gas's molar mass: mg/m³ = ppm × molar mass ÷ 24.45, where 24.45 is the volume one mole of any gas fills at room conditions. Select the gas, type a value in either unit, and the converter lands the other. Carbon monoxide at 25 ppm weighs 28.6 mg/m³; carbon dioxide at 5,000 ppm weighs a full 9,000.
This is the industrial-hygiene conversion: workplace exposure limits are published in both units - OSHA's PELs and NIOSH's RELs quote ppm for convenience and mg/m³ for the weight-based toxicology - and the two must agree before anyone signs a monitoring report. The table carries the common gases at their familiar limits, so the scale starts calibrated: a few ppm of hydrogen sulfide is a nose event, thousands of ppm of CO2 is a crowded meeting.
How to use
- Select the gas from the list - each has its own molar mass, which is the whole conversion.
- Type the concentration in ppm from your detector or the exposure table, and mg/m³ appears instantly - or run it backwards from a lab report in mg/m³.
- Check the table rows against the workplace limits: CO at 25 ppm, CO2 at 5,000, ammonia at 25 - the conversions behind the signs on the wall.
Frequently asked questions
How do you convert ppm to mg/m³ for a gas?
Multiply by the gas's molar mass and divide by 24.45: mg/m³ = ppm × MW ÷ 24.45. The 24.45 is liters per mole - the volume one mole of any ideal gas occupies at 25 °C and one atmosphere - so the formula converts a count of molecules into a weight of them. Carbon monoxide (28.01 g/mol) at 25 ppm works out to 28.6 mg/m³; run the same 25 ppm of chlorine (70.90 g/mol) and you get 72.5 mg/m³, nearly three times heavier per molecule.
Why does 24.45 depend on temperature and pressure?
Because it is the molar gas volume at 25 °C and one atmosphere - cooler gas packs tighter and pressurized gas packs denser, so the moles-per-cubic-meter changes. Industrial hygiene converts at 25 °C and 760 mmHg by convention, which is what the 24.45 encodes. For hot stacks or compressed lines the constant shifts (22.4 at 0 °C), and that difference is exactly why monitoring reports state their reference conditions.
What is the difference between ppm and mg/m³?
PPM is a molecule count - how many of every million air molecules are the gas in question - which is why it survives altitude and weather. Mg/m³ is a weight per volume, which is what toxicology doses by: the body responds to mass inhaled, not molecular census. Both describe the same air; ppm is what detectors print and noses notice, mg/m³ is what exposure limits and lab reports weigh.
Where do workplace exposure limits come from?
Two American agencies publish the numbers most signs quote: OSHA's Permissible Exposure Limits (legal ceilings) and NIOSH's Recommended Exposure Limits (the health-based advice). Both are published in ppm and mg/m³ side by side in NIOSH's Pocket Guide to Chemical Hazards - the reference shelf this converter's table borrows from. Other jurisdictions set their own numbers, so always match the limits to your regulator, not your neighbor's.
Why does the same ppm feel different for different gases?
Toxicity is chemistry, not arithmetic. The conversion's molar mass only translates units - 25 ppm of carbon monoxide is dangerous because CO binds hemoglobin, while 25 ppm of methane is a non-event. That is why the converter makes you pick the gas first: the molar mass is the honest bridge between the units, but the danger line comes from the gas's own toxicology, published per substance in the exposure tables.