Gas concentration converter
Convert between ppm and mg/m³ for a gas in air. Pick the gas first: heavier molecules weigh more at the same ppm.
That's 2× as strong as a workplace exposure limit, while a detector alarm level is 4× higher. That is about one molecule of it in every 20,000 of air.
What it means
What is in the air?
Weight-per-volume figures assume 25 °C and one atmosphere. A hot or high-altitude measurement needs its own molar volume.
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Learn
Three things worth knowing
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mg/m³ = ppm × molar mass ÷ 24.45
50 ppm of carbon monoxide is 57 mg/m³. The 24.45 is the volume one mole of any gas fills at 25 °C, in liters.
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Avogadro did the hard part
Equal volumes of any gas hold equal numbers of molecules. That single fact is why one constant works for every gas, with only the molar mass changing.
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ppm by volume, not by weight
For gases, ppm nearly always means by volume. Treating it as by weight is wrong by the ratio of molar masses, which for methane is almost double.
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Background
About air concentrations
Gas detectors read ppm by volume; exposure limits are often written in mg/m³ by weight. Converting between them needs the molar mass, because a million molecules of methane weigh far less than a million of benzene.