How do you convert ppm to mg/m³?

Multiply the ppm value by the molar mass of the gas and divide by the molar volume, 24.45 l/mol at 25 °C and 1013 hPa: c [mg/m³] = c [ppm] · M / 24.45. For CO₂ (M = 44.01 g/mol), 1,000 ppm therefore equals 1,000 · 44.01 / 24.45 ≈ 1,800 mg/m³. The calculator above also takes temperature and air pressure into account.