Molar volume of an ideal gas at 25 °C and 1 atm

Room-temperature work rarely happens at 0 °C, so industrial hygiene, air quality and HVAC calculations use 25 °C and 1 atm: often labelled NTP, sometimes SATP. One mole then fills 24.4654 L. This is the number hiding inside the standard conversion between ppm by volume and mg/m³ for a gaseous contaminant: mg/m³ = ppm × M / 24.4654.

Be warned that the labels are not standardised. NIST's NTP is 20 °C and 1 atm, giving 24.0555 L/mol; IUPAC's SATP is 25 °C and 1 bar, giving 24.7896 L/mol; and the reference conditions written into gas-sales contracts are different again, often 60 °F and 14.696 psia. Always carry the temperature and pressure with the number rather than the acronym.