STP Calculator

Standard temperature and pressure is a bookkeeping convention, not a law of nature. Because the ideal gas law rearranges to Vm = R*T/P, fixing T and P fixes the volume that one mole of any ideal gas occupies, and every gas amount can then be converted to volume by simple multiplication. At 273.15 K and 101.325 kPa, the older 1 atm convention still used throughout general chemistry teaching, Vm = 22.414 L/mol. At 273.15 K and exactly 100 kPa, the standard IUPAC adopted in 1982, Vm = 22.711 L/mol. SATP at 298.15 K and 100 kPa gives 24.789 L/mol, and NTP at 293.15 K and 1 atm gives about 24.06 L/mol. Since the 2019 SI redefinition R is exact at 8.31446261815324 J/(mol*K). Two cautions matter in practice. First, real gases deviate from Vm through the compressibility factor Z = P*Vm/(R*T). Carbon dioxide, with strong attractions, has Z near 0.993 at 0 degrees C and 1 atm and so occupies about 22.26 L/mol, while helium and hydrogen sit fractionally above the ideal value; deviations of a few tenths of a percent are typical at 1 atm and grow rapidly above about 10 atm. Second, the standard must always be stated. European industrial practice quotes normal cubic metres referenced to 0 degrees C and 101.325 kPa, while much US gas metering uses 60 degrees F and 14.696 psia, a difference of roughly 6 percent in reported volume for identical quantities of gas. The gas density at any chosen standard is just the molar mass divided by that molar volume.

The idea that equal volumes of gases contain equal numbers of particles came from Amedeo Avogadro in 1811, building on Joseph Louis Gay-Lussac's 1808 law of combining volumes. Avogadro's hypothesis was largely ignored for half a century until Stanislao Cannizzaro circulated his pamphlet reorganising atomic weights around it at the Karlsruhe Congress of 1860, which finally gave chemists a consistent basis for molar quantities. The gas law itself was assembled piecewise from Robert Boyle's 1662 pressure-volume relation, the temperature work of Jacques Charles and Gay-Lussac, and the combined statement written by Benoit Clapeyron in 1834; Dmitri Mendeleev is usually credited with putting it in the modern molar form with a universal constant in the 1870s. Once the constant was known, 22.4 litres per mole became one of the most repeated numbers in chemistry teaching. The definition shifted in 1982 when IUPAC moved standard pressure from 1 atm to exactly 1 bar, changing the molar volume to 22.711 L/mol, though textbooks and examination boards adopted the change unevenly and both figures remain in circulation. The mole became an SI base unit in 1971, and the 2019 revision redefined it by fixing the Avogadro constant at exactly 6.02214076e23 entities per mole, which removed the last dependence on a physical artefact.