Standard Temperature and Pressure Calculator

There is no single STP — the 22.4 L everyone memorised is the pre-1982 definition.

Molar volume22.711 L/molIUPAC STP, since 1982

Volume of 1 mol22.711 L

Temperature273.15 K0 °C

Pressure100 kPa0.98692 atm

IUPAC STP, before 198222.414 L/mol0 °C, 101.325 kPa

NIST NTP24.0551 L/mol20 °C, 101.325 kPa

Standard ambient (SATP)24.7896 L/mol25 °C, 100 kPa

ISO 13443, natural gas23.6448 L/mol15 °C, 101.325 kPa

US EPA standard24.0551 L/mol20 °C, 101.325 kPa

The familiar 22.4 L/mol belongs to 0 °C and one atmosphere — the pre-1982 IUPAC definition. Under the current one it is 22.711, and the spread across all the conventions here is close to nine per cent.

The formula

PV = nRT, evaluated at whichever standard you mean

There is no single STP

IUPAC changed the standard pressure from 1 atm to 100 kPa in 1982, which moved the molar volume from 22.414 to 22.711 L. NIST uses 20 °C and 1 atm and calls it NTP. The gas industry uses 15 °C. Textbooks print whichever was current when they were written, and the 22.4 figure most people memorised is the pre-1982 one.

The differences are not small

Between the coldest and warmest of these conventions the molar volume varies by about nine per cent. For a gas sold by volume that is a commercial quantity, which is exactly why the gas industry pins down its reference conditions in the contract rather than relying on a word.

Where these models stop

Michaelis-Menten assumes a single substrate and a steady state; Langmuir assumes one layer on identical sites with no interaction between them; Stokes-Einstein assumes a hard sphere much larger than the solvent molecules around it. Each is an idealisation that happens to describe real systems well over a useful range, and each fails predictably outside it. Knowing which assumption a number rests on is usually more valuable than the number.