Molar gas constant
| Value | 8.31446261815324 J/(mol·K) |
| Status | Exact by definition — no uncertainty |
| Source | SI Brochure, 9th edition (2019); CODATA 2022 |
| Categories | ThermodynamicChemistryUniversal & Atomic |
| joule per mole kelvin | 8.3144626 J/(mol·K) |
| kilojoule per mole kelvin | 0.0083144626 kJ/(mol·K) |
| calorie per mole kelvin | 1.9872043 cal/(mol·K) |
Learning zone
The molar gas constant is the Boltzmann constant scaled up to laboratory quantities: R = k · NA. It is the constant of proportionality in the ideal gas law PV = nRT, the coefficient in the Nernst and Arrhenius equations, and the natural unit of molar entropy — which is why the site types it as J/(mol·K) and will convert it to BTU/(lb-mol·°R) or cal/(mol·K) for you.
Its value came out of nineteenth-century gas metrology, above all Henri Victor Regnault's obsessively careful measurements of gas densities and compressibilities in Paris in the 1840s, which Clausius and others folded into a single universal constant once Avogadro's hypothesis was accepted. For a century afterwards R was determined experimentally, usually by acoustic thermometry in argon; the best measurements agreed to a few parts per million and were the raw material for the 2019 redefinition. Now that both k and NA are fixed by definition, their product is exact: 8.31446261815324 J/(mol·K), full stop.