Specific gas constant for dry air
| Value | 287.0528 J/(kg·K) |
| Status | Conventional / typical value |
| Source | ISO 2533:1975 / ICAO Standard Atmosphere |
| Categories | ThermodynamicEngineering & Trade |
| joule per kilogram-kelvin | 287.0528 J/(kg·K) |
| kilojoule per kilogram-kelvin | 0.2870528 kJ/(kg·K) |
| calorie per gram-Celsius | 0.068607266 cal/(g·°C) |
| BTU per pound-Fahrenheit | 0.068561383 BTU/(lb·°F) |
Learning zone
Engineers rarely count moles of air; they weigh it. Dividing the molar gas constant by the molar mass of dry air gives a gas constant per kilogram, so the ideal gas law becomes p = ρRairT. That single line gives you air density directly: at 101.325 kPa and 20 °C, ρ = 101325/(287.05 × 293.15) = 1.204 kg/m³, the number behind every fan, duct and drag calculation.
The value quoted is the ICAO/ISO standard atmosphere's, based on a dry-air molar mass of 28.9644 g/mol. Real air is never quite this: humidity lowers density, because water vapour at 18 g/mol is lighter than the nitrogen and oxygen it displaces, so moist air has a slightly larger effective gas constant. Rising CO₂ has also nudged the molar mass upward in the fourth decimal since the standard was written.