Air Density Calculator
Enter temperature, altitude and RH to get air density and the correction factor that fan curves and the 1.08 equation quietly assume away.
Last updated: 2026-09-28
How the calculation works
- Ideal-gas mixture: dry air and water vapor each contribute by their partial pressure and molar mass.
- Barometric pressure falls with altitude on the standard atmosphere — the dominant density effect.
- The correction factor compares against standard air (1.204 kg/m³, 20 °C, sea level).
Formula
ρ = (p_d·M_d + p_v·M_v) / (R·T)
| Symbol | Meaning | Unit |
|---|---|---|
ρ | Moist air density | kg/m³ |
p_d, p_v | Partial pressures (dry air, vapor) | kPa |
Worked example
Interpreting the result
Standard air assumptions hide three corrections: altitude (the big one — every 1000 m costs ~9% density), temperature (hot air is thin air), and humidity (wet air is LIGHTER than dry — counterintuitive but real, since water molecules weigh less than N₂/O₂). Fan volume CFM is nearly constant while mass varies, so high-altitude sites get less cooling per CFM and less combustion air — equipment derating tables exist because of this factor.
Assumptions
- Ideal-gas mixture, standard atmosphere lapse for altitude.
- Magnus saturation pressure (accurate to ~0.2% over the range).
Limitations
- Not for pressurized systems — only atmospheric conditions.
- Fan performance correction also involves pressure ratios, not just density.
Frequently asked questions
What is the density of air at sea level?
About 1.204 kg/m³ (0.075 lb/ft³) at 20 °C — the 'standard air' that fan curves and the 1.08 sensible-heat factor assume. Colder is denser; higher is thinner.
Does humid air weigh more?
No — it weighs LESS. Water vapor (18 g/mol) displaces heavier dry-air molecules (29 g/mol) at the same pressure and temperature. Humid air is slightly thinner, which is why humid days marginally help aircraft wings and hurt cooling towers' opposite way.