Density comes straight from the ideal gas law
Pressure P is in pascals, T is absolute temperature in kelvin (C + 273.15), and R is the specific gas constant for air, 287 J/(kg·K). Density rises with pressure and falls as the air warms. At 15 C and one atmosphere this gives the standard 1.225 kg/m3. Humidity applies a small downward correction because water vapor is lighter than the air it displaces.
How density falls with altitude
Pressure and temperature both drop as you climb, and density drops with them. These are the Standard Atmosphere values, useful when you know your elevation but not the local pressure.
| Altitude | Temp | Pressure | Density |
|---|---|---|---|
| 0 m (sea level) | 15 C | 101.3 kPa | 1.225 kg/m3 |
| 1,000 m | 8.5 C | 89.9 kPa | 1.112 kg/m3 |
| 2,000 m | 2 C | 79.5 kPa | 1.007 kg/m3 |
| 5,000 m | −17.5 C | 54.0 kPa | 0.736 kg/m3 |
| 10,000 m | −50 C | 26.5 kPa | 0.414 kg/m3 |
At 5,000 m the air holds only about 60% of the mass it does at sea level, which is why high-altitude takeoffs need longer runways.
What moves the number
- Temperature. Density is inversely proportional to absolute temperature. Air at 35 C is about 6% less dense than at 15 C.
- Pressure. Density is directly proportional to pressure, so a low-pressure day is a lower-density day at the same temperature.
- Humidity. Moist air is lighter, not heavier. At 80% relative humidity air is roughly 0.4% less dense than dry air.
- You need pressure to be exact. Temperature and humidity alone are not enough. If you only know altitude, the Standard Atmosphere model estimates the pressure for you.
Common questions
What is the density of air at sea level?
Under standard conditions (15 C, 101,325 Pa, dry air) air density is 1.225 kg/m3. That is the reference value aircraft instruments and engine specs assume.
Is humid air heavier or lighter than dry air?
Lighter. A water molecule weighs 18 against 28 for nitrogen and 32 for oxygen, so replacing dry air with water vapor lowers the mass. The effect is small, roughly 0.5% less density per 10% rise in relative humidity.
Why does my engine lose power at altitude?
Thinner air holds fewer oxygen molecules per intake stroke. A naturally aspirated engine loses about 1% of its power for every 300 m of elevation gain. Forced induction compensates, but only up to a point.
