Density of Air: 1.225 kg/m³ at Sea Level — Temperature, Altitude, and Formula
Dry air at sea level and 15°C has a density of 1.225 kg/m³ under standard atmospheric conditions. This is the International Standard Atmosphere reference used in aviation engineering and a common default for HVAC and meteorological calculations.
Unlike most solids and liquids, air density is highly sensitive to temperature, pressure, and altitude. It falls by roughly 10% for every 1,000 m of altitude gain, and it also changes with humidity. This page gives standard values, temperature and altitude tables, humidity effects, and the ideal-gas calculation method, with a path back to the material density calculator.
Key values
Air Density: Key Values
kg/m³
1.225 kg/m³
ISA standard: 15°C, 101,325 Pa, sea level
g/cm³
0.001225 g/cm³
Same condition
lb/ft³
0.0765 lb/ft³
U.S. standard atmosphere reference
ISA = International Standard Atmosphere (ICAO standard).
At 0°C (STP): 1.293 kg/m³. At 20°C (NTP): 1.204 kg/m³.
Humid air is slightly less dense than dry air at the same temperature and pressure.
Temperature effect
Air Density at Different Temperatures
Temperature is the most direct factor affecting air density. According to the ideal gas law, when temperature rises, gas expands and the mass inside the same volume decreases, lowering density.
This is why warm air rises and cold air sinks.
| Temperature | Density | Notes |
|---|---|---|
| −40°C | 1.514 kg/m³ | Extreme cold climate |
| −20°C | 1.395 kg/m³ | Cold winter conditions |
| 0°C | 1.293 kg/m³ | STP (Standard Temperature and Pressure) |
| 10°C | 1.247 kg/m³ | Cool conditions |
| 15°C | 1.225 kg/m³ | ISA standard atmosphere |
| 20°C | 1.204 kg/m³ | NTP (Normal Temperature and Pressure) |
| 25°C | 1.184 kg/m³ | Common lab reference |
| 35°C | 1.146 kg/m³ | Hot summer conditions |
| 100°C | 0.946 kg/m³ | Boiling point of water |
At constant pressure, air density decreases by approximately 0.4% per °C rise in temperature. A 20°C increase from 15°C to 35°C reduces density by about 6.5%.
For comparison with a much denser liquid reference, see density of water.
Altitude effect
Air Density vs Altitude
As altitude increases, atmospheric pressure falls and air density falls with it. This directly affects aircraft performance, athletic performance at altitude, and internal combustion engine efficiency.
| Altitude | Pressure | Temperature | Air Density | vs Sea Level |
|---|---|---|---|---|
| 0 m (sea level) | 101,325 Pa | 15°C | 1.225 kg/m³ | 100% |
| 500 m | 95,461 Pa | 11.8°C | 1.167 kg/m³ | 95.3% |
| 1,000 m | 89,876 Pa | 8.5°C | 1.112 kg/m³ | 90.8% |
| 1,500 m | 84,560 Pa | 5.3°C | 1.058 kg/m³ | 86.4% |
| 2,000 m | 79,501 Pa | 2.0°C | 1.007 kg/m³ | 82.2% |
| 3,000 m | 70,121 Pa | −4.5°C | 0.909 kg/m³ | 74.2% |
| 5,000 m | 54,048 Pa | −17.5°C | 0.736 kg/m³ | 60.1% |
| 8,849 m (Everest) | 31,460 Pa | −36.6°C | 0.467 kg/m³ | 38.1% |
| 10,000 m (cruising) | 26,500 Pa | −50°C | 0.414 kg/m³ | 33.8% |
At cruising altitude (~10,000 m), air density is only about one-third of sea-level density. This is why jet engines use turbocompressors and why cabin pressurisation is essential for passenger comfort and safety.
For broader material comparisons, open the density table.
Formula
How to Calculate Air Density: The Formula
Air density can be calculated accurately from the ideal gas law when temperature and pressure are known. For the base concept of mass per unit volume, see what is density; for rearranging density equations in general, use the density formula guide.
The ideal gas law states:
PV = nRT
Where P is pressure (Pa), V is volume (m³), n is amount of substance (mol), R is the universal gas constant (8.314 J/mol·K), and T is temperature (Kelvin).
ρ = PM / RT
Rearranging for density gives mass per unit volume directly from pressure, molar mass, the gas constant, and absolute temperature.
Where
- ρ = air density (kg/m³)
- P = absolute pressure (Pa)
- M = molar mass of dry air = 0.028964 kg/mol
- R = universal gas constant = 8.314 J/(mol·K)
- T = absolute temperature (Kelvin = °C + 273.15)
Worked example
Calculate air density at 25°C and standard sea-level pressure (101,325 Pa):
T = 25 + 273.15 = 298.15 K
ρ = (101,325 × 0.028964) / (8.314 × 298.15)
ρ = 2,934.0 / 2,478.8
ρ = 1.184 kg/m³
This matches the table value for 25°C above.
Humidity
How Humidity Affects Air Density
Humid air is less dense than dry air at the same temperature and pressure. This surprises many people because water vapour feels "heavy." The reason is molecular mass: water vapour (H₂O, molar mass 18 g/mol) is lighter than the nitrogen (N₂, 28 g/mol) and oxygen (O₂, 32 g/mol) molecules it displaces. When water vapour replaces heavier gas molecules, the overall density drops.
At 30°C and 100% relative humidity, air density is approximately 1.146 kg/m³, compared to 1.165 kg/m³ for dry air at the same temperature — a reduction of about 1.6%. While small, this effect is measurable and matters in precision applications such as aircraft performance calculations and meteorological modelling.
In baseball and cricket, humid air reduces aerodynamic drag on the ball slightly, allowing it to travel marginally farther. In aviation, high humidity on hot days ("hot and high" conditions) reduces air density enough to noticeably affect takeoff distance and climb rate. HVAC engineers account for humidity when calculating air mass flow rates through duct systems.
Applications
Practical Applications of Air Density
Aviation and aerodynamics
Lift, drag, and engine thrust all depend directly on air density. Pilots use density altitude — the altitude in the standard atmosphere corresponding to the actual air density — to predict aircraft performance, especially at high-elevation airports on hot days.
HVAC and ventilation engineering
Air density determines the mass flow rate of air through ducts and fans. At higher elevations or temperatures, lower-density air carries less thermal energy per cubic metre, so HVAC systems must move larger volumes to achieve the same heating or cooling effect.
Weather and meteorology
Density differences between warm and cold air masses drive atmospheric circulation, wind patterns, and storm formation. Cold, dense air sinks and displaces warmer, less dense air upward — the fundamental mechanism behind most weather systems.
Sports performance at altitude
At 2,000 m elevation, air density is about 18% lower than at sea level, reducing aerodynamic drag. This is why world records in sprint events and throwing disciplines are more commonly set at high-altitude venues, and why endurance athletes train at altitude to adapt to lower oxygen availability.
FAQ
Frequently Asked Questions
What is the density of air in kg/m³?
Dry air at sea level and 15°C (the International Standard Atmosphere reference condition) has a density of 1.225 kg/m³. At 0°C (STP), it is 1.293 kg/m³, and at 20°C (NTP), it is 1.204 kg/m³. Air density decreases with rising temperature, increasing altitude, and increasing humidity.
What is the density of air at STP?
At STP (Standard Temperature and Pressure: 0°C and 101,325 Pa), dry air has a density of 1.293 kg/m³. Note that different organisations define STP slightly differently — IUPAC defines STP as 0°C and 100,000 Pa, which gives a density of 1.275 kg/m³. The ISA standard (15°C, 101,325 Pa) gives 1.225 kg/m³.
How does altitude affect air density?
Air density decreases with altitude because atmospheric pressure drops as the weight of the air column above decreases. At 1,000 m, air density is about 91% of sea-level density. At 5,000 m, it drops to about 60%. At the cruising altitude of a commercial aircraft (~10,000 m), air density is only about 34% of sea-level density.
Why is humid air less dense than dry air?
Water vapour molecules (molar mass 18 g/mol) are lighter than the nitrogen (28 g/mol) and oxygen (32 g/mol) molecules they replace in humid air. When water vapour displaces heavier gas molecules, the average molecular mass of the air mixture decreases, reducing density. At 30°C and full saturation, humid air is about 1.5–2% less dense than dry air.
What is the formula for air density?
Air density is calculated using the ideal gas law rearranged as ρ = PM / (RT), where P is absolute pressure in Pascals, M is the molar mass of dry air (0.028964 kg/mol), R is the universal gas constant (8.314 J/mol·K), and T is absolute temperature in Kelvin. At 25°C and 101,325 Pa, this gives 1.184 kg/m³.