For dry air, an ideal-gas form is:

Humid air is treated as a mixture of dry air and water vapor, each contributing a partial pressure. Temperature must be absolute and pressure must be absolute.

Choose whether to enter pressure directly or estimate it from altitude. Enter temperature and relative humidity. Use the result for comparison, HVAC correction or educational atmospheric work, and verify project-specific standards where fan ratings or aviation data require defined reference conditions.

“Standard air density” depends on the standard being used. A commonly quoted value near sea level is about 1.225 kg/m³ at 15°C under standard-atmosphere conditions. Room-temperature values are lower at the same pressure because warmer air is less dense.

At approximately constant pressure, warmer air is less dense. This is central to natural convection, hot-air balloons, combustion calculations and fan performance. The live chart shows the trend instead of hiding it behind a single lookup value.

Atmospheric pressure decreases with altitude, so air density generally decreases as elevation increases. Standard-atmosphere formulas are convenient reference models, but actual weather pressure and temperature can differ from the standard profile.

At the same temperature and total pressure, humid air is slightly less dense than dry air because water vapor has a lower molar mass than the dry-air mixture it displaces. This surprises people because “humid air feels heavy,” but thermodynamics has declined to consult intuition.

Specific volume is v=1/ρ. Specific weight is γ=ρg. These are useful in fluid mechanics, ventilation, fan calculations and engineering reference work.

If volume is known, air mass is m=ρV. A 100 m³ space contains roughly 120 kg of air near ordinary indoor conditions, depending on temperature, pressure and humidity.

A useful table must state both temperature and pressure. Values at 20°C, 25°C, 60°F or 300 K are not universal if atmospheric pressure changes. Altitude tables are likewise reference-model values unless current weather pressure is used.

Do not enter gauge pressure into an absolute-pressure formula, do not treat altitude alone as a complete weather model, and do not assume humidity always increases gas-mixture density.

These are orientation values only. Humidity and actual barometric pressure shift the result.

At 25°C and 101.325 kPa, dry air density from the ideal-gas relationship is roughly 1.184 kg/m³. Adding humidity changes the mixture because water vapor replaces some heavier dry-air molecules. The calculator performs that partial-pressure split automatically.

Air density appears in fan selection, HVAC airflow correction, aerodynamic force, combustion, drone and aircraft performance, meteorology and natural convection. Many equipment ratings are referenced to a standard density, so a density ratio can be more useful than the raw kg/m³ value when correcting performance.