Enter dry-bulb temperature, absolute pressure, and relative humidity to calculate moist-air density and specific volume.
Concepts behind this calculation
Calculate total moist-air mass density from dry-bulb temperature, total absolute barometric pressure and relative humidity.
Air density depends on the air state rather than duct dimensions. Dry-bulb temperature, total absolute pressure and relative humidity establish water-vapor partial pressure and humidity ratio in this perfect-gas moist-air model. The specific volume describes mixture volume per unit dry-air mass. Its reciprocal therefore gives dry-air mass per mixture volume, while adding the vapor mass gives total moist-air density.
Those two density outputs serve different mass balances. A calculation involving total air mass needs total moist-air density; a psychrometric calculation based on pounds of dry air needs the separately labeled dry-air quantity. Relative humidity must belong to the same temperature and pressure state. Duct static or fan pressure alone is not the total absolute barometric pressure required here, and the tool does not derive local pressure from elevation.
How to use this calculator
Enter Air dry-bulb temperature and Relative humidity for one measured or defined air state.
Enter Total absolute barometric pressure at that same state, using an absolute pressure reference.
Review total moist-air density, dry-air mass per mixture volume, specific volume, humidity ratio and vapor partial pressure before transferring a property to another calculation.
pv/pabs = vapor/total absolute pressure (psia); W = humidity ratio (lb water/lb dry air); v = specific volume (ft³/lb dry air). ρdry = dry-air mass per actual ft³; ρmoist = total moist-air mass per actual ft³.
Input definitions and units
Air dry-bulb temperature (°F)
Temperature of the air at the stated density condition. Supported range −60 to 120°F for the stated perfect-gas model.
Total absolute barometric pressure (psia)
Use the total local barometric absolute pressure, not gauge pressure, duct static pressure or fan pressure. No altitude or weather lookup is performed.
Relative humidity (%)
Relative humidity at the entered dry-bulb temperature and absolute pressure.
How to interpret the result
Choose the density output that matches the downstream mass basis and preserve the echoed temperature, pressure and humidity. The calculated property does not establish airflow, fan capacity or pressure loss at that state.
Worked examples
Worked example
Inputs: Constructed state: 70°F dry bulb, 14.696 psia total absolute pressure and 50% RH.
Calculation: The ASHRAE perfect-gas moist-air relations give W = 0.00778327 lb water/lb dry air and v = 13.520079 ft³/lb dry air. Total moist-air density is (1+W)/v; dry-air mass per mixture volume is 1/v.
Result: 0.074540 lbm moist air/ft³ total density and 0.073964 lbm dry air/ft³ dry-air contribution. This constructed state is not a universal standard-air default.
Assumptions and limits
Uses the perfect-gas moist-air approximation. The ASHRAE comparison bounds the listed humidity-ratio, enthalpy and specific-volume errors under the stated standard-pressure conditions; it is not a separate guaranteed error bound for every density result.
Use for ordinary atmospheric HVAC air, not compressed-air service, high-pressure gas mixtures or real-gas density correction.
Temperature must be −60 to 120°F, relative humidity 0–100%, total absolute pressure positive and finite, and water-vapor partial pressure below total absolute pressure.
Pressure is total absolute barometric pressure. No elevation, station pressure, weather, gas composition or instrument factor is inferred.
Total moist-air density includes water vapor. Dry-air mass per mixture volume and specific volume per unit dry-air mass are different, explicitly labeled quantities.
This property calculator does not determine CFM, fan operating point, duct pressure loss, heat transfer, enthalpy, wet-bulb temperature, dew point, code compliance or equipment capacity.
FAQs
Why is total moist-air density different from dry-air mass per volume?
Both refer to the same mixture volume, but total density includes water vapor as well as dry air. The dry-air quantity excludes that vapor mass and pairs with specific volume per pound of dry air.
Does entering relative humidity also give dew point or wet-bulb temperature?
This form uses humidity to determine the stated density properties. Its outputs do not include a dew-point or wet-bulb solution, and those quantities should not be inferred from density alone.
Sources
ASHRAE Handbook—Fundamentals 2025, IP Chapter 1: Psychrometrics, Equations 10–11 and 21–25Equations 10–11 define moist-air density from total mixture mass and specific volume; Equations 21–25 provide perfect-gas humidity ratio, relative-humidity relation and specific volume per unit dry-air mass. The chapter states the perfect-gas comparison range; Table 2 supplies the independently compared 70°F dry and saturated density values at 14.696 psia.
Omni Calculator: Air Density CalculatorHTTP 200 and page controls inspected 2026-11-15. The peer exposes air pressure, air temperature, dry/moist selection, relative humidity or dew point, and air density. Direct task/control evidence only; no live numeric result or equation is copied.
Engineering ToolBox: Air Density, Specific Weight, and Thermal ExpansionHTTP 200 and page metadata/controls inspected 2026-11-15. The page identifies air-density calculation across temperature and pressure and includes an imperial/SI calculator; it does not establish the moist-air RH method used here. No peer numeric result or equation is copied.
