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Psychrometric Calculator

The full state of moist air from its dry-bulb temperature and one other property (relative humidity, wet-bulb temperature, dew point or humidity ratio) at the pressure of the entered altitude: humidity ratio, relative humidity, wet-bulb and dew-point temperatures, enthalpy, specific volume, density, vapor pressure and the saturation values, by the ASHRAE perfect-gas relations with the Hyland and Wexler saturation pressure. The state is drawn on a psychrometric chart with the saturation curve and the relative-humidity lines. Temperatures are entered as plain numbers in °F (USCS) or °C (SI).

  • State from dry bulb plus relative humidity, wet bulb, dew point or humidity ratio, at any altitude

  • Humidity ratio, enthalpy, specific volume, density, vapor and saturation pressures, degree of saturation

  • Psychrometric chart with the state point, saturation curve and relative-humidity lines

Psychrometric Calculator

Unit System: /

State (temperatures in °F for USCS, °C for SI)





Moist-Air Relations | Method page
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Psychrometric Calculator

Moist-Air Relations

Moist air is treated as a mixture of two perfect gases, dry air and water vapor, at the total pressure p of the altitude. The humidity ratio is the mass of vapor per mass of dry air, W = 0.621945 pw/(p − pw), with pw the vapor's partial pressure; at saturation pw equals the saturation pressure pws(T) of Hyland and Wexler, and the relative humidity is pw/pws. The enthalpy per unit of dry air, h = 1.006 t + W(2501 + 1.86 t) kJ/kg with t in °C, carries the sensible heat of the air and vapor and the latent heat of the vapor; ASHRAE's USCS tables take 0 °F dry air as the datum and the tool reports Btu/lb on that basis. The specific volume per unit of dry air follows from the gas law.

The wet-bulb temperature is found from the adiabatic-saturation relation (ASHRAE eq. 35) by iteration; the dew point from the inverse of the saturation curve (eq. 39 and 40). Given the dry bulb and any one of relative humidity, wet bulb, dew point or humidity ratio, the state is fixed.

Altitude matters: at 5,000 ft the pressure is 12.2 psia and the same dry bulb and relative humidity hold 20 percent more water per pound of air than at sea level, which is why the chart is drawn at the entered pressure.

Temperatures are entered as plain numbers in the units of the selected system because their scales have offsets; every other quantity carries its unit dropdown.



References

American Society of Heating, Refrigerating and Air-Conditioning Engineers (2021). ASHRAE Handbook: Fundamentals. ASHRAE, Peachtree Corners, GA.

Hyland, R. W., and Wexler, A. (1983). "Formulations for the Thermodynamic Properties of the Saturated Phases of H2O from 173.15 K to 473.15 K." ASHRAE Transactions, Vol. 89, No. 2A, pp. 500–519.

Worked Examples

Published examples this tool reproduces; each page lists the inputs to enter and the printed answers.

All worked examples

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