How this calculation works
Relative humidity compares vapor pressure with saturation at the current temperature. Humidity ratio instead expresses water mass per mass of dry air. Heating air without adding water can change RH while leaving humidity ratio essentially unchanged at constant composition.
pv = RH × psaturation; w = 0.621945pv/(P − pv); h = 1.006T + w(2501 + 1.86T)A worked example
At 25 °C, fifty percent RH and 101.325 kPa, vapor pressure is about half the liquid-water saturation pressure. The humidity ratio is approximately 9.86 grams per kilogram of dry air. Keeping temperature and RH fixed while lowering absolute pressure raises humidity ratio. Keeping pressure and water ratio fixed while changing temperature is a different comparison; it would generally require a corresponding RH change.
Good to know
Ideal-gas moist-air relations with approximate saturation pressure; no full ASHRAE solver, comfort score or equipment rating. The formula does not account for droplets, fog loading or high-temperature steam mixtures. Use a property method suitable for conditions outside the entered range.
Reference: ASHRAE — psychrometric definitions and ideal-gas relations (opens in a new tab) ↗
What the model answers
Relative humidity compares vapor pressure with saturation at the current temperature. Humidity ratio instead expresses water mass per mass of dry air. Heating air without adding water can change RH while leaving humidity ratio essentially unchanged at constant composition.
This page calculates several related ideal moist-air properties on a dry-air basis. The choice of basis matters when comparing a volume flow, an enthalpy difference or a moisture-removal calculation.
The Cooling coil condensate and air load calculator compares two entered moist-air states. It uses humidity-ratio and enthalpy differences, rather than treating an RH difference as a water mass.
Measure and enter consistently
Enter total absolute pressure, not gauge pressure. Altitude can change this input enough to alter humidity ratio and specific volume, so a standard sea-level value is an assumption rather than a location lookup.
RH is a percentage between zero and one hundred. Saturation pressure uses a liquid-water Magnus approximation in the supported temperature range. It is not the complete set of equations used by every professional psychrometric chart or property package.
Interpret the result
Humidity ratio is displayed in grams per kilogram of dry air, while the enthalpy equation internally uses kilograms per kilogram. Specific volume likewise refers to one kilogram of dry air and includes the water vapor occupying the mixture.
Enthalpy uses a conventional zero-degree reference. Its absolute numeric value is meaningful within that convention; differences between consistently calculated states are the quantities used in many air-conditioning balances.
Work through the numbers
At 25 °C, fifty percent RH and 101.325 kPa, vapor pressure is about half the liquid-water saturation pressure. The humidity ratio is approximately 9.86 grams per kilogram of dry air.
Keeping temperature and RH fixed while lowering absolute pressure raises humidity ratio. Keeping pressure and water ratio fixed while changing temperature is a different comparison; it would generally require a corresponding RH change.
Where this model stops
Ideal-gas moist-air relations with approximate saturation pressure; no full ASHRAE solver, comfort score or equipment rating.
The formula does not account for droplets, fog loading or high-temperature steam mixtures. Use a property method suitable for conditions outside the entered range.
A few questions, answered
Why is the denominator dry air?
The water-to-dry-air basis makes water and energy balances consistent when moisture is added or removed.
Does 50 percent RH mean half the air is water?
No. It means vapor pressure is half the saturation pressure at that temperature, not a mass or volume fraction.