Psychrometric Calculator

Written by Great Medico

Updated September 25, 2026 •3 min read

Share Print Save

Air-property calculations are tightly coupled: temperature, moisture, pressure, and density can all move together. Psychrometric Calculator focuses that system on the page’s humidity ratio, so the number can be traced back to the exact atmospheric inputs rather than treated as a generic weather value.

What this calculator does

The Psychrometric Calculator brings together Altitude (Z), Atmospheric pressure (Pa), Saturated vapor pressure at dry bulb (Ps), Partial vapor pressure (Pv), Saturated vapor pressure at wet bulb (Pw) around the page’s humidity ratio. The formula section below identifies which values actually drive that result and which fields are supporting or derived quantities, so you can check the page without assuming every visible box is an independent input.

How to use it

The main fields on this page are Altitude (Z), Atmospheric pressure (Pa), Saturated vapor pressure at dry bulb (Ps), Partial vapor pressure (Pv), Saturated vapor pressure at wet bulb (Pw). Enter the quantities you actually know, keep their units consistent, and leave derived/output-style fields blank unless the formula explicitly allows solving in the opposite direction. Use dry-bulb temperature, atmospheric pressure, and relative humidity as the primary state inputs; the remaining psychrometric fields can serve as derived/support values. For a clean check of humidity ratio, change one driving quantity at a time and confirm that the result moves in the direction predicted by the equation.

How the calculation works

From dry-bulb temperature, atmospheric pressure, and relative humidity, the page estimates saturation vapor pressure Ps, partial vapor pressure Pv = RH·Ps, humidity ratio w = 0.62198Pv/(p−Pv), dew point, and moist-air enthalpy h ≈ 1.006T + w(2501 + 1.86T).

Example

For example, start with the page’s populated scenario: Altitude (Z) = 0; Atmospheric pressure (Pa) = 101.325; Dry bulb temperature (Tdb) = 25; Wet bulb temperature (Twb) = 20. Apply the equation above using the units shown on the page, then compare the calculated humidity ratio with the displayed result. As a second check, change one physical input while holding the others fixed and confirm that the direction of change makes sense for this formula.

How to interpret the result

Interpret humidity ratio as an estimate for the entered air state and the specific approximation described above. Compare it with the related supporting values, and remember that a numerically precise result can still be wrong if pressure type, temperature scale, or humidity basis is inconsistent.

Limitations and notes

Atmospheric formulas simplify a variable real environment. Sensor error, nonuniform air, altitude-dependent pressure, radiation, wind, and local microclimate can dominate small calculated differences. Recheck units and use measured/local data when an operational decision depends on the result. For Psychrometric Calculator, keep that general caution tied to the exact fields and equation shown here rather than carrying assumptions over from a different calculator.