Psychrometric Calculator

Dew point, GPP, vapor pressure from temperature and RH.

Run the calculator

Example

You enter

Temperature (°F) 75
Relative humidity (percent) 50

You get

Dew point 55.1 F
Grains per pound 64.5
Vapor pressure 14.79

Details, formula, and sources

Saturation vapor pressure es(T) = 6.112 × exp((17.62 × T) / (243.12 + T)) mb (August-Roche-Magnus). RH × es = e (actual vapor pressure). Dew point Td = (243.12 × ln(e/6.112)) / (17.62 - ln(e/6.112)). GPP = 7000 × (0.622 × e) / (P - e).

August-Roche-Magnus formulation by name; ASHRAE Handbook (Fundamentals) Chapter 1 (Psychrometrics) by name.

Psychrometric formulas free in published engineering texts; ASHRAE Handbook licensed.

Estimate. AHJ and licensed professional govern.

Field names used by the API: temperature_F, RH_percent, dew_point_F, GPP, vapor_pressure_hPa

Atmospheric pressure 29.92 in Hg (1013.2 mb sea level) unless user suppliesICAO Standard Atmosphere
Conversion 7000 gr/lb physical fact (1 lb water = 7000 grains)Avoirdupois weight system