Psychrometric Calculator
Dew point, GPP, vapor pressure from temperature and RH.
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
Checked against published worked examples from: ASHRAE Handbook (Fundamentals).
Details, formula, and sources
Formula or tableSaturation vapor pressure es(T) = 6.1094 × exp((17.625 × T) / (243.04 + T)) mb (August-Roche-Magnus, Alduchov-Eskridge 1996 coefficients). RH × es = e (actual vapor pressure). Dew point Td = (243.04 × ln(e/6.1094)) / (17.625 - ln(e/6.1094)). GPP = 7000 × (0.622 × e) / (P - e).
Source and editionAugust-Roche-Magnus formulation by name; ASHRAE Handbook (Fundamentals) Chapter 1 (Psychrometrics) by name.
Free accessPsychrometric formulas free in published engineering texts; ASHRAE Handbook licensed.
What governsEstimate. AHJ and licensed professional govern.
API field namestemperature_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