Wet Bulb / Dry Bulb / RH Calculator

Solve an air state from dry bulb plus RH, wet bulb, or dew point.

Inputs

DB + RH
DB + Wet Bulb
DB + Dew Point
Inputs, results, and method — as an Excel workbook or plain CSV.

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About This Calculator

This wet bulb / dry bulb / RH calculator solves the complete psychrometric state from any two known conditions. Choose dry bulb plus relative humidity, dry bulb plus wet bulb, or dry bulb plus dew point, add your site elevation, and it returns the other temperatures along with humidity ratio (in grains per pound) and moist-air enthalpy.

It is handy for converting field readings, sizing evaporative and cooling-tower equipment by wet bulb, checking economizer changeover, and confirming the moisture content behind a coil. Because two independent properties fix the air state, the tool fills in whichever value you do not have.

Formula & Method

Saturation pressurePws: Hyland–Wexler equations (over ice below 32 °F, over liquid water above)
Barometric pressureP = 14.696 · (1 − 6.8754×10−6 · Z)5.2559 psia, Z = elevation (ft)
Humidity ratioW = 0.621945 · Pw ÷ (P − Pw), Pw = RH · Pws
Dew pointPws(Tdp) = Pw, solved numerically (frost point below 32 °F)
Wet bulbW = [(1093 − 0.556·Twb)·Ws,wb − 0.240·(T − Twb)] ÷ (1093 + 0.444·T − Twb) (ice form below 32 °F)
Enthalpyh = 0.240 · T + W · (1061 + 0.444 · T)

All relations are the ASHRAE Handbook—Fundamentals (2021) Chapter 1 moist-air equations in IP units (T in °F, P in psia) — the same set as the AIM Works psychrometric calculator, so one air state reads the same on every page. Barometric pressure comes from your elevation through the standard atmosphere. Whichever pair you enter, the tool finds the humidity ratio first, then solves the saturation curve for dew point and the wet-bulb equation for wet bulb. Wet bulb here is the thermodynamic (adiabatic-saturation) wet bulb.

Frequently Asked Questions

What is wet-bulb temperature?
Wet-bulb temperature is what a thermometer reads when its bulb is wrapped in a wet wick and air moves across it, so evaporation cools the bulb. It always falls between dew point and dry bulb. The gap between dry bulb and wet bulb, called the wet-bulb depression, indicates how dry the air is: a large depression means low humidity, and zero depression means the air is saturated.
Which two inputs do I need?
Pick any one of the three input modes: dry bulb plus relative humidity, dry bulb plus wet bulb, or dry bulb plus dew point. Two independent properties fully fix the moist-air state, so the calculator then solves for every remaining value, including the ones you did not enter.
How is humidity ratio found from wet bulb?
In the dry-bulb plus wet-bulb mode the tool uses the ASHRAE adiabatic-saturation energy balance from Handbook—Fundamentals (2021) Chapter 1: W = [(1093 − 0.556·Twb)·Ws,wb − 0.240·(T − Twb)] ÷ (1093 + 0.444·T − Twb), where Ws,wb is the saturation humidity ratio at the wet bulb and barometric pressure for your elevation (an ice form applies below 32 °F). This is the thermodynamic wet bulb, which a well-ventilated sling psychrometer reads closely. In the other two modes the same equation is solved backward for the wet bulb.
Why does my space sensor read RH but cooling towers are rated at wet bulb?
Space sensors report relative humidity because that is what occupants and humidistats care about. Evaporative equipment such as cooling towers and swamp coolers is limited by wet-bulb temperature, since wet bulb is the lowest temperature evaporation can reach. This calculator converts freely between the two so you can use whichever your data source provides.

Related Tools

Psychrometric Calculator Dew Point Calculator Humidity Ratio Enthalpy Calculator Coil Leaving Air
AIM Works · launches Oct 15, 2026

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How the calculations work →

Results are design estimates for preliminary sizing. Verify final designs against applicable codes and standards — engineering judgment and a licensed professional engineer’s review are required.