Wet Bulb / Dry Bulb / RH Calculator

Solve for any psychrometric condition. Enter two known values — dry bulb + RH, dry bulb + wet bulb, or dry bulb + dew point — and get all the rest.

🌡️ Inputs

DB + RH
DB + Wet Bulb
DB + Dew Point
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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, 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 = 0.6112 · exp(17.67 · T ÷ (T + 243.5)) kPa (T in °C)
Humidity ratioW = 0.622 · Pw ÷ (P − Pw)
Dew pointMagnus inverse of RH and T (a = 17.67, b = 243.5)
Wet bulb (DB+WB)Pw = Pws,wb − 0.000660 · (1 + 0.00115·Twb) · (Tdb − Twb) · P
Enthalpyh = 0.240 · T + W · (1061 + 0.444 · T)

Saturation vapor pressure Pws uses a Magnus-type formula, and humidity ratio follows W = 0.622 · Pw ÷ (P − Pw) at standard sea-level pressure (P ≈ 101.325 kPa, 14.696 psia). In the dry-bulb plus wet-bulb mode, vapor pressure comes from the psychrometer equation; in the other modes wet bulb is estimated from the dry-bulb-to-dew-point depression. Dew point is the Magnus inverse, and enthalpy uses the standard moist-air relation. These follow ASHRAE Fundamentals Chapter 1 psychrometrics.

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 psychrometer equation, computing vapor pressure as the saturation pressure at the wet bulb minus a psychrometric-constant term proportional to the wet-bulb depression and barometric pressure, then humidity ratio from W = 0.622 times Pw divided by (P minus Pw). This follows ASHRAE Fundamentals Chapter 1 psychrometrics.
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
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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.