Wet Bulb Calculator

Enter the air (dry-bulb) temperature and relative humidity to find the wet-bulb temperature using the Stull (2011) empirical formula.

Quick Facts

Formula
Stull (2011) empirical approximation
Matches the exact psychrometric wet-bulb solution to within about ±1°C, without iteration.
Valid range
-20°C to 50°C, 5% to 99% RH
Assumes standard sea-level pressure (1013.25 hPa / 101.325 kPa).
At 100% humidity
Tw = T (dry-bulb)
Saturated air cannot evaporate any more water, so no evaporative cooling occurs.
Survivability limit
~35°C (95°F) wet-bulb
Sherwood & Huber (2010): sustained exposure above this approaches the human heat-tolerance limit.

Your Results

Calculated
Wet-Bulb Temperature
-
Lowest temperature reachable by evaporative cooling
Wet-Bulb Depression
-
Dry-bulb minus wet-bulb temperature
Relative Humidity
-
As entered, for reference
Heat-Stress Guidance
-
Based on wet-bulb temperature level

Ready

Enter a temperature and relative humidity, then press Calculate.

About Wet-Bulb Temperature

Wet-bulb temperature (Tw) is the lowest temperature that can be reached by evaporating water into air at constant pressure — physically, it's the steady-state reading of a thermometer whose bulb is wrapped in a wet wick and exposed to moving air. It sits between the dew point and the dry-bulb (ordinary air) temperature: Tdew ≤ Tw ≤ Tdry. This calculator uses the Stull (2011) formula, an empirical fit that computes wet-bulb temperature directly from dry-bulb temperature and relative humidity without the iterative psychrometric calculations traditionally required.

Understanding the formula

With T as the dry-bulb temperature in °C and RH% as relative humidity expressed as a number from 0 to 100:

Tw = T·atan[0.151977·(RH% + 8.313659)^0.5] + atan(T + RH%) − atan(RH% − 1.676331) + 0.00391838·(RH%)^1.5·atan(0.023101·RH%) − 4.686035

Roland Stull published this regression in the Journal of Applied Meteorology and Climatology (2011) after fitting it against the full psychrometric equations. It reproduces the exact solution to within roughly ±1°C across dry-bulb temperatures from -20°C to 50°C and relative humidity from 5% to 99% at standard sea-level pressure (1013.25 hPa), which is accurate enough for weather, HVAC, agriculture, and heat-safety use without needing a lookup chart or iterative solver.

Reading the result

The wet-bulb depression (dry-bulb minus wet-bulb temperature) shows how much evaporative cooling is available: a large depression means dry air that can cool effectively through evaporation (useful for evaporative "swamp" coolers), while a depression near zero means the air is already close to saturated and evaporative cooling won't help much. When relative humidity reaches 100%, the wet-bulb and dry-bulb temperatures become equal because saturated air cannot absorb any more moisture. This calculator also flags a general heat-stress guidance level, since the human body cools itself primarily by sweat evaporation — a high wet-bulb temperature limits how effectively you can lose heat even in the shade with plenty of water, regardless of how hot the air itself feels.

Frequently Asked Questions

What is wet-bulb temperature?
Wet-bulb temperature is the lowest temperature air can reach through evaporative cooling alone, such as water evaporating from a wet cloth wrapped around a thermometer bulb. It depends on both the air temperature and the relative humidity — drier air lets more evaporation happen, so the wet-bulb temperature drops further below the dry-bulb (regular) air temperature.
What formula does this calculator use?
This calculator uses the Stull (2011) empirical formula, Tw = T·atan[0.151977(RH%+8.313659)^0.5] + atan(T+RH%) − atan(RH%−1.676331) + 0.00391838(RH%)^1.5·atan(0.023101·RH%) − 4.686035, where T is dry-bulb temperature in °C and RH% is relative humidity as a number from 0-100. It is accurate to within about ±1°C for temperatures from -20°C to 50°C and relative humidity from 5% to 99% at standard sea-level pressure, without needing an iterative psychrometric solution.
Why is wet-bulb temperature always lower than air temperature?
Evaporating water absorbs latent heat from its surroundings, cooling the wet surface below the surrounding air temperature. The only exception is when relative humidity is 100%, meaning the air is already saturated and no net evaporation — and therefore no evaporative cooling — can occur, so the wet-bulb and dry-bulb temperatures become equal.
Why does wet-bulb temperature matter for heat stress?
The human body relies on sweat evaporation to shed heat, so a high wet-bulb temperature limits how effectively you can cool down even in the shade with unlimited water. Research (Sherwood & Huber, 2010) indicates that sustained wet-bulb temperatures around 35°C (95°F) approach the theoretical limit of human heat tolerance, which is why wet-bulb temperature — not just air temperature — is used to judge outdoor heat-stress risk.

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