What is wet-bulb temperature?

Wet-bulb temperature (Tw) is the temperature a parcel of air would have if cooled adiabatically to saturation by evaporating water into it at constant pressure. It is always equal to or less than the dry-bulb temperature and represents the theoretical minimum air temperature achievable by evaporative cooling.

Which formula does this tool use?

This tool uses the Stull (2011) empirical polynomial: Tw = T * atan(0.151977 * sqrt(RH + 8.313659)) + atan(T + RH) - atan(RH - 1.676331) + 0.00391838 * RH^1.5 * atan(0.023101 * RH) - 4.686035. It is accurate to within 0.3 °C for temperatures of -20 °C to 50 °C and relative humidity of 5% to 100%.

Why is a wet-bulb temperature of 35 °C dangerous?

The human body cools itself by sweating. For evaporation to carry heat away, the surrounding air must be cooler and drier than the skin surface (~35 °C when sweating). At a wet-bulb temperature of 35 °C and above, evaporation cannot occur fast enough to prevent core temperature from rising, leading to hyperthermia and death within hours even for healthy resting adults.

What is the difference between wet-bulb and dew-point temperature?

Dew point is the temperature at which air must be cooled at constant pressure for condensation to begin. Wet-bulb temperature is the temperature reached by cooling air through evaporation and is always between the dew point and the dry-bulb temperature. Wet-bulb is used for heat-stress and cooling engineering; dew point is used for condensation and frost risk.

Can the wet-bulb temperature equal the dry-bulb temperature?

Yes — when relative humidity is 100%, no evaporation can occur, so wet-bulb temperature equals dry-bulb temperature. In practice this means the air is saturated with moisture (fog, rain, or heavy overcast conditions).

Wet-Bulb Temperature Calculator

Enter dry-bulb temperature and relative humidity to instantly calculate wet-bulb temperature with a heat-stress rating.

Enter Conditions

Temperature Unit

Dry-Bulb Temperature (°F)

Relative Humidity (%)

5% Dry 50% 100% Saturated

Enter temperature and humidity, then click
Calculate Wet-Bulb Temperature.

Wet-Bulb Temperature

-- °F

-- °C

Dry-Bulb

Humidity

--%

Depression

Wet-Bulb Heat-Stress Scale

Safe

Below 18 °C (64 °F)

Caution

18–24 °C (64–75 °F)

High Risk

24–28 °C (75–82 °F)

Extreme

28–35 °C (82–95 °F)

Potentially Fatal

Above 35 °C (95 °F)

Formula: Stull (2011) empirical polynomial, accurate to ±0.3 °C for RH 5–100% and T −20 to 50 °C.

Summary

The Wet-Bulb Temperature Calculator uses the Stull (2011) empirical formula to compute wet-bulb temperature from dry-bulb temperature and relative humidity. Wet-bulb temperature is the lowest temperature air can reach through evaporative cooling, making it critical for assessing heat stress, evaporative cooler performance, and cooling tower design. When wet-bulb temperature approaches body temperature (35 °C / 95 °F), the human body can no longer cool itself effectively — a life-threatening condition. Results are shown in both Celsius and Fahrenheit alongside a color-coded heat-stress rating.

How it works

Enter the dry-bulb (air) temperature in Fahrenheit or Celsius.
Enter the relative humidity as a percentage (5–100%).
The Stull (2011) polynomial approximation computes wet-bulb temperature in Celsius.
The result is converted to Fahrenheit and classified by heat-stress level.
A color-coded banner and scale show where conditions fall on the heat-stress spectrum.

Use cases

Assess outdoor heat stress risk for workers and athletes.
Size evaporative (swamp) coolers for maximum cooling potential.
Estimate achievable supply-air temperature from a direct evaporative cooler.
Evaluate cooling-tower performance and approach temperature.
Support military and emergency-management heat-casualty prevention planning.
Understand why high humidity reduces the body's ability to sweat and cool down.
Verify WBGT (Wet-Bulb Globe Temperature) component inputs.
Compare conditions across weather stations using a humidity-adjusted temperature.

Frequently Asked Questions

Last updated: 2026-06-13 · Reviewed by Nham Vu