Wet Bulb Calculator
Calculate wet-bulb temperature from air temperature and relative humidity using the Stull formula. Free online wet-bulb calculator with dew point, WBGT, charts, and detailed breakdowns.
About This Calculator
The Wet Bulb Calculator helps you estimate the wet-bulb temperature — the lowest temperature air can reach through evaporative cooling — using only two inputs: air temperature and relative humidity. This tool is essential for meteorologists, HVAC engineers, athletes, industrial hygienists, and anyone concerned with heat stress and human thermal comfort. Simply enter your temperature and humidity values to get instant wet-bulb temperature results.
This calculator uses the Stull formula, a widely validated approximation accurate for temperatures between -20°C and 50°C and relative humidities between 5% and 99%. The formula derives the wet-bulb temperature from the dry-bulb temperature and relative humidity through a series of arctangent and square-root terms that model the thermodynamic relationship between air temperature, moisture content, and evaporative cooling. Results include wet-bulb temperature in both Celsius and Fahrenheit, dew point temperature, and Wet-Bulb Globe Temperature (WBGT) for both outdoor and indoor conditions. The breakdown table shows intermediate values, and the temperature comparison chart visualizes how the wet-bulb temperature relates to the dry-bulb temperature, dew point, and WBGT.
About Wet-Bulb Temperature and Heat Safety
Wet-bulb temperature is a critical measure for human safety in hot environments. Unlike air temperature alone, wet-bulb temperature accounts for humidity, which directly affects the body's ability to cool itself through sweating. A wet-bulb temperature of 35°C (95°F) is considered the survivability limit for healthy humans — beyond this point, the body cannot dissipate heat regardless of hydration or access to shade. The Wet-Bulb Globe Temperature (WBGT) extends this concept by also incorporating solar radiation effects. In the United States, the military and OSHA use WBGT thresholds to recommend work-rest cycles and water intake. In India and the UK, heat action plans increasingly reference wet-bulb temperature for issuing heat wave warnings and protecting vulnerable populations during extreme heat events.
Frequently Asked Questions
What is wet-bulb temperature?
Wet-bulb temperature is the lowest temperature air can reach through evaporative cooling alone. It is measured by a thermometer wrapped in a water-soaked cloth and ventilated. When humidity is 100%, the wet-bulb temperature equals the air temperature. As humidity decreases, the wet-bulb temperature falls below the air temperature.
How is wet-bulb temperature calculated?
This calculator uses the Stull formula: Tw = T × atan(0.151977 × sqrt(RH + 8.313659)) + atan(T + RH) - atan(RH - 1.676331) + 0.00391838 × RH^(3/2) × atan(0.023101 × RH) - 4.686035. It is accurate for temperatures between -20°C and 50°C and relative humidity between 5% and 99% with an error range of -1°C to +0.65°C.
Why is wet-bulb temperature important for human health?
Wet-bulb temperature measures how effectively sweat can evaporate to cool the body. When wet-bulb temperature exceeds 35°C (95°F) for an extended period, humans cannot cool down through sweating, leading to hyperthermia and potentially fatal heat stroke regardless of hydration. This threshold is critical for heat safety guidelines worldwide.
What is the difference between wet-bulb and dew point?
Wet-bulb temperature is the lowest temperature achievable through evaporative cooling, which depends on both temperature and humidity. Dew point is the temperature at which air becomes saturated with water vapor and condensation begins. The dew point is always lower than or equal to the wet-bulb temperature, which is always lower than or equal to the dry-bulb (air) temperature.
What is Wet-Bulb Globe Temperature (WBGT)?
Wet-Bulb Globe Temperature (WBGT) is a heat stress index used by the military, athletics, and industrial hygienists. It combines wet-bulb temperature, black-globe temperature, and air temperature. Outdoors with sunlight: WBGT = 0.7 × Tw + 0.2 × Tg + 0.1 × T. Indoors or shade: WBGT = 0.7 × Tw + 0.3 × T. It provides vital guidance for physical activity safety on hot days.
What are practical applications of wet-bulb temperature?
Wet-bulb temperature has several practical uses: construction (material curing depends on humidity and temperature), snowmaking (snow production requires low wet-bulb temperatures), meteorology (forecasting rain, snow, or freezing rain), aviation (engine performance calculations), and agriculture (crop water stress and irrigation scheduling).
What is a dangerous wet-bulb temperature for humans?
A wet-bulb temperature of 35°C (95°F) is considered the theoretical upper limit for human survivability. At this level, the body cannot dissipate heat through sweating. In practice, heat stress becomes severe above 30°C (86°F) wet-bulb, where physical labor becomes dangerous. These thresholds apply globally regardless of geographic location.
Is wet-bulb the same as heat index?
No. Both Wet-Bulb Globe Temperature (WBGT) and Heat Index measure heat stress, but they differ: WBGT is measured in direct sunlight and accounts for temperature, humidity, wind speed, and solar radiation. Heat Index is calculated for shady conditions with only temperature and humidity. WBGT is the standard used by the US military and occupational safety organizations for heat stress guidelines.