Free wet-bulb temperature calculator using Stull's formula. Gauge heat-stress risk and the 35°C human survivability limit from temperature and humidity.
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Wet-bulb temperature is the single truest measure of how dangerous heat and humidity are together. It is the lowest temperature air can reach through evaporation — the same cooling your sweat relies on. As the wet-bulb temperature climbs toward 35°C (95°F), sweat stops evaporating and the body can no longer shed heat, which is why it defines the limit of human heat survivability. Enter the air temperature and humidity to see the wet-bulb temperature and its heat-stress category.
Wet-bulb temperature is the reading of a thermometer wrapped in a wet cloth with air blowing over it. Because water evaporating from the cloth cools it, the wet-bulb temperature is always at or below the air (dry-bulb) temperature, and the gap between them — the wet-bulb depression — shrinks as humidity rises. It matters because humans cool themselves the same way: by evaporating sweat. Once the wet-bulb temperature reaches about 35°C (95°F), the air is too warm and moist for sweat to evaporate, so body temperature rises uncontrollably. This 35°C threshold is the theoretical limit of human survivability, even for a healthy person resting in the shade with water.
Stull's Formula (2011)
Trainers use wet-bulb readings to pause or cancel practice in dangerous humid heat.
Armed forces and contractors set work-rest cycles from it to protect people from heat illness.
Researchers map where and how often the 35°C survivability limit is approached.
Engineers use the wet-bulb temperature to size evaporative coolers and cooling towers.
See how close conditions are to the 35°C wet-bulb limit beyond which the body cannot cool itself.
Decide when outdoor labor, training, or games become unsafe in humid heat.
Track how often extreme humid-heat events approach the limits of human habitability.
Wet-bulb temperatures above about 26°C (79°F) start to stress people doing hard work. Above 30-32°C (86-90°F) is dangerous even with rest, and 35°C (95°F) is the theoretical survivability limit — beyond it the body cannot cool itself at all.
Skin sits near 35°C, so it can only shed heat to air that is cooler or drier. At a 35°C wet-bulb temperature the air is already saturated at skin temperature, sweat stops evaporating, and core temperature climbs until organs fail. Sherwood and Huber first proposed this limit in a 2010 PNAS study.
Wet-bulb temperature is the pure thermodynamic limit of evaporative cooling — the survivability measure. Heat index is a shade 'feels-like' temperature from heat and humidity. WBGT (wet-bulb globe temperature) adds direct sun and wind and is used for work-rest and sports guidelines. This tool computes the wet-bulb temperature itself.
Yes — weather stations near the Persian Gulf and in South Asia have briefly touched or exceeded 35°C wet-bulb since 2000. The events are short and local for now, but climate change is making them more frequent and longer.
This calculator uses Stull's 2011 formula, which estimates the wet-bulb temperature from air temperature and relative humidity at sea-level pressure to within about 0.3°C. Traditionally it is measured directly with a sling psychrometer — a thermometer with a water-soaked wick that is whirled through the air.