Estimate the wet-bulb temperature from just two numbers — the air temperature and the relative humidity — using the widely used Stull formula. The wet-bulb temperature tells you the lowest temperature air can reach by evaporating water into it, and it is a key measure of how dangerous heat and humidity are for the human body.
The wet bulb calculator works out the wet-bulb temperature of the air from two simple readings: the air temperature and the relative humidity. The wet-bulb temperature is one of the most important — and most misunderstood — numbers in atmospheric physics and heat safety. It represents the lowest temperature to which air can be cooled by evaporating water into it, and it sets a hard limit on how well the human body can cool itself by sweating. As heatwaves become more frequent and severe, understanding wet-bulb temperature is increasingly a matter of life and death, not just meteorological curiosity.
This page explains what the wet-bulb temperature is, the formula the calculator uses, worked examples you can reproduce, why a wet-bulb temperature of 35 °C is considered the limit of human survival, and answers to the questions people ask most. Whether you are a student, an athlete, a worker in a hot environment, or simply someone trying to stay safe in summer, this tool gives you a fast, reliable estimate.
The wet-bulb temperature is the temperature a thermometer would show if its bulb were wrapped in a wet cloth and exposed to moving air. As water evaporates from the cloth, it absorbs heat and cools the thermometer — exactly the way sweat cools your skin. The drier the air, the more water can evaporate and the lower the wet-bulb temperature falls below the actual air temperature. When the air is completely saturated (100% relative humidity), no more water can evaporate, so the wet-bulb temperature equals the air temperature. The wet-bulb temperature is therefore always lower than or equal to the ordinary "dry-bulb" air temperature, and the gap between them tells you how dry the air is.
Calculating the exact wet-bulb temperature involves complex thermodynamics, but in 2011 Roland Stull published a simple, accurate empirical formula that needs only temperature and relative humidity. This calculator uses that formula:
Tw = T · atan[0.151977 · (RH + 8.313659)1/2] + atan(T + RH) − atan(RH − 1.676331) + 0.00391838 · RH3/2 · atan(0.023101 · RH) − 4.686035
where Tw is the wet-bulb temperature in degrees Celsius, T is the air temperature in degrees Celsius, RH is the relative humidity as a percentage, and the arctangent (atan) functions are evaluated in radians. The formula is accurate to within about −1 °C to +0.65 °C for relative humidities between 5% and 99% and air temperatures between −20 °C and 50 °C, which covers virtually every real-world weather situation. The calculator handles the conversion between Celsius, Fahrenheit, and Kelvin for you, always computing internally in Celsius as the formula requires.
Suppose the air temperature is 25 °C and the relative humidity is 40%. Plugging these into the Stull formula gives a wet-bulb temperature of about 16.384 °C. In other words, on a warm, moderately dry day, a wet cloth (or sweaty skin) can cool to roughly 16 °C — about 9 degrees below the air temperature — because the relatively dry air readily absorbs evaporating moisture. These are the calculator's default inputs, so you can press Calculate to confirm the result and then try your own conditions. Raise the humidity toward 100% and you will see the wet-bulb temperature climb toward the air temperature, because the muggier the air, the less cooling evaporation can provide.
The human body maintains a core temperature of about 37 °C, and it sheds excess heat mainly by sweating: as sweat evaporates from the skin, it carries heat away. But evaporation only works if the surrounding air can accept more moisture — and the wet-bulb temperature is precisely the measure of that capacity. Once the wet-bulb temperature reaches about 35 °C, the air is so warm and humid that sweat can no longer evaporate fast enough to cool the body. At that point even a fit, healthy person resting in the shade with unlimited water will overheat, and prolonged exposure becomes fatal within a few hours. This 35 °C threshold is a theoretical ceiling; in practice, dangerous conditions begin well below it. Wet-bulb temperatures above about 32 °C are extremely hazardous for anyone doing physical work, and vulnerable people — the elderly, the very young, and those with health conditions — can suffer at even lower values.
Several numbers describe how hot and humid conditions feel, and it helps to keep them straight. The heat index (or "feels-like" temperature) estimates how hot the air feels to a person and is usually higher than the actual temperature. The dew point is the temperature to which air must be cooled for water to start condensing; it is a direct measure of how much moisture the air holds. The wet-bulb temperature sits between the dry-bulb (actual) temperature and the dew point, and uniquely captures the cooling power available through evaporation. While the heat index tells you about comfort, the wet-bulb temperature tells you about survival: it is the figure scientists and physiologists use to judge when heat and humidity together become physically unbearable. All three depend on temperature and humidity, but each answers a different question.
The wet-bulb temperature is used far more widely than most people realize. Meteorologists use it in forecasting, in identifying the potential for severe weather, and in estimating how precipitation will fall — a wet-bulb temperature near or below freezing can turn rain into snow or ice. Engineers rely on it to design and rate cooling towers, evaporative ("swamp") coolers, and air-conditioning systems, because the wet-bulb temperature sets the coldest temperature evaporative cooling can achieve. In occupational health and sport, a related measure called the Wet-Bulb Globe Temperature (WBGT) combines wet-bulb temperature with radiant heat and air temperature to set safe limits for military training, athletics, and outdoor labor. Farmers and livestock managers watch it to protect animals from heat stress. In every case, the wet-bulb temperature provides information that the ordinary thermometer reading alone cannot.
You may encounter the Wet-Bulb Globe Temperature (WBGT), which is related to but distinct from the plain wet-bulb temperature this calculator estimates. WBGT is a composite index that blends three readings: the natural wet-bulb temperature (which reflects humidity and wind), the globe temperature (which reflects radiant heat from the sun), and the ordinary dry-bulb air temperature. Outdoors in sunlight it is typically calculated as 0.7 times the wet-bulb temperature plus 0.2 times the globe temperature plus 0.1 times the dry-bulb temperature. Because it accounts for sunshine and wind as well as heat and humidity, WBGT is the standard used by the military, sports bodies, and workplace-safety regulators to decide when activity should be limited or stopped. The plain wet-bulb temperature from this calculator is the single most important ingredient in that index, and on its own it is an excellent first indicator of heat danger.
When wet-bulb temperatures climb, ordinary heat-safety advice becomes critical. Seek air-conditioned or well-ventilated spaces, because in very humid heat a fan alone may not help — if the air is near skin temperature, moving it around does little to cool you. Stay hydrated, but understand that water alone cannot protect you once evaporation fails. Avoid strenuous activity during the hottest, most humid part of the day, wear light and loose clothing, and check on vulnerable neighbors and relatives. Recognize the early warning signs of heat illness — dizziness, nausea, a rapid pulse, confusion, and a failure to sweat — and treat them as emergencies. Because dangerous wet-bulb temperatures can occur even when the ordinary thermometer reading does not look extreme, using a wet-bulb calculator to check the real risk is a simple but potentially life-saving habit during humid heatwaves.
The single biggest factor separating the wet-bulb temperature from the ordinary thermometer reading is humidity, and it is worth building an intuition for how the two relate. On a hot but dry day — say 35 °C with 20% humidity — the wet-bulb temperature is far lower than the air temperature, around 21 °C, because the parched air greedily absorbs evaporating sweat and cools the body efficiently; this is why desert heat, though intense, is survivable. Take that same 35 °C and raise the humidity to 70%, and the wet-bulb temperature jumps to about 30 °C, dangerously close to the limit at which sweating fails. Push the humidity higher still and the wet-bulb temperature keeps climbing toward the air temperature. This is the crucial insight the calculator makes visible: two days with identical thermometer readings can pose completely different levels of danger depending on the moisture in the air. It also explains why humid regions near coasts and tropics are far more vulnerable to deadly heat than dry inland areas at the same temperature.
For a long time, scientists believed the human survivability threshold of a 35 °C wet-bulb temperature was almost never reached on Earth. That reassurance is fading. Analyses of weather-station data have found that some locations around the Persian Gulf, in South Asia, and along other hot, humid coastlines have already briefly touched or approached this limit, decades earlier than climate models once predicted. Even wet-bulb temperatures in the low 30s, which occur far more often, are enough to kill during prolonged exposure, especially among vulnerable populations without access to cooling. As global temperatures rise, the frequency and geographic spread of dangerous wet-bulb conditions are expected to grow, turning what was once a theoretical ceiling into a real and increasing public-health threat. Tracking wet-bulb temperature, rather than air temperature alone, is becoming an essential tool for understanding the true danger of modern heatwaves.
You do not need sophisticated equipment to measure a wet-bulb temperature directly. The classic instrument is a sling psychrometer, which holds two thermometers side by side — one with a dry bulb and one whose bulb is wrapped in a wet wick. Whirling the device through the air for a minute or two maximizes evaporation from the wet wick, and once its reading stops falling, that is the wet-bulb temperature; the dry thermometer gives the ordinary air temperature. The difference between the two, called the wet-bulb depression, can then be used with a chart to read off the relative humidity — the reverse of what this calculator does. Modern electronic weather stations compute the same values from humidity sensors. Whether you measure it with a simple wet wick or estimate it from temperature and humidity here, the wet-bulb temperature remains one of the most practical and revealing measurements in all of meteorology.
It is the lowest temperature air can reach by evaporating water into it — the reading a wet-cloth-wrapped thermometer would show. It reflects how effectively sweat can cool the body.
This calculator uses the Stull formula, which estimates the wet-bulb temperature from air temperature and relative humidity. For 25 °C and 40% humidity, the result is about 16.384 °C.
At a wet-bulb temperature of about 35 °C, sweat can no longer evaporate fast enough to cool the body, so even healthy people resting in shade with water can fatally overheat.
Yes, except at 100% relative humidity, where the two are equal. The drier the air, the further the wet-bulb temperature falls below the air temperature.
Both depend on humidity, but the dew point is where water starts to condense, while the wet-bulb temperature is the cooling limit from evaporation. The wet-bulb value lies between the air temperature and the dew point.
It is accurate to within about −1 °C to +0.65 °C for relative humidity between 5% and 99% and temperatures from −20 °C to 50 °C, which covers almost all real weather.
This Wet Bulb Calculator is provided for general informational and educational purposes and uses the empirical Stull approximation. It is not a substitute for official weather data or professional heat-safety guidance. In extreme heat, follow the advice of local authorities and health professionals.