Wet Bulb Temperature Calculator
Enter the air (dry-bulb) temperature and relative humidity to calculate the wet-bulb temperature using the Stull (2011) formula. The calculator also outputs the dew point, indoor and outdoor Wet Bulb Globe Temperature (WBGT), and the corresponding heat stress category. Switch between Celsius and Fahrenheit at any time.
What is wet-bulb temperature?
Wet-bulb temperature is the lowest temperature a surface can reach by evaporating water into the surrounding air at constant pressure. It is measured with a thermometer whose sensing bulb is wrapped in a water-soaked wick, mimicking the cooling effect of sweating. Because evaporation slows as humidity rises, the wet-bulb reading climbs toward the dry-bulb reading at 100% relative humidity. The gap between the two is called the wet-bulb depression, and it directly reflects how effectively the human body can shed heat through perspiration. A wet-bulb temperature above 35 °C (95 °F) exceeds the body's ability to cool itself through sweating and is considered unsurvivable for prolonged unshielded outdoor exposure, regardless of fitness level.
Stull formula and how this calculator works
This calculator uses the Stull (2011) polynomial approximation, which gives an error of less than 1 °C across the valid range of -20 °C to 50 °C and 5% to 99% relative humidity. The formula requires only dry-bulb temperature and relative humidity, making it practical for weather station data and smartphone apps. The dew point is calculated with the Magnus formula, which relates vapor pressure to temperature and is accurate to within 0.35 °C for everyday atmospheric conditions. For the Wet Bulb Globe Temperature (WBGT), the calculator applies the ISO 7243 indoor formula (0.7 x wet-bulb + 0.3 x dry-bulb) and, when a globe temperature is provided, the outdoor formula (0.7 x wet-bulb + 0.2 x globe + 0.1 x dry-bulb). The globe temperature accounts for radiant heat from the sun and surroundings, making outdoor WBGT more realistic for workers and athletes in direct sunlight.
Wet-bulb temperature, heat stress, and safety
Heat stress guidelines from OSHA, NIOSH, the American College of Sports Medicine (ACSM), and ISO 7243 use WBGT or wet-bulb temperature to set work-rest schedules and activity limits. At a wet-bulb temperature of 18-22 °C, strenuous activity is generally safe for acclimatized adults with normal hydration. From 23-27 °C, work intensity should be reduced and water breaks taken every 15-20 minutes. Above 28 °C, mandatory rest breaks are required, and outdoor sports events are typically cancelled or postponed. Above 32-35 °C, all vigorous activity should stop. Vulnerable groups including the elderly, children, pregnant women, and people with cardiovascular disease face significant risks at lower thresholds. Humidity is as important as temperature: a dry 38 °C day may feel survivable, while a humid 32 °C day with a wet-bulb above 28 °C can be as dangerous.
Dew point vs wet-bulb temperature
The dew point is the temperature at which the air becomes saturated and water vapor begins to condense. It is a measure of the absolute amount of moisture in the air and does not change with temperature unless moisture is added or removed. Wet-bulb temperature lies between the dew point and the dry-bulb temperature: when the air is fully saturated the three values are equal, and as the air dries out the wet-bulb and dew point diverge below the dry-bulb reading. Dew point above 16 °C (60 °F) starts to feel uncomfortable to most people; above 21 °C (70 °F) is considered oppressive. Wet-bulb temperature is more directly relevant to heat stress because it reflects how well the body can cool itself in those exact conditions.
Wet-bulb temperature and heat stress levels (ISO 7243 / ACSM)
| Wet-bulb (°C) | Wet-bulb (°F) | Heat stress level | Risk and guidance |
|---|---|---|---|
| Below 18 | Below 64 | Low | Comfortable for all activity levels |
| 18-22 | 64-72 | Moderate | Comfortable at rest; limit strenuous outdoor work |
| 23-27 | 73-81 | High | Reduce work intensity; hydrate every 15-20 min |
| 28-31 | 82-88 | Very High | High heat illness risk; mandatory rest breaks |
| 32-35 | 90-95 | Extreme | Stop vigorous activity; heat stroke risk is severe |
| Above 35 | Above 95 | Unsurvivable (prolonged) | Exceeds human thermoregulatory capacity |
Categories based on indoor WBGT (approximated by wet-bulb temperature when globe temperature is not available). Thresholds apply to moderately active adults.
Frequently asked questions
What is a dangerous wet-bulb temperature for humans?
Research and field studies indicate that a sustained wet-bulb temperature of 35 °C (95 °F) is the theoretical upper limit for human survival without air conditioning, because sweating can no longer cool the body below core temperature. In practice, heat stroke and death can occur at lower wet-bulb readings (around 28-32 °C) for people engaged in strenuous activity, the elderly, or those with health conditions. OSHA and NIOSH recommend stopping vigorous outdoor work when WBGT exceeds 28 °C (82 °F) for unacclimatized workers.
How is wet-bulb temperature different from heat index?
Heat index (also called apparent temperature or "feels like") is an empirical scale designed to describe how hot it feels to a person standing in the shade with a light breeze. Wet-bulb temperature is a physical measurement tied to evaporative cooling capacity and thermodynamic principles. Wet-bulb temperature is more useful for assessing physiological heat strain and is the basis for most occupational health standards (ISO 7243, NIOSH, ACSM). Heat index tends to understate risk at very high humidity and overstate it in breezy conditions.
What relative humidity range does the Stull formula work for?
The Stull (2011) polynomial is accurate within about 1 °C for relative humidity between 5% and 99% and dry-bulb temperatures between -20 °C and 50 °C. Outside these ranges, the approximation can introduce larger errors. At 100% humidity the wet-bulb equals the dry-bulb by definition, so the formula is not needed at that extreme.
What is WBGT and how is it used?
Wet Bulb Globe Temperature (WBGT) was developed by the US military in the 1950s to set safe training conditions. It combines the wet-bulb temperature, the dry-bulb temperature, and the globe temperature (radiant heat) into one index. The indoor formula is 0.7 x wet-bulb + 0.3 x dry-bulb; the outdoor formula adds a 20% globe component. Sports federations, armies, and occupational health bodies use WBGT thresholds to decide when to cancel events or mandate rest breaks. ISO 7243 sets limits based on WBGT and metabolic work rate.
Does wind affect wet-bulb temperature?
Wind speeds up the evaporation of water from the wick of a sling or aspirated psychrometer, but the Stull formula used in this calculator assumes standard still-air conditions. In very calm air a standard mercury psychrometer may read slightly high because evaporation is restricted. For field measurements, a sling psychrometer (whirled through the air) or an aspirated psychrometer with a fan gives more accurate readings. Wind does reduce heat strain on the human body by increasing convective and evaporative cooling, but this benefit decreases as air temperature rises above skin temperature.
Can I use this calculator for altitude corrections?
The Stull formula does not require atmospheric pressure as an input, so no altitude correction is needed for the wet-bulb estimate itself. However, at high altitudes the lower air pressure reduces the boiling point of water and slightly changes evaporative dynamics. For most practical purposes (below about 3000 m), the error introduced is small compared to the formula's own uncertainty. If you need precise psychrometric calculations at high altitude, use a full psychrometric chart corrected for the local atmospheric pressure.