Wet-Bulb Temperature Calculator
Enter air temperature and relative humidity to get the wet-bulb temperature instantly via the Stull (2011) empirical formula — or work backwards from temperature and wet-bulb to find relative humidity. With heat-stress zones, the 35 °C human survivability limit, and WBGT context.
Scope boundary — read this first
- This tool = a fast, single-purpose wet-bulb calculator using the closed-form Stull (2011) empirical approximation, plus heat-stress context (stress zones, the 35 °C survivability limit, WBGT introduction) and a reverse solver for relative humidity.
- Not this tool = full moist-air psychrometrics. That lives in our Psychrometric Calculator, which computes wet-bulb by iterating the ASHRAE adiabatic-saturation equations and additionally returns dew point, humidity ratio, enthalpy, specific volume, vapor pressure, air density and absolute humidity, with a pressure input for altitude.
- Rule of thumb: quick wet-bulb answer or heat-stress check → stay here. Engineering-grade moist-air numbers or anything above sea level → use the psychrometric calculator.
Calculator
Wet-bulb and heat stress: what the number means
Wet-bulb temperature is the best single number for how hard it is for a human body to cool itself: sweat cools only when it evaporates, and evaporation collapses as air approaches saturation. The table below gives rough population-average bands (not medical advice — children, the elderly and people with health conditions are affected at lower values).
| Wet-bulb | Band | What it feels like |
|---|---|---|
| < 21 °C | Comfortable | Typical indoor conditions; most people feel fine. |
| 21–25 °C | Muggy | Feels sticky; exercise starts to strain. |
| 25–28 °C | Moderate heat stress | Prolonged exertion becomes risky; hydrate and rest. |
| 28–32 °C | High / dangerous | Outdoor work and sport are dangerous for hours; heat illness risk climbs steeply. |
| 32–35 °C | Extreme | Even resting in the shade is hard; survival requires active cooling. |
| ≥ 35 °C | Beyond human survivability | Sherwood & Huber (2010): the body cannot shed metabolic heat even at rest in shade with unlimited water. A few hours can be fatal. |
Wet-bulb is not WBGT
Occupational standards use the wet-bulb globe temperature (WBGT), which adds radiant heat and air movement:
So a wet-bulb reading alone never justifies a safety decision. Real heat-stress management needs a calibrated WBGT meter at the worksite plus work–rest tables from OSHA or ACGIH. This tool is a screening aid.
The Stull (2011) formula
A closed-form empirical fit that skips the iterative psychrometric equations — average error below about 0.3 °C inside its calibrated range:
Worked example
T = 35 °C, RH = 70 % (a humid-extreme case)
Wet-bulb depression: 35 − 30.2534 = 4.7466 °C — very little evaporative cooling left. This falls in the high / dangerous band (28–32 °C).
Frequently asked questions
What is wet-bulb temperature?
Wet-bulb temperature is the lowest temperature air can reach through evaporative cooling alone. It is what a thermometer would read if its bulb were wrapped in a wet wick and exposed to moving air. It combines the effect of air temperature and humidity: dry air lets water evaporate freely so the wet-bulb reading sits far below the air temperature, while saturated air evaporates almost nothing, so wet-bulb nearly equals the dry-bulb temperature.
What formula does this calculator use?
This calculator uses the Stull (2011) empirical approximation: 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, with T in degrees Celsius, RH in percent and atan in radians. It is valid for -20 C to 50 C and 5% to 99% relative humidity. Inputs outside that range trigger a warning because the fit was not calibrated there.
How accurate is the Stull approximation?
Stull reports an average error below about 0.3 C against iterative psychrometric calculation over its calibrated range, which is fine for heat-stress screening, comfort checks and education. If you need higher precision, or extra properties like enthalpy, dew point and humidity ratio, use the full ASHRAE iterative psychrometric calculator instead, which this page links below.
How do I find relative humidity from temperature and wet-bulb?
Switch to the reverse mode (T + Tw to RH). Wet-bulb rises monotonically as relative humidity rises at a fixed temperature, so the calculator solves for RH with a numerical bisection that inverts the Stull formula. If the wet-bulb you enter is higher than the air temperature or otherwise unreachable at the given temperature, the tool reports that no solution exists.
What is the 35 C wet-bulb survivability limit?
Research by Sherwood and Huber (2010) argues that above a wet-bulb temperature of about 35 C, even a healthy, acclimatized person resting in the shade with unlimited water cannot shed metabolic heat to the environment, because sweat no longer cools the skin enough. A few hours of exposure can be fatal. Such events were once thought centuries away but have already been recorded in parts of South Asia and the Persian Gulf.
Is wet-bulb temperature the same as WBGT?
No. Wet-bulb is one input to the wet-bulb globe temperature (WBGT), the metric occupational standards actually use. Outdoors in sun: WBGT = 0.7*Twb + 0.2*Tg + 0.1*Tdb, where Tg is the black-globe temperature. Indoors or in shade without solar load: WBGT = 0.7*Twb + 0.3*Tdb. Wet-bulb alone does not account for radiant heat or wind, so for workplace safety follow WBGT-based guidelines from ACGIH or OSHA, not this screening tool.
What wet-bulb values are dangerous?
As rough guidance: below about 21 C most people feel comfortable; 21-25 C feels muggy and exercise starts to strain; 25-28 C is moderate heat stress; 28-32 C is high and dangerous for prolonged exertion; 32-35 C is extreme, where even resting in shade is hard; and 35 C and above is beyond the human survivability threshold. These bands describe population averages, not medical advice: children, the elderly and people with health conditions are affected at lower values.
Why does sweat stop cooling me when it is humid?
Sweat cools you only when it evaporates, and evaporation needs air that is not already saturated with moisture. As humidity rises, the air approaches saturation, the evaporation rate collapses, and the wet-bulb temperature climbs toward the air temperature. That is why 35 C at 20% humidity can feel manageable while 32 C at 90% humidity can feel oppressive: the difference is how much room the air still has to absorb water.
Can wet-bulb temperature exceed the air temperature?
No. Wet-bulb temperature can never be higher than the dry-bulb (air) temperature; it nearly equals it when the air is saturated at 100% relative humidity, because no evaporation is possible. (This calculator uses the Stull approximation, which can deviate by up to about ±0.3 °C, so it may show a value a fraction of a degree off from the air temperature at saturation.) If your inputs produce a wet-bulb above the air temperature, either the reading or the calculation is wrong.
Does altitude affect wet-bulb temperature?
The Stull formula used here assumes near-sea-level atmospheric pressure. At high altitude, lower air pressure changes the psychrometric ratio, so the true adiabatic-saturation wet-bulb is slightly lower than Stull predicts. For altitude-corrected results, use the full psychrometric calculator linked below, which takes pressure into account.
Can I use Fahrenheit instead of Celsius?
Yes. Toggle the unit switch at the top of the calculator. The Stull formula itself is defined in Celsius, so the tool converts your Fahrenheit inputs to Celsius internally, computes, and shows the result in both units.
How is this different from your psychrometric calculator?
This page is a fast, single-purpose wet-bulb calculator: it uses the closed-form Stull empirical approximation and adds heat-stress context, the 35 C survivability limit and comfort tables. The psychrometric calculator computes wet-bulb by iterating the ASHRAE adiabatic-saturation equations and additionally returns dew point, humidity ratio, enthalpy, specific volume, vapor pressure, air density and absolute humidity, with pressure input for altitude. Use this page for quick wet-bulb answers, and the psychrometric calculator for full moist-air analysis.
What input ranges are valid?
The Stull fit is calibrated for air temperature from -20 C to 50 C (-4 F to 122 F) and relative humidity from 5% to 99%. The calculator accepts inputs slightly outside that window but shows a warning, because results there are extrapolations the formula was not fitted to. The reverse solver works within the same window.
Can I use this tool for occupational safety decisions?
No. This tool is a screening and education aid, not a safety instrument. Real occupational heat-stress decisions require calibrated WBGT meters placed at the worksite, acclimatization schedules, work-rest tables and local regulations such as OSHA or ACGIH guidance. Do not plan work, events or emergency responses from a web approximation.
Related tools
Free tool by NRTechStudio