About the Wet Bulb Temperature Calculator
This wet bulb temperature calculator finds the lowest temperature air can be cooled to by evaporating water into it – what a thermometer wrapped in a wet cloth reads when air blows over it. Enter the air (dry-bulb) temperature, the relative humidity and, if you are not near sea level, the air pressure. It returns the wet-bulb temperature, the wet-bulb depression, the dew point and an estimate from Stull's popular formula for comparison.
Wet-bulb temperature matters for heat stress (sweat cools you the same way the wet wick cools), for evaporative and swamp cooler performance, cooling-tower design, snowmaking, crop and livestock management, and checking psychrometer readings. The table shows how the wet bulb changes across the humidity range at your temperature.
The main result solves the standard psychrometer equation for a ventilated wet bulb, using Magnus saturation vapour pressure, so it works at any pressure. Stull's 2011 formula has a mean absolute error under about 0.3 °C at sea level for 5–99% RH and −20 to 50 °C. Note that the wet-bulb globe temperature (WBGT) used for workplace and sports heat rules is a different index that also includes sun and wind.
With the default inputs, the wet-bulb temperature is 23.9 °. Change any value above to recalculate instantly.
How to use the wet bulb temperature calculator
- 1Choose °C or °F.
- 2Enter the air temperature and relative humidity.
- 3If you are well above sea level, enter the local station pressure.
- 4Read the wet-bulb temperature, depression and dew point.
- 5Check the table to see how humidity changes the result.
Formula and method
The wet-bulb temperature is the temperature at which the heat taken by evaporation exactly balances the heat supplied by the surrounding air. The psychrometer equation expresses that balance: the vapour pressure in the air equals the saturation vapour pressure at the wet bulb minus a term proportional to pressure and the wet-bulb depression. Saturation vapour pressure uses the Magnus formula, eₛ(T) = 6.1094·exp(17.625T/(T + 243.04)) hPa.
Because T_w appears on both sides, the calculator solves the equation numerically by bisection between −60 °C and the dry-bulb temperature. For comparison it also shows Stull's 2011 empirical fit, Tw = T·atan(0.151977·√(RH + 8.313659)) + atan(T + RH) − atan(RH − 1.676331) + 0.00391838·RH^1.5·atan(0.023101·RH) − 4.686035, which assumes sea-level pressure.
- T
- Dry-bulb air temperature (°C)
- T_w
- Wet-bulb temperature (°C)
- RH
- Relative humidity (%)
- P
- Station pressure (hPa)
- eₛ
- Saturation vapour pressure (hPa)
Worked examples
Hot humid afternoon: 30 °C at 60% RH
e = 0.60 × 42.37 = 25.42 hPa. Solving the psychrometer equation at 1013.25 hPa gives T_w = 23.9 °C, a 6.1 °C depression; Stull's formula gives 24.0 °C.
Mild day: 20 °C at 50% RH
e = 11.67 hPa and the wet bulb settles at 13.9 °C. The Stull approximation is 13.7 °C, well within its published error range.
Dangerous heat: 35 °C at 75% RH
With e = 42.24 hPa the wet bulb is 31.0 °C, so sweat can barely cool the body. Conditions like this occur in parts of South Asia and the Persian Gulf during heatwaves.
Dry high-altitude air: 25 °C, 30% RH, 850 hPa
Lower pressure speeds evaporation, so the wet bulb is 13.9 °C – about 0.7 °C cooler than Stull's sea-level estimate of 14.6 °C. Great conditions for an evaporative cooler.
95 °F at 50% RH
95 °F is 35 °C. At 50% RH the wet bulb is 26.3 °C, or 79.3 °F – a 15.7 °F depression.
Frequently asked questions
What is wet-bulb temperature?+
It is the temperature a thermometer covered in a water-soaked cloth reads when air flows over it. Evaporation cools the bulb until it balances the heat from the air. It is always between the dew point and the air temperature, and equals both at 100% humidity.
Why is a 35 °C wet-bulb temperature considered deadly?+
Skin is normally around 35 °C. When the wet-bulb temperature reaches that level, sweat can no longer carry heat away, so even a resting person in shade would overheat. Sherwood and Huber proposed this theoretical limit in 2010.
Is the real survivable limit lower than 35 °C?+
Lab studies at Penn State in 2021–2022 found that young, healthy volunteers lost the ability to control core temperature at wet-bulb values around 30–31 °C during light activity. Older people, children and those with health conditions are at risk at lower values.
What is the difference between wet-bulb temperature and WBGT?+
Wet-bulb temperature uses only air temperature and humidity. The wet-bulb globe temperature (WBGT) used by OSHA, the military and sports bodies combines natural wet-bulb, black-globe (sun) and air temperature, so it rises in direct sun.
How accurate is the Stull wet-bulb formula?+
Stull (2011) reported errors mostly within −1 to +0.65 °C, with mean absolute error about 0.3 °C, for RH 5–99% and temperatures −20 to 50 °C at standard sea-level pressure. It is not valid at high altitude.
This tool gives general estimates and is not medical advice. Talk to a doctor or qualified professional about your health, diet or training.