Weather & Climate
Wet-bulb temperature calculator.
Wet-bulb temperature is what a thermometer with a wetted wick reads when air flows over it, and it is always at or below the air temperature. At 20°C and 50% humidity at sea-level pressure it is 13.9°C. It is not a heat-stress rating.
Air conditions
Wet-bulb temperature
Fill the form and press Calculate.
Worked example
Air at 20°C and 50% relative humidity has a vapour pressure of 11.68 hPa. At 1013.25 hPa the psychrometric equation is satisfied by a wet-bulb temperature of 13.9°C, a depression of 6.1°C below the air temperature. The dew point is 9.3°C. The same air at 800 hPa, for example at altitude, has a wet-bulb of 13.3°C.
How it works
The calculator first finds the vapour pressure from the air temperature and humidity, e = RH × 6.112 × exp[17.67T / (T + 243.5)] hPa. It then finds the wet-bulb temperature Tw that satisfies the National Weather Service equation e = ew − P × (T − Tw) × 0.00066 × (1 + 0.00115 Tw), where ew is the saturation vapour pressure at Tw and P is the station pressure in hPa. The NWS page steps toward the answer; this tool bisects to the same equation.
Wet-bulb temperature is not the same as WBGT (wet-bulb globe temperature), which also accounts for sun and radiation and is what occupational heat-stress limits use. This tool gives no heat-stress limits.
Sources and basis (all opened 2026-10-08): US National Weather Service El Paso, “Relative Humidity and Dewpoint from Wet-bulb” calculator and its formula notes (PDF) (e = ew − P × (T − Tw) × 0.00066 × (1 + 0.00115 Tw)).
FAQ
What is wet-bulb temperature?
The temperature a thermometer reads when its bulb is covered by a wet wick and air moves over it. Evaporation cools it, so it is at or below the air temperature, and the two are equal at 100% humidity.
What is the wet-bulb temperature at 20 degrees C and 50% humidity?
About 13.9 degrees C at 1013.25 hPa.
Does pressure change the wet-bulb temperature?
Slightly. The same 20 degrees C and 50% air gives 13.3 degrees C at 800 hPa instead of 13.9 degrees C at 1013.25 hPa.
Is wet-bulb temperature the same as wet-bulb globe temperature?
No. WBGT also includes the effect of sun and radiant heat. Use official heat warnings and workplace heat-stress guidance for safety decisions.
Which station pressure should I enter?
The actual pressure at your location. Weather reports often give pressure adjusted to sea level, which overstates it at altitude. If you do not know it, 1013.25 hPa is a reasonable default near sea level.
Estimate only; not a heat-safety tool
This calculates a wet-bulb temperature from a published equation. It is not wet-bulb globe temperature and gives no safe-exposure limits. For heat safety, rely on official heat alerts and occupational guidance.