Wet Bulb Calculator
This wet bulb calculator finds the wet-bulb temperature from air temperature, relative humidity and air pressure. At 86 °F and 60 % humidity it is 75.0 °F. The calculation uses the psychrometric formula published by the National Weather Service and also shows dew point and vapor pressure.
Example: Air temperature 30 · Unit: °C · Relative humidity 60 % · Air pressure at the location 1,013.25 hPa → Wet-bulb temperature: Wet-bulb temperature 23.9 °C. Source: NWS El Paso, Wxcalc: Relative Humidity and Dewpoint Temperature from Temperature and Wet Bulb Temperature – es = 6.112·e^(17.67T/(T+243.5)), e = ew − P·(T−Tw)·0.00066·(1+0.00115·Tw), Td formula (read 10.10.2026). Updated: .
How it is calculated
The wet-bulb temperature Tw is the temperature at which e = 6.112 · e^(17.67·Tw/(Tw + 243.5)) − P · (T − Tw) · 0.00066 · (1 + 0.00115 · Tw) equals the actual vapor pressure of the air (T and Tw in °C, pressures in hPa). This is the equation the National Weather Service (NWS) gives for converting wet-bulb readings into humidity. The calculator turns it around: from temperature and relative humidity it first finds the vapor pressure e = humidity × 6.112 · e^(17.67·T/(T + 243.5)) and then searches for the wet-bulb temperature that fits.
Wet-bulb temperature table (sea level)
| Air temperature | Humidity | Wet bulb | Dew point |
|---|---|---|---|
| 68 °F | 50 % | 57.0 °F | 48.7 °F |
| 77 °F | 50 % | 64.4 °F | 57.0 °F |
| 77 °F | 80 % | 72.4 °F | 70.4 °F |
| 86 °F | 40 % | 68.4 °F | 58.9 °F |
| 86 °F | 60 % | 75.0 °F | 70.5 °F |
| 95 °F | 50 % | 79.3 °F | 73.5 °F |
| 95 °F | 75 % | 87.8 °F | 85.8 °F |
| 104 °F | 30 % | 77.6 °F | 66.5 °F |
Calculated for 1013.25 hPa (29.92 inHg). The wet bulb always lies between the dew point and the air temperature; at 100 % humidity all three are equal. The drier the air, the larger the gap between air temperature and wet bulb.
Why the pressure matters
The pressure P appears in the NWS equation. At 77 °F and 50 % humidity the wet bulb is 64.4 °F at 1013.25 hPa but 63.6 °F at a station pressure of 850 hPa. Enter the station pressure at your location, not the sea-level value from a weather report.
- The dew point comes from the same NWS document: Td = 243.5 · ln(e/6.112) ÷ (17.67 − ln(e/6.112)).
- The calculator gives a temperature, not a heat-stress rating.
- Inputs in °F are converted with °C = (°F − 32) ÷ 1.8 (NIST).
The dew point alone is shown by the dew point calculator; how hot humid air feels is estimated by the heat index calculator.
Frequently asked questions
How do you calculate the wet-bulb temperature?
With the NWS equation e = es(Tw) − P · (T − Tw) · 0.00066 · (1 + 0.00115 · Tw): the vapor pressure e follows from temperature and humidity, and Tw is the value that satisfies the equation. The calculator solves it numerically.
What is the wet bulb at 86 °F and 60 % humidity?
About 75.0 °F (23.9 °C) at sea-level pressure. The dew point is 70.5 °F.
Can the wet bulb be higher than the air temperature?
No. It lies between dew point and air temperature and equals the air temperature only at 100 % relative humidity.
Which pressure should I enter?
The actual air pressure at your location (station pressure). At sea level the standard is 1013.25 hPa; higher up it is lower, which lowers the wet bulb slightly.
Is the wet-bulb temperature the same as the heat index?
No, the heat index is a different quantity with its own formula. This calculator only gives the wet-bulb temperature; for heat warnings follow your weather service.
Sources and legal basis
- NWS El Paso, Wxcalc: Relative Humidity and Dewpoint Temperature from Temperature and Wet Bulb Temperature – es = 6.112·e^(17.67T/(T+243.5)), e = ew − P·(T−Tw)·0.00066·(1+0.00115·Tw), Td formula (read 10.10.2026)
- Stull, Practical Meteorology (UBC), ch. 4 Water Vapor: saturation over liquid water and ice, supercooled water down to −40 °C, Fig. 4.2 es over water minus es over ice (read 10.10.2026)
- NIST SP 811, Appendix B.8: degree Fahrenheit, hectopascal (read 10.10.2026)
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