Wet Bulb Temperature Calculator
Enter the air temperature and relative humidity to get the wet-bulb temperature, the lowest temperature evaporation can cool something to. It uses Stull's formula with an exact psychrometric cross-check, and rates the heat stress so you can judge how dangerous hot, humid conditions are.
Last updated:
for the psychrometric cross-check; lower at altitude
How to use the Wet Bulb Temperature Calculator
- Pick °F or °C.
- Enter the air temperature (the normal dry-bulb reading).
- Enter the relative humidity from a weather report or hygrometer.
- Optionally set the air pressure for the exact psychrometric value at altitude.
- Press Calculate to see the wet-bulb temperature, heat-stress level and the working.
Wet-bulb temperature table (°C, Stull formula)
| Air temp ↓ / RH → | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 20 °C (68 °F) | 7.0 | 9.1 | 10.8 | 12.3 | 13.7 | 15.0 | 16.3 | 17.5 | 18.8 |
| 25 °C (77 °F) | 9.8 | 12.5 | 14.6 | 16.4 | 18.0 | 19.5 | 20.9 | 22.3 | 23.7 |
| 30 °C (86 °F) | 12.7 | 15.9 | 18.4 | 20.4 | 22.3 | 24.0 | 25.6 | 27.1 | 28.6 |
| 35 °C (95 °F) | 15.6 | 19.3 | 22.1 | 24.5 | 26.6 | 28.5 | 30.3 | 31.9 | 33.5 |
| 40 °C (104 °F) | 18.5 | 22.7 | 25.9 | 28.6 | 30.9 | 33.0 | 34.9 | 36.7 | 38.5 |
| 45 °C (113 °F) | 21.4 | 26.1 | 29.7 | 32.6 | 35.2 | 37.5 | 39.6 | 41.5 | 43.4 |
Read across from the air temperature to the humidity column. A 40 °C (104 °F) day at 60% humidity already has a wet bulb above 32 °C, deep in the danger zone, while 45 °C at 10% is far safer at under 23 °C.
Heat-stress guide used here
| Wet bulb | Level | What it means |
|---|---|---|
| Below 18 °C (64 °F) | Low | Humidity adds little heat stress |
| 18–24 °C (64–75 °F) | Moderate | Normal precautions for hard exercise |
| 24–28 °C (75–82 °F) | High | Sweat cools less; schedule breaks, drink water |
| 28–31 °C (82–88 °F) | Very high | Dangerous for prolonged exertion |
| 31–35 °C (88–95 °F) | Extreme | Beyond what lab studies found young adults can tolerate for long, even at light activity |
| 35 °C (95 °F) and above | Beyond survivable limit | Theoretical limit for survival over several hours, even resting in shade |
These bands are a practical guide built around the published 31 °C and 35 °C thresholds, not an official warning scale. For work and sport, follow your local heat-safety guidance, which usually uses WBGT or the heat index. Older adults, children, and people with health conditions are affected at lower values.
Why wet bulb matters
Your body cools itself mostly by evaporating sweat, which works like the wet cloth on a wet-bulb thermometer. When humidity is high, the wet bulb gets close to the air temperature and sweat stops working. That is why 32 °C in a humid coastal city can be more dangerous than 42 °C in a dry desert.
Frequently asked questions
What is the wet-bulb temperature?
It is the temperature a thermometer wrapped in a wet cloth reaches when air flows over it, as evaporation cools it. It is the lowest temperature reachable by evaporative cooling, and it combines heat and humidity into one number: the drier the air, the further the wet bulb falls below the air temperature.
What is the Stull formula?
Roland Stull (2011) published an empirical formula for wet-bulb temperature from air temperature T (°C) and relative humidity RH (%): 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. It is valid at sea-level pressure for −20 to 50 °C and 5–99% RH, with errors of about −1 to +0.65 °C.
Why is a 35 °C wet-bulb temperature considered deadly?
Your skin is normally around 35 °C. When the wet-bulb temperature reaches that level, sweat can no longer carry heat away, so even a healthy person resting in shade with water cannot cool down. Laboratory studies suggest the real limit for young, healthy adults doing light activity is lower, around 31 °C, and lower still for older people.
Is wet-bulb temperature the same as WBGT?
No. Wet-bulb globe temperature (WBGT), used for work and sports safety, combines the natural wet-bulb temperature with a black-globe reading for sun and radiant heat and the air temperature. In shade with no radiant heat, WBGT is roughly 0.7 × wet bulb + 0.3 × air temperature; in direct sun it is higher.
How is wet-bulb temperature different from heat index?
The heat index is an apparent "feels like" temperature for a person in the shade and is always at or above the air temperature. The wet-bulb temperature is a physical temperature that is always at or below the air temperature. For example, 95 °F at 50% RH has a heat index around 105 °F and a wet bulb of about 80 °F.
What does the wet-bulb depression tell me?
The difference between air temperature and wet bulb shows how much cooling evaporation can provide. A large depression (dry air) means swamp coolers and sweating work well; near zero means the air is close to saturated.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.