Heat stress · humidity
Wet Bulb Temperature Calculator
The wet-bulb temperature is the lowest temperature evaporation can cool air to. It is what a thermometer wrapped in a wet cloth reads, and it is the best single number for how well sweat can still cool you.
Formula
- Tw = T·atan[0.151977·(RH + 8.313659)^½] + atan(T + RH) − atan(RH − 1.676331) + 0.00391838·RH^(3/2)·atan(0.023101·RH) − 4.686035
- T in °C, RH in percent (50 for 50%), angles in radians.
Source: Roland Stull, “Wet-Bulb Temperature from Relative Humidity and Air Temperature”, Journal of Applied Meteorology and Climatology, 2011.
Valid range
- Relative humidity 5% to 99% and air temperature -20°C to 50°C (-4°F to 122°F). Outside that box the calculator still shows a number but flags it, because the fit can drift by more than a degree or two.
- At exactly 100% humidity the calculator returns the air temperature, which is the physical answer: nothing can evaporate, so the wet bulb cannot cool.
- The formula assumes sea-level pressure (1013.25 hPa). At altitude the true wet-bulb temperature is slightly lower than shown.
What the wet-bulb number tells you
Your body sheds heat mainly by evaporating sweat. The wet-bulb temperature measures how much cooling evaporation can still do in the current air. In dry heat it sits far below the air temperature; as humidity climbs it closes in on it, and at 100% humidity the two are equal.
A wet-bulb value of 35°C (95°F) is often cited as the theoretical upper limit for human survival, from Sherwood and Huber (2010): beyond it, even a resting person in the shade cannot shed metabolic heat. Laboratory work since then, notably Vecellio and colleagues at Penn State (2022), found that young, healthy volunteers lost the ability to hold a stable core temperature at much lower wet-bulb values, around 31°C (88°F) in humid conditions. Older people, children and anyone working hard reach their limit sooner still.
Wet-bulb temperature is not the same as WBGT (wet-bulb globe temperature), the index used for outdoor work and sports guidance. WBGT also accounts for sunshine and wind, and needs measurements this calculator does not have.
Wet-bulb temperature by air temperature and humidity (°F, sea-level pressure)
Generated from the same equation as the calculator above.
| Temperature (°F) / Humidity (%) | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% | 99% |
|---|---|---|---|---|---|---|---|---|---|---|
| 50° | 34 | 36 | 38 | 40 | 41 | 43 | 45 | 46 | 48 | 50 |
| 60° | 40 | 43 | 45 | 48 | 50 | 52 | 54 | 56 | 58 | 60 |
| 70° | 46 | 50 | 53 | 56 | 58 | 61 | 63 | 65 | 68 | 70 |
| 80° | 51 | 57 | 61 | 64 | 67 | 70 | 72 | 75 | 78 | 80 |
| 90° | 57 | 63 | 68 | 72 | 76 | 79 | 82 | 85 | 87 | 90 |
| 100° | 63 | 70 | 76 | 80 | 84 | 88 | 91 | 94 | 97 | 100 |
| 110° | 69 | 77 | 83 | 88 | 93 | 97 | 100 | 104 | 107 | 110 |
| 120° | 75 | 84 | 91 | 96 | 101 | 106 | 110 | 113 | 117 | 120 |
Wet-bulb temperature by air temperature and humidity (°C, sea-level pressure)
Generated from the same equation as the calculator above.
| Temperature (°C) / Humidity (%) | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% | 99% |
|---|---|---|---|---|---|---|---|---|---|---|
| 10° | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
| 15° | 4 | 6 | 7 | 8 | 9 | 11 | 12 | 13 | 14 | 15 |
| 20° | 7 | 9 | 11 | 12 | 14 | 15 | 16 | 18 | 19 | 20 |
| 25° | 10 | 12 | 15 | 16 | 18 | 20 | 21 | 22 | 24 | 25 |
| 30° | 13 | 16 | 18 | 20 | 22 | 24 | 26 | 27 | 29 | 30 |
| 35° | 16 | 19 | 22 | 25 | 27 | 28 | 30 | 32 | 34 | 35 |
| 40° | 19 | 23 | 26 | 29 | 31 | 33 | 35 | 37 | 38 | 40 |
| 45° | 21 | 26 | 30 | 33 | 35 | 37 | 40 | 42 | 43 | 45 |
| 50° | 24 | 30 | 33 | 37 | 39 | 42 | 44 | 46 | 48 | 50 |
Wet-bulb temperature questions
- What is a dangerous wet-bulb temperature?
- There is no single official threshold. 35°C (95°F) is the widely quoted theoretical limit of survivability for a healthy person at rest in the shade (Sherwood and Huber, 2010). Laboratory studies since then put the practical limit for young, healthy adults closer to 31°C (88°F) in humid heat, and lower for older people or anyone doing physical work. Long before those values, heat illness is a real risk.
- How is wet-bulb temperature different from the heat index?
- The heat index is a “feels like” temperature for a person in the shade, built from a model of the human body. The wet-bulb temperature is a physical property of the air: how far evaporation can cool it. The heat index is easier to relate to everyday experience; the wet-bulb value is the one used when asking whether sweating still works.
- Why is the wet-bulb temperature always lower than the air temperature?
- Because evaporation takes heat away. Unless the air is completely saturated, some water can always evaporate from the wet wick, which cools it below the air temperature. At 100% humidity the two are equal.
- How accurate is the Stull formula?
- Stull reports errors mostly between about −1°C and +0.65°C inside its valid range, against the full psychrometric calculation at sea-level pressure. That is plenty for weather use. It is not a substitute for a calibrated psychrometer.
- Is wet-bulb temperature the same as WBGT?
- No. Wet-bulb globe temperature (WBGT) combines the wet-bulb reading with a black-globe thermometer (sunshine) and air temperature. It is the measure used in many work and sports heat guidelines, and in direct sun it can be several degrees higher than the plain wet-bulb temperature.