Calculator

Wet bulb temperature calculator

Air temperature 90°F

Relative humidity 60%

Dew point 74°F

Used instead of humidity when the Dew point mode is selected above.

78.8°F Wet bulb temperature · Raised
Evaporative cooling still works, but less efficiently. This is a normal summer figure in a humid climate.

Below the 35°C limit by: 16.2°F
Heat index: 100°F
Dew point: 74°F

The numbers above are the answer for the values the page loaded with. Changing the controls needs JavaScript — without it the calculator cannot recalculate, so read the figures as one worked example rather than a live result.

How do you calculate wet bulb temperature?

Wet bulb temperature is the lowest temperature air can be cooled to by evaporating water into it. Enter air temperature and either humidity or dew point above, and the calculator applies Stull's regression. At 90°F and 60% humidity the wet bulb is 78.8°F (26.0°C) — about 9°C below the 35°C limit at which sweating stops working.

What wet bulb temperature is

Wrap a thermometer bulb in wet muslin and blow air across it. Water evaporates, evaporation takes heat, and the thermometer settles below the air temperature. Where it settles is the wet bulb temperature.

It is the coldest anything can get by evaporation alone in that air — a swamp cooler, a wet shirt, a sweating person. In dry air the gap is large: at 100°F and 15% humidity the wet bulb is around 68°F. In saturated air there is no gap at all, because no more water will evaporate.

That is why it matters more than the thermometer reading. The dry bulb tells you how hot the air is. The wet bulb tells you whether you can do anything about it.

Reading the answer

The number this calculator gives is the psychrometric wet bulb: shaded and ventilated. It is what people mean in conversation and in the climate literature, and it is not WBGT — that is a different, weighted quantity that includes sunshine. If you need the one that safety rules are written against, use the WBGT estimator.

Worked example: the same day, two ways

Phoenix, dry heat: 108°F, 15% humidity → wet bulb 74°F
Houston, humid heat: 92°F, 75% humidity → wet bulb 84°F

10°F Houston's wet bulb is ten degrees higher despite being sixteen degrees cooler on the thermometer. Evaporative cooling has far less room to work there.

Which is why "but it's a dry heat" is not only a cliché. It is thermodynamics.

Where the 35°C limit comes from

In 2010 Sherwood and Huber published a limit based on a simple observation: human skin sits near 35°C while the body is shedding heat, so if the wet bulb temperature of the surrounding air reaches 35°C there is nowhere for that heat to go. Not shade, not a fan, not drinking water. Only cooler or drier air.

Two things are worth knowing about that number:

It is a theoretical ceiling, not a safety threshold. Later experimental work at Penn State found that critical environmental limits for young, healthy, resting adults sit closer to 31°C wet bulb, and lower again for older people or anyone doing work.
It is nearly unreached, but not entirely. Analyses of station data have found brief 35°C wet bulb events along the Persian Gulf and in parts of Pakistan, lasting hours rather than days.

So a wet bulb of 31°C is not "comfortably below the limit". It is already at the edge of what published human trials describe as survivable for a resting adult.

What this calculator assumes

Standard sea-level pressure (1013.25 hPa). At altitude the true wet bulb is slightly lower.
Stull's fitted range: −20°C to 50°C, 5% to 99% humidity. Outside it, the page tells you rather than pretending.
Shade and ventilation. A wet bulb in direct sun reads higher, which is the natural wet bulb that goes into WBGT.

For the derivation and the physical picture, see what wet bulb temperature is and why 35°C matters. For how all three thermometers relate, see dry bulb, wet bulb and globe.

Common questions

What is the difference between wet bulb temperature and WBGT?

Wet bulb is one thermometer: a shaded, ventilated, water-wrapped bulb. WBGT is a weighted average of three — a natural wet bulb in the open, a black globe measuring radiant heat, and the ordinary air temperature. Outdoors WBGT is 0.7 natural wet bulb plus 0.2 globe plus 0.1 air temperature, so it includes sunshine and wind while wet bulb does not.

Why is 35°C wet bulb the limit for human survival?

Skin sits at roughly 35°C when the body is shedding heat. If the wet bulb temperature of the air reaches that value, evaporation can no longer move heat from skin to air, because there is nowhere cooler for it to go. Core temperature then rises regardless of shade, water or rest. Studies since have suggested the practical limit for many people is nearer 31°C.

Can wet bulb temperature be higher than the air temperature?

No. Evaporation only cools, so the wet bulb is always at or below the dry bulb. The two are equal at 100% relative humidity, when no further evaporation is possible.

How accurate is this calculator?

It uses Stull's 2011 regression, which is stated as accurate to about 0.3°C over the range it was fitted for: −20°C to 50°C and 5% to 99% humidity. Outside that range the calculator says so rather than presenting an extrapolation as a result. It assumes standard sea-level pressure.

What wet bulb temperature is dangerous?

There is no single official band table for wet bulb the way there is for the heat index. As a rough guide, sustained heavy work becomes difficult above about 28°C, prolonged exposure is dangerous even at rest above about 31°C, and 35°C is the theoretical limit. The bands on this page are a reading aid built from that literature, not a published standard.

Sources

Stull, R. (2011). Wet-Bulb Temperature from Relative Humidity and Air Temperature — Journal of Applied Meteorology and Climatology 50(11) — the regression used here
Sherwood, S. & Huber, M. (2010). An adaptability limit to climate change due to heat stress — PNAS — the origin of the 35°C figure
Vecellio et al. (2022). Evaluating the 35°C wet-bulb temperature adaptability threshold — Journal of Applied Physiology — experimental limits nearer 31°C

Published 30 July 2026