Wet Bulb Temperature Calculator
Wet bulb temperature folds heat and humidity into one number: the temperature a ventilated wet thermometer would read, and the practical limit of evaporative (sweat) cooling. Enter air temperature and relative humidity.
Example: with Air temperature (dry bulb) 90 · Temperature unit °F (Fahrenheit) · Relative humidity (%) 60 → Wet-bulb temperature: 78.8 °F.
Computed by the calculator below using its default values. Change any input to see your own numbers.
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Check it outWhat wet-bulb temperature tells you
The wet-bulb temperature is what a thermometer wrapped in a wet wick reads as water evaporates off it — the lowest temperature evaporative cooling, including your sweat, can reach. It equals the air temperature only at 100% humidity; drier air evaporates more and reads further below the dry-bulb number. At 90 °F and 60% relative humidity the wet-bulb temperature is about 78.8 °F (26.0 °C); cut the humidity to 20% and it drops to roughly 63 °F.
Wet-bulb readings anchor heat-safety science because they track how well sweating can still cool you. Prolonged exposure around 31 °C (88 °F) wet-bulb is dangerous even for fit, acclimated people, and 35 °C (95 °F) is the theoretical survival limit. Don’t confuse it with WBGT, which blends wet-bulb (70%) with globe (20%) and dry-bulb (10%) readings for work-rest guidelines.
How it’s calculated
Wet-bulb temperature Tw uses the Stull (2011) approximation: 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, with T in °C and RH in percent. The fit assumes roughly sea-level pressure (~1013 hPa) and is valid for RH 5–99% and T from −20 °C to 50 °C, typically within ±0.3–1 °C of exact psychrometric values; outside that range this tool shows no result. Fahrenheit inputs convert via °C = (°F − 32) × 5/9.
Results update as you type and are estimates, not professional advice — verify important decisions with a qualified professional.
Common mistakes
- Confusing wet-bulb with heat index — heat index models perceived temperature; wet-bulb is a physical evaporation limit.
- Using the formula outside its fit — Stull’s approximation covers RH 5–99% and −20 to 50 °C at roughly sea-level pressure.
- Reading a 95 °F forecast as a 95 °F wet-bulb — hitting 35 °C wet-bulb takes extreme heat and extreme humidity together.
Frequently asked questions
What is wet bulb temperature?
The lowest temperature air can be cooled to by evaporating water into it, measured with a wetted, ventilated thermometer. It rises with both heat and humidity, which is why it is used for heat-stress warnings.
How do you calculate wet bulb temperature?
From air temperature and relative humidity with Stull’s 2011 formula, as this calculator does: at 20 °C and 50% RH it gives 13.7 °C. Exact psychrometric methods also require air pressure.
What wet bulb temperature is dangerous?
Research puts the survivable limit near 35 °C (95 °F) wet-bulb, and lab studies of young, healthy adults found heat balance failing around 31 °C. Strenuous work becomes risky well below those levels.
Is wet bulb the same as WBGT?
No. Outdoor WBGT = 0.7 × wet-bulb + 0.2 × globe + 0.1 × dry-bulb, so it also captures sun and radiant heat. Wet-bulb is just its biggest ingredient.
What happens at 100% humidity?
Wet-bulb, dry-bulb, and dew point temperatures all converge — no net evaporation is possible, so sweating stops cooling you.