Tool
Wet Bulb Calculator
Enter a dry bulb temperature and humidity to get the wet bulb temperature — or work the other way and turn a wet bulb reading back into relative humidity. Both directions use the same formulas weather services do, and every result comes with a heat-stress band so you know what the number actually means.
Enter two values — the result appears here with a heat-stress reading.
Wet bulb uses the Stull (2011) approximation, accurate to about ±0.3°C in normal weather. The reverse direction uses the psychrometric equation at standard sea-level pressure (1013.25 hPa).
What Is the Wet Bulb Temperature Today?
Type a city to see the current wet bulb reading, worked out from live temperature and humidity.
Live readings from Open-Meteo via the same weather API as Dew Point Today. Wet bulb is derived with the same Stull formula used in the calculator above.
What Are Dry Bulb and Wet Bulb Temperature?
Dry bulb is simply the air temperature — what you see on any ordinary thermometer, read in the shade with the sun off the sensor. It is called “dry” only to distinguish it from the other bulb, not because the air is dry.
Wet bulb is the temperature you get when you wrap that same thermometer in a wet cloth and let air move past it. Water evaporates, evaporation pulls heat out, and the reading drops below the dry bulb. It settles at the point where the air around the wick is fully saturated — which is why wet bulb is the practical measure of how much cooling evaporation can still do. When the gap between dry bulb and wet bulb is wide, sweat works. When it closes, sweat doesn't.
What a Wet Bulb Thermometer Measures
A sling psychrometer is the classic instrument: two thermometers side by side, one dry, one with a muslin wick, swung through the air until the wet one stops falling. Modern weather stations compute the same number from temperature and humidity, which is what this calculator does — no swinging required. The reason field readings are fussy is that the wet bulb depends on air speed past the wick: too little ventilation and it reads warm, too much and it reads cool.
How to Calculate Wet Bulb Temperature from Dry Bulb and Humidity
The shortcut used almost everywhere is the Stull (2011) approximation, with temperature in °C and relative humidity in percent:
Tw = T·atan(0.151977·√(RH + 8.313659))
+ atan(T + RH)
− atan(RH − 1.676331)
+ 0.00391838·RH^1.5·atan(0.023101·RH)
− 4.686035It is accurate to roughly ±0.3°C between −20°C and 50°C, which is well inside the uncertainty of the humidity reading it starts from. The reverse direction — wet bulb back to humidity — uses the psychrometric equation instead: e = es(Tw) − 0.00066·P·(T − Tw), then RH = e / es(T) × 100, with P the station pressure. This page uses the standard sea-level value of 1013.25 hPa; at altitude the true wet bulb runs slightly lower.
Wet Bulb Temperature Chart
Same formula, laid out as a table. Read across for humidity, down for dry bulb — the cells are the wet bulb in °F. Notice how the number barely moves with humidity until the air gets humid, then climbs fast: at 90°F the wet bulb spans 20 degrees across this range.
| Dry bulb | 20% RH | 40% RH | 60% RH | 80% RH |
|---|---|---|---|---|
| 90°F | 63°F | 72°F | 79°F | 85°F |
| 95°F | 67°F | 76°F | 83°F | 89°F |
| 100°F | 70°F | 80°F | 88°F | 94°F |
| 105°F | 74°F | 84°F | 92°F | 99°F |
Generated with the Stull formula above, not hand-copied — so the table and the calculator always agree.
What Wet Bulb Temperature Is Dangerous?
| Wet bulb (°C) | Level | What it means |
|---|---|---|
| Below 10 | Comfortable | No heat stress. Normal outdoor activity. |
| 10 – 20 | Caution | Sustained exertion starts to feel heavy. Hydrate, take breaks. |
| 20 – 25 | Extreme caution | Cramps and exhaustion likely with prolonged activity. Move work out of midday. |
| 25 – 28 | Danger | Heat exhaustion probable. Reschedule outdoor work and hard training. |
| 28 – 31 | Extreme danger | Heat stroke likely with exertion. Do not exercise outdoors. |
| Above 35 | Beyond human limits | Sweat can no longer cool the body at all — the survivability threshold, even in shade with unlimited water. |
A wet bulb of 35°C is the famous line, but it is not the only one that matters. Most heat illness happens well below it — the bands above 25°C are where workers and athletes actually get into trouble, because the reading still looks like an ordinary summer day on the dry bulb thermometer.
WBGT: When You Need the Globe Temperature Too
Wet bulb tells you how well evaporation cools the air, but it ignores the sun. WBGT — wet bulb globe temperature — combines wet bulb, a black globe thermometer for solar radiation, and dry bulb, which is why sports leagues, the military and outdoor-work rules are all written in WBGT rather than wet bulb. In full sun WBGT runs several degrees above the wet bulb reading; in shade the two converge. If you are setting a training or worksite policy, use the WBGT calculator — this page gives you the wet bulb input it is built from.
Wet Bulb Questions, Answered
What is the difference between wet bulb and dry bulb temperature?
Dry bulb is the ordinary air temperature a thermometer reads. Wet bulb is the lowest temperature air can reach by evaporating water into it, measured with a thermometer wrapped in a wet wick. The gap between the two is how much cooling evaporation can still provide: a small gap means the air is nearly saturated and sweat barely works, a large gap means it works well.
Can wet bulb temperature be higher than dry bulb?
No. Wet bulb is always at or below dry bulb, because evaporation can only remove heat, never add it. The two are equal only at 100% relative humidity. If a reading gives a wet bulb above the dry bulb, the wick has dried out or air is moving too fast past it.
How do you calculate wet bulb temperature from dry bulb and humidity?
The standard approach is 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. It is accurate to about ±0.3°C for normal weather conditions between −20°C and 50°C.
Why is 35°C wet bulb called the survivability limit?
At a wet bulb of 35°C, air is so saturated that skin can no longer shed heat by sweating, no matter how much water you drink or how much shade you find. Core temperature rises until the body fails. That threshold has been reached only rarely, but it is the line public-health agencies plan around.
What is the wet bulb rule?
The wet bulb rule is the practical version of the 35°C limit: when wet bulb climbs above roughly 28°C, outdoor work and exercise should move to cooler hours, and above 31°C they should stop. Different agencies set slightly different cut-offs, so this page shows the band that applies to your reading instead of a single number.
What is the difference between wet bulb and WBGT?
Wet bulb measures how well evaporation cools the air. WBGT (wet bulb globe temperature) adds solar radiation and wind, which is why it is the number used for sports and military heat policy. In full sun, WBGT runs several degrees above the wet bulb reading; in shade, the two are close. Use the WBGT calculator when you need the sun-inclusive reading.
Related
- WBGT Calculator — wet bulb globe temperature from temp and humidity, with ISO 7243 formulas and flag bands.
- Dew Point Calculator — temperature and humidity to dew point, with the Magnus formula and a comfort scale.
- Dew Point Today — live dew point and comfort level for your location.
- Dew Point Chart — the full comfort scale in °F and °C, and why dew point beats relative humidity as a comfort measure.
Formulas: Stull, R. (2011), “Wet-Bulb Temperature from Relative Humidity and Air Temperature”, Journal of Applied Meteorology and Climatology; Bolton, D. (1980) for saturation vapour pressure. Live readings from Open-Meteo.