Free wet bulb temperature calculator
Enter the air temperature and relative humidity to get the wet-bulb temperature in °C and °F, plus a heat-risk band — using the Stull (2011) formula, updated live, as you type.
On this page15 sections
An estimate of risk, not safety advice. How accurate is this?
Results are estimates. Consult a professional.
How the wet bulb temperature calculator works
Wet-bulb temperature is the lowest temperature air can reach by evaporating water into it. It is the reading a thermometer settles at when its bulb is wrapped in a wet wick and air blows over it. The calculator takes two readings you already have — the air (dry-bulb) temperature and the relative humidity — and returns the wet-bulb temperature in both Celsius and Fahrenheit, plus a plain-language heat-risk note.
The figure matters because sweating cools your body the same way the wet wick cools the thermometer: by evaporation. When the air is already moist, less sweat evaporates, so the wet-bulb temperature rises and your body loses its main way to shed heat.
Why 35 °C wet bulb is the survivability limit
Your skin sits near 35 °C, and your body must constantly move heat outward to that surface and away. The only way to lose heat once the air is hot is to evaporate sweat. A sustained wet-bulb temperature of about 35 °C (95 °F) is the theoretical point where that stops working: the air is so warm and moist that sweat will not evaporate fast enough to carry your own metabolic heat away, even resting in the shade with unlimited water.
That 35 °C figure is a ceiling, not a comfort line. Serious heat stress builds long before it. Researchers note real harm to healthy people at wet-bulb readings in the high 20s, and recent work argues the practical limit for many people — especially older adults — is several degrees below 35 °C. Treat the limit as a worst case and the warning bands above it as the part you will more likely meet.
What determines wet bulb temperature
Two inputs set the result, and they pull in different ways. Understanding each one explains why a muggy 32 °C day can be more dangerous than a dry 40 °C day.
Air temperature (dry bulb)
This is the ordinary temperature a weather report gives. Higher air temperature raises the wet-bulb temperature, but not one-for-one — some of the heat is spent evaporating water rather than warming the wet bulb. That gap between dry-bulb and wet-bulb is the cooling that evaporation buys you.
Relative humidity
This is the lever that turns heat dangerous. Dry air leaves plenty of room for sweat to evaporate, so the wet-bulb temperature sits far below the air temperature. As humidity climbs, that room shrinks. At 100% humidity the air holds no more water, evaporation stops, and the wet-bulb temperature equals the air temperature — there is no cooling left to give.
A worked example using the wet bulb calculator
Maya checks her weather app during a heatwave: the air temperature is 35 °C (95 °F) and relative humidity is 60%. The dry-bulb number alone does not tell her how dangerous it is, so she wants the wet-bulb temperature.
Step 1 — Enter the two readings
Air temperature 35 °C, relative humidity 60%. Both sit inside the formula's valid range, so the estimate is reliable.
Step 2 — Apply the Stull formula
Feeding 35 and 60 into the equation gives a wet-bulb temperature of 28.5 °C, which is 83.3 °F.
Step 3 — Read the risk band
28.5 °C falls in the caution band — well below the 35 °C limit, but high enough that heat stress is real for anyone working, exercising, or without shade. It is also far below the 35 °C dry-bulb reading, showing how much cooling the 40% of "dry" air is still providing.
Wet bulb temperature chart by temperature and humidity
This chart gives the wet-bulb temperature (°C) for common combinations of air temperature and relative humidity. Read down to your air temperature, across to your humidity. Notice how each row climbs sharply to the right — that is humidity tightening its grip.
| Air temp | 40% RH | 60% RH | 80% RH | 100% RH |
|---|---|---|---|---|
| 25 °C (77 °F) | 16.4 | 19.5 | 22.3 | 25.0 |
| 30 °C (86 °F) | 20.4 | 24.0 | 27.1 | 30.1 |
| 35 °C (95 °F) | 24.5 | 28.5 | 31.9 | 35.1 |
| 40 °C (104 °F) | 28.6 | 33.0 | 36.7 | 40.2 |
Wet-bulb temperature in °C from the Stull (2011) formula at sea-level pressure. At 100% humidity the wet bulb equals the air temperature, because evaporation can no longer cool. Values at 40 °C with high humidity reach or exceed the ~35 °C survivability limit.
What wet bulb temperatures are dangerous?
There is no single switch from safe to unsafe — risk rises steadily as the wet-bulb temperature climbs. The bands below are a general guide for healthy adults at rest in shade. Exertion, direct sun, age, and existing illness all shift the danger earlier.
| Wet bulb | Risk band | What it means |
|---|---|---|
| Below 26 °C | Comfortable | Sweating cools the body easily; little heat-stress risk at rest. |
| 26–30 °C | Caution | Heat stress likely during work or exercise; take breaks and hydrate. |
| 30–32 °C | Extreme caution | Dangerous for prolonged exposure or exertion; limit time outdoors. |
| 32–35 °C | Danger | Heat illness likely; rest, shade, and cooling are essential. |
| 35 °C and above | Survivability limit | The body cannot shed its own heat even at rest — life-threatening. |
Bands match the calculator's risk output. They are an estimate for general guidance, not medical or safety advice; thresholds for vulnerable people are lower.
How to use the wet bulb temperature calculator
- Get the air temperature. Use the dry-bulb (ordinary) temperature from a thermometer or weather app, in °C or °F — toggle the unit and the slider follows.
- Get the relative humidity. Most weather apps and home hygrometers report it as a percentage. Enter the current value, not the daily high.
- Read the wet-bulb result. The hero shows the wet-bulb temperature in your chosen unit, with the °F (or °C) equivalent beside it.
- Check the risk band. The note translates the number into a comfort-to-danger band so you know how seriously to take it.
- Compare scenarios. Nudge the humidity up while holding temperature steady to see how fast wet bulb — and risk — climbs.
For related views of the same heat, see the dew point and heat index tools linked at the foot of this page. Dew point describes the moisture in absolute terms; the heat index estimates how hot it feels.
Wet bulb temperature vs. WBGT and heat index
Three "heat" numbers are easy to confuse. They measure related but different things, and this calculator computes the first.
- Wet-bulb temperature (Tw) — what this tool returns. A physical temperature: the limit of evaporative cooling, set by air temperature and humidity only.
- Wet-bulb globe temperature (WBGT) — a workplace and sports heat-stress index that blends wet-bulb, a black-globe reading (radiant sun), and dry-bulb temperature, typically weighted 70/20/10. It needs sun and a globe sensor, so it is not derivable from temperature and humidity alone.
- Heat index — an estimate of how hot it feels, also from temperature and humidity, but a perceived-temperature figure rather than a physical one.
If you want the "feels-like" number, use the heat index calculator. If you want the moisture itself, use the dew point calculator. WBGT is a separate measurement that requires field instruments.
Wet bulb temperature definitions
How accurate is this wet bulb temperature calculator?
The calculator uses Stull's 2011 empirical formula, the standard single-equation method for estimating wet-bulb temperature from air temperature and humidity. Across its valid range — about 5–99% relative humidity and −20 to +50 °C at sea-level pressure — its error runs from roughly −1.0 to +0.65 °C, with a mean absolute error under 0.3 °C. For everyday heat-safety judgment that is more than precise enough.
Two limits are worth knowing. First, the formula assumes standard sea-level pressure; at high altitude the true wet-bulb temperature shifts slightly. Second, accuracy degrades for the extreme cold-and-dry corner outside the stated range. The risk bands are a general guide for healthy adults, not a medical assessment — vulnerable people reach danger at lower readings, and sun, wind, and exertion all matter. Treat the result as a solid estimate, and rely on your local weather service and health authority during an actual heat event.
Frequently asked questions about the free wet bulb temperature calculator
About this Wet Bulb Temperature calculator
This calculator runs entirely in your browser. Nothing you type is sent anywhere — the wet-bulb temperature and heat-risk band are computed locally from the Stull (2011) formula as you adjust the air temperature and humidity.
It is one of our weather calculators. Browse the full set of free calculators for more heat, climate, and everyday tools.