Wet Bulb Calculator
At 35°C and 50% relative humidity, this formula estimates a wet-bulb temperature of 26.6°C and a dew point of 23.0°C. This calculator uses dry-bulb air temperature and relative humidity to estimate wet-bulb temperature, dew point, and a broad risk label. The result comes from the Stull approximation and is accepted only from 5°C to 50°C and 5% to 99% humidity. It is not wet-bulb globe temperature, heat index, a weather-station observation, or a medical assessment. The label is based only on estimated wet-bulb temperature. Sun, radiant heat, wind, workload, clothing, medications, age, health, cooling access, and acclimatization can make conditions more dangerous than the label suggests.
Quick answer
Wet-bulb temperature is the lowest temperature air can reach through evaporation alone.
What this tells you
- •Wet-bulb temperature is the lowest temperature air can reach through evaporation alone.
- •It combines heat and humidity into one number that reflects how well the body can cool itself.
- •A very high wet-bulb value signals severely limited evaporative cooling, but no single number is a universal safe-versus-fatal boundary.
- •The implementation accepts only 5°C to 50°C and 5% to 99% relative humidity.
- •Wet-bulb and dew point display to 1 decimal place after the formulas are evaluated.
- •The risk label does not account for direct sun, exertion, clothing, health, or access to cooling.
How to Use
- 1Enter the air temperature in degrees Celsius.
- 2Enter the relative humidity as a percentage.
- 3Click Calculate to get the wet-bulb temperature, dew point, and a heat stress risk level.
- 4Check whether the inputs fall inside the supported approximation range.
- 5Use an on-site WBGT instrument and an applicable heat-safety plan for work or organized activity decisions.
- 6Act on heat-illness symptoms rather than waiting for the calculator to reach a particular label.
How It Works
Formula
Wet-bulb is computed using the Stull approximation, a validated empirical formula for temperatures between 5 and 50 degrees C and humidity between 5 and 99 percent.The wet-bulb temperature is estimated from the dry-bulb temperature and relative humidity using the Stull formula, which models the cooling effect of evaporation. The dew point uses the Magnus-Tetens approximation. Both are widely used in meteorology and occupational heat safety.
Calculation note: values are processed in the order shown above, using the current input units.
Worked Examples
Hot and humid day
The approximation returns 26.6°C, which falls between the code's 24°C and 28°C thresholds and receives the Moderate label. Hot-weather activity can still be hazardous because the label omits sun, workload, clothing, and personal risk factors.
Hot and dry day
The approximation returns 22.7°C and the code labels values below 24°C Comfortable. That label describes the modeled wet-bulb band only. A 40°C air temperature remains severe heat and can cause illness despite lower humidity.
Warm and humid conditions
At 30°C and 70% humidity, the wet-bulb estimate is 25.6°C and dew point is 23.9°C. Both show that evaporation is restricted compared with drier air.
Extreme modeled combination
The formula produces 41.5°C, above the code's 35°C threshold. This describes an emergency-level modeled condition, but an approximation should not replace verified observations and immediate protective action.
Wet-Bulb Risk Levels
Health risk levels based on wet-bulb temperature.
| Wet Bulb (C) | Risk Level | Description |
|---|---|---|
| Below 24 | Comfortable | Normal outdoor activity is fine |
| 24 to 28 | Moderate | Take breaks and stay hydrated during exertion |
| 28 to 32 | High | Risk of heat illness during extended activity |
| 32 to 35 | Very High | Dangerous for extended outdoor exposure |
| Above 35 | Extreme | Potentially fatal after several hours without cooling |
What this approximation does and does not measure
A ventilated wet-bulb thermometer wraps a wet wick around a temperature sensor. Evaporation cools the sensor, and the final reading depends on air temperature, moisture, airflow, and pressure. The Stull equation estimates that reading from dry-bulb temperature and relative humidity without measuring it directly.
Wet-bulb globe temperature, or WBGT, is a different heat-stress index. Outdoor WBGT incorporates natural wet-bulb, globe temperature, and dry-bulb measurements, which gives radiant heat and sun a role. A simple psychrometric wet-bulb estimate cannot substitute for an occupational WBGT limit.
Dew point is calculated separately with the Magnus-Tetens approximation. It describes atmospheric moisture and does not set the risk label. The code assigns that label only from wet-bulb thresholds at 24, 28, 32, and 35°C.
Heat illness can happen below any listed threshold. Work intensity, protective clothing, dehydration, chronic conditions, medications, age, pregnancy, poor sleep, and lack of acclimatization all matter. Confusion, collapse, seizures, or altered consciousness during heat exposure require emergency help and rapid cooling according to local guidance.
Common mistakes
- Confusing wet-bulb temperature with the feels-like or heat index. Wet-bulb temperature is a thermodynamic measure specific to evaporative cooling, not a comfort index.
- Using WBGT instead of wet-bulb. Wet Bulb Globe Temperature includes solar radiation and is different from the simple wet-bulb temperature this calculator provides.
- Assuming a low air temperature means low wet-bulb risk. High humidity can push the wet-bulb temperature into a risky range even when the air temperature is moderate.
- Calling this output WBGT even though it does not use globe temperature or solar radiation
- Treating the Comfortable label as proof that strenuous work is safe in very hot, dry air
- Entering humidity as 0.50 instead of 50 for 50% relative humidity
- Using measurements outside the supported formula domain
- Waiting for a threshold instead of responding to symptoms or an established heat-safety plan
Limitations
This calculator uses the Stull approximation only within 5 to 50°C and 5 to 99% relative humidity. Dew point uses Magnus-Tetens constants. Inputs are assumed to describe the same air sample and accurate instruments. The model does not account for atmospheric pressure, sensor exposure, direct sun, radiant surfaces, local wind, workload, clothing, hydration, medications, age, pregnancy, illness, acclimatization, or access to cooling. Its fixed risk bands are editorial labels, not regulatory exposure limits or individual medical thresholds. Estimated values can differ from a ventilated wet-bulb instrument or WBGT monitor.
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