Wet-Bulb Temperature Calculator
Estimate wet-bulb temperature from dry-bulb air temperature and relative humidity with the Stull approximation.
Inputs
Required. Enter a decimal point, not a decimal comma. Supported model range is – 20 to 50 °C.
Required. Use a value from 5% through 99%. Enter 40 for 40% relative humidity.
Live results
Wet-bulb temperature
16.38 °C
Estimated at standard sea-level pressure within the published approximation range.
Wet-bulb depression
8.62 °C
Dry-bulb temperature
25.00 °C
Model range
Within range
Temperature comparison
| Measure | Celsius | Fahrenheit | Kelvin |
|---|---|---|---|
| Dry-bulb temperature | 25.00 °C | 77.00 °F | 298.15 K |
| Wet-bulb temperature | 16.38 °C | 61.49 °F | 289.53 K |
| Wet-bulb depression | 8.62 °C | 15.51 °F | 8.62 K |
The table presents the same canonical calculation in three temperature scales. Kelvin differences use the same numerical magnitude as Celsius differences.
How to use the wet-bulb temperature calculator
What this calculator does
This calculator estimates the wet-bulb temperature of air from two measured conditions: dry-bulb air temperature and relative humidity. Wet-bulb temperature represents the approximate lowest temperature a ventilated, water-wetted surface can reach through evaporation under the stated conditions. The calculation uses the empirical Stull approximation at standard sea-level pressure. It is useful for screening and education, but it is not a substitute for a ventilated psychrometer, a full psychrometric calculation at the actual pressure, or a certified heat-stress instrument.
When to use it
Use the estimate when comparing how efficiently evaporative cooling may work in dry versus humid air, when checking weather conditions that may favor rain or snow transitions, when planning snowmaking or evaporative cooling, or when teaching the relationship among dry-bulb temperature, humidity, and wet-bulb temperature. For workplace or athletic heat decisions, do not confuse this value with Wet-Bulb Globe Temperature. The National Weather Service explanation of WBGT shows that WBGT also accounts for wind and solar radiation.
How to calculate
- The calculator opens with a complete demonstration: an air temperature of 25 °C and relative humidity of 40%. The result and a validated Excel workbook are available immediately.
- Replace Air temperature with your measured dry-bulb value. Choose °C or °F; changing the unit converts the current entry rather than merely relabeling it.
- Enter Relative humidity as a percentage from 5 through 99. Use a decimal point for fractions, such as 62.5. A decimal comma, scientific notation, or an added percent sign is rejected so the value cannot be misread.
- Read Wet-bulb temperature first, then review Wet-bulb depression, Dry-bulb temperature, and Model range. The comparison table gives the same values in Celsius, Fahrenheit, and Kelvin.
- Select Download Excel to create a current-state OOXML workbook with Summary, Inputs, and Notes sheets. Select Reset to clear the demonstration data. Reset may disable the download until both required inputs are complete and valid again.
Input guide
Air temperature is a required decimal temperature measured by a normal, shaded thermometer. The selected Temperature unit determines whether the entry is interpreted as Celsius or Fahrenheit. The supported Stull range is – 20 to 50 °C, equivalent to – 4 to 122 °F. A realistic entry is 25 °C. Higher air temperature usually raises the wet-bulb estimate, while changing units should not change the physical condition. Common mistakes include entering a sun-heated surface temperature, typing a comma as a decimal separator, or entering a Fahrenheit number while °C is selected.
Relative humidity is required and is entered as a percentage, not a fraction. Enter 40 for 40%, not 0.40. The supported range is 5% through 99%. At a fixed air temperature, higher humidity generally raises the wet-bulb temperature because evaporation becomes less effective. A common mistake is to use a humidity value from a different time or location than the temperature measurement.
Output guide
Wet-bulb temperature is the primary estimate in the selected display unit. It is driven by both air temperature and humidity. A value close to the dry-bulb temperature indicates humid air and limited evaporative cooling; a much lower value indicates drier air and greater evaporative cooling potential. It is an empirical estimate, not a direct safety recommendation.
Wet-bulb depression is dry-bulb temperature minus wet-bulb temperature. A depression near zero corresponds to nearly saturated air. A larger positive depression indicates greater evaporative cooling potential. Near the published boundaries, the approximation can produce a very small negative depression because of fitting error; treat that as a limitation of the approximation rather than a physical claim.
Dry-bulb temperature repeats the canonical converted input so you can compare it directly with the wet-bulb estimate. Model range confirms whether the inputs are inside the supported domain. The Temperature comparison table has rows for dry-bulb temperature, wet-bulb temperature, and wet-bulb depression, with columns for Celsius, Fahrenheit, and Kelvin. These are exact unit conversions of the same model result, not separate calculations.
Worked example
With the startup values of 25 °C and 40% relative humidity, the Stull equation gives a wet-bulb temperature of 16.3841 °C, displayed as 16.38 °C. The wet-bulb depression is 25 – 16.3841 = 8.6159 °C, displayed as 8.62 °C. Converting the wet-bulb result gives 61.49 °F and 289.53 K. The independently published formula and its validity range are described in Roland Stull's wet-bulb approximation paper.
How the approximation works
The Stull equation is an empirical fit that estimates wet-bulb temperature directly from temperature and relative humidity at standard sea-level pressure. It avoids an iterative psychrometric solution, making it practical for a lightweight browser calculator. The model is published for temperatures from – 20 to 50 °C and relative humidity from 5% to 99%. Its reported errors are larger in colder, drier conditions, so precision shown to hundredths should not be interpreted as measurement-grade accuracy.
Here, T and Tw are in degrees Celsius, and RH is the relative humidity percentage. Fahrenheit inputs are converted to Celsius before the equation is evaluated, then outputs are converted back for display.
Interpreting wet-bulb temperature responsibly
A wet-bulb thermometer has a water-wetted wick and is ventilated so evaporation cools the sensing bulb. The National Weather Service dry- and wet-bulb training page explains this physical measurement. At very high relative humidity, evaporation slows and wet-bulb temperature approaches dry-bulb temperature. In drier air, evaporation is more effective and the wet-bulb reading falls farther below the dry-bulb reading.
Wet-bulb temperature alone does not include sunlight, radiant heat, wind speed, clothing, activity, hydration, age, or medical vulnerability. For occupational heat programs, the CDC and NIOSH heat-stress guidance notes that WBGT-based limits are preferred when radiant heat or other complex exposures matter. Use local official warnings and appropriate instruments for safety-critical decisions.