Wet Bulb Calculator
Calculate thermodynamic wet-bulb temperature from dry-bulb temperature and relative humidity, with optional pressure or elevation correction.
Calculator is for informational purposes only. Terms and Conditions
The calculator solves the ASHRAE psychrometric relationship iteratively and accounts for atmospheric pressure.
Choose the calculation setup
Choose what you know and your preferred display units.
Enter the known values
Use measured air conditions when available. Pressure settings are available under Advanced Options.
The default example is 86°F dry bulb and 50% relative humidity at standard sea-level pressure.
Result
Thermodynamic result first, followed by related psychrometric properties and calculation checks.
Psychrometric properties
- Check—
Show calculation steps Review pressure, vapor pressure, humidity ratio, iteration, and checks
- Enter valid values to see the complete calculation.
Air-State Temperature Relationship
Compare dry bulb, thermodynamic wet bulb, and dew point for the calculated air state.
For unsaturated air, dew point ≤ wet bulb ≤ dry bulb.
Method, Sources, and Assumptions
Calculation basis, reference equations, pressure handling, limits, and verification guidance.
Uses the ASHRAE perfect-gas psychrometric relationships implemented by PsychroLib, with thermodynamic wet-bulb temperature solved numerically by bisection.
- Standard pressure is 101.325 kPa unless elevation or station pressure is selected.
- The implemented equations use the ASHRAE perfect-gas formulation; current ASHRAE guidance also describes non-ideal saturation enhancement-factor refinements for higher-precision work.
- Thermodynamic wet-bulb temperature is not the same quantity as WBGT or natural wet-bulb temperature used in heat-stress measurements.
- Use measured local station pressure for the best pressure correction; elevation mode uses a standard-atmosphere estimate.
Calculator guide
Wet Bulb Temperature Guide
The Wet Bulb Calculator determines thermodynamic wet-bulb temperature from dry-bulb temperature and relative humidity, or from dry bulb and dew point. It can also work in reverse to estimate relative humidity from dry-bulb and wet-bulb temperatures. Atmospheric pressure matters, so the calculator can use standard sea-level pressure, elevation-based pressure, or a measured station pressure.
Wet-bulb temperature describes the evaporative-cooling state of moist air. For ordinary unsaturated air, it lies between dew point and dry-bulb temperature. As relative humidity rises, evaporation becomes less effective and wet bulb moves closer to dry bulb; at saturation, all three temperatures converge.
- Fastest input pair
- Dry-bulb temperature + relative humidity
- Primary output
- Thermodynamic wet-bulb temperature
- Key physical check
- Dew point ≤ wet bulb ≤ dry bulb
How to Use the Wet Bulb Calculator
For the most common calculation, select “Wet bulb from dry bulb + RH,” enter the measured air temperature and relative humidity, and read the wet-bulb result. The calculator updates automatically and keeps U.S. and SI unit selections physically equivalent.
-
Choose the calculation
Use Wet bulb from dry bulb + RH when you have a temperature-and-humidity sensor reading. Use Relative humidity from dry bulb + wet bulb for a psychrometer-style measurement. Use Wet bulb from dry bulb + dew point when weather or process data provides dew point instead of RH.
-
Enter values from the same air condition
Dry bulb, RH, wet bulb, or dew point should describe the same location and time. Mixing an indoor temperature with outdoor humidity, or readings taken before a process stabilized, can create a precise-looking but meaningless state.
-
Set the pressure basis when it matters
Leave pressure at standard sea level for a general comparison near sea level. Choose elevation when you need the calculator to estimate pressure from the standard atmosphere, or choose station pressure when an actual absolute barometric pressure is available.
-
Check the related psychrometric results
Use dew point, wet-bulb depression, humidity ratio, enthalpy, vapor pressure, specific volume, and atmospheric pressure as consistency checks. The wet-bulb result is the primary answer; the secondary values help explain the same moist-air state.
Inputs and Results Explained
Wet-bulb temperature is not determined by air temperature alone. The calculator needs a second humidity property and a pressure basis because moisture content, temperature, and total pressure jointly define the psychrometric state.
- Dry-bulb temperature
- The ordinary air temperature measured by a dry thermometer or temperature sensor. Do not substitute heat index, apparent temperature, or a hot surface temperature.
- Relative humidity
- The ratio of actual water-vapor partial pressure to saturation vapor pressure at the same dry-bulb temperature. Enter it as 0% to 100%, not as a decimal fraction.
- Wet-bulb temperature
- The thermodynamic wet-bulb temperature associated with evaporative saturation at the selected pressure. In reverse mode, the calculator uses dry bulb and wet bulb to determine the implied RH.
- Dew point
- The saturation temperature corresponding to the actual water-vapor pressure. For an ordinary unsaturated state, dew point cannot exceed dry-bulb temperature.
- Atmospheric pressure
- Total absolute air pressure used in the humidity-ratio and wet-bulb relationships. Standard sea-level pressure is 101.325 kPa; actual station pressure is preferable when pressure accuracy matters.
- Wet-bulb depression
- The temperature difference between dry bulb and wet bulb. A larger depression means greater evaporative-cooling potential under the same general airflow and equipment conditions.
- Humidity ratio
- Mass of water vapor per mass of dry air. The calculator displays it as g/kg dry air in SI or grains/lb dry air in U.S. units.
- Enthalpy and specific volume
- Related moist-air properties calculated from the same pressure-corrected state. They are useful for HVAC process checks but are secondary to the wet-bulb result on this page.
How Wet Bulb Temperature Is Calculated
The calculator uses ASHRAE-based perfect-gas psychrometric relationships and solves wet-bulb temperature numerically. It first establishes the moisture content of the air, then finds the wet-bulb temperature whose saturated wet-bulb relationship produces that same humidity ratio at the selected pressure.
Humidity ratio from vapor pressure
In plain language: the calculator converts water-vapor partial pressure into a mass ratio of water vapor to dry air. Total atmospheric pressure appears in the denominator, which is why elevation and station pressure can change the result.
Wet-bulb relationship above freezing
The calculator varies \(T_{wb}\) until this wet-bulb expression matches the humidity ratio established by the known air condition. Below 0°C wet bulb, it switches to the ASHRAE ice-bulb branch used by the calculator.
ASHRAE states that perfect-gas relationships are suitable for most air-conditioning calculations, while more detailed real-gas formulations are available where greater precision is required.
- \(T_{db}\)
- Dry-bulb temperature Ordinary air temperature for the moist-air state.
- \(T_{wb}\)
- Wet-bulb temperature Unknown thermodynamic wet-bulb temperature when wet bulb is being solved.
- \(W\)
- Humidity ratio Mass of water vapor divided by mass of dry air.
- \(W_s^*\)
- Saturated humidity ratio at wet bulb Humidity ratio of saturated air evaluated at the candidate wet-bulb temperature and selected pressure.
- \(p_w\)
- Water-vapor partial pressure Actual portion of total pressure contributed by water vapor.
- \(P\)
- Total atmospheric pressure Absolute pressure for the air state, from standard pressure, elevation, or station pressure.
Why not use only the Stull approximation?
Roland Stull’s 2011 equation is a useful explicit approximation when only dry-bulb temperature and RH are available. The published correlation is for standard sea-level pressure, 5% to 99% RH, and roughly −20°C to 50°C, with a reported mean absolute error below 0.3°C over its stated valid region. This calculator instead uses pressure-aware psychrometric relationships so elevation or measured station pressure can be included rather than silently assuming 101.325 kPa for every calculation.
Worked Example: 86°F and 50% RH
This example reproduces the calculator’s default air state: 86°F dry bulb, 50% relative humidity, and standard sea-level pressure.
Pressure-aware psychrometric procedure
- At 30°C, the calculator evaluates saturation vapor pressure as approximately 4.246 kPa.
- At 50% RH, actual water-vapor partial pressure is approximately 2.123 kPa.
- Using \(W=0.621945p_w/(P-p_w)\), the humidity ratio is approximately 0.01331 kg/kg dry air, or 93.17 grains/lb dry air.
- The wet-bulb solver searches between dew point and dry bulb until the ASHRAE wet-bulb humidity ratio matches 0.01331 kg/kg.
- The converged wet bulb is approximately 22.01°C, which is 71.61°F.
Result
Wet bulb ≈ 71.61°F (22.01°C)
The corresponding wet-bulb depression is about 14.39°F. The calculator also gives a dew point of about 65.20°F, confirming the expected ordering of dew point below wet bulb below dry bulb.
How to Interpret Wet Bulb Temperature
A wet-bulb result is most useful when compared with the dry-bulb temperature, dew point, and the process you are evaluating. The number is not a generic comfort score; it is a thermodynamic property of the moist-air state.
Wet-bulb depression
Calculate \(T_{db}-T_{wb}\). A larger depression means the air can support more evaporative cooling. As the depression approaches zero, the air approaches saturation and direct evaporation has little remaining temperature-driving potential.
Humidity sensitivity
Hold dry bulb at 90°F and standard pressure: this calculator gives about 62.8°F wet bulb at 20% RH, 71.2°F at 40%, 78.3°F at 60%, 84.5°F at 80%, and 90°F at 100%. Higher RH pushes wet bulb toward dry bulb.
Sanity check
For ordinary unsaturated air, dew point should not exceed wet bulb and wet bulb should not exceed dry bulb. If wet bulb equals dry bulb, the state is at or extremely near 100% RH.
| Dry Bulb | 20% RH | 40% RH | 60% RH | 80% RH | 100% RH |
|---|---|---|---|---|---|
| 60°F | 43.4°F | 48.0°F | 52.3°F | 56.3°F | 60.0°F |
| 70°F | 50.0°F | 55.8°F | 61.0°F | 65.7°F | 70.0°F |
| 80°F | 56.4°F | 63.5°F | 69.6°F | 75.1°F | 80.0°F |
| 90°F | 62.8°F | 71.2°F | 78.3°F | 84.5°F | 90.0°F |
| 100°F | 69.1°F | 79.0°F | 87.1°F | 94.0°F | 100.0°F |
HVAC, Evaporative Cooling, and Cooling Towers
Wet bulb is useful because it connects the moisture condition of the air to evaporative heat transfer. The same result has different practical meanings in an evaporative cooler, cooling tower, HVAC coil check, or field psychrometer measurement.
Evaporative cooling
Pacific Northwest National Laboratory’s Building America guidance uses wet-bulb temperature as the ideal evaporative-cooling limit for the entering air state and notes that cooler effectiveness depends strongly on the dry-bulb-to-wet-bulb difference. A real direct evaporative cooler generally leaves air above thermodynamic wet bulb because effectiveness is less than 100%.
Cooling towers
For an evaporative cooling tower, “approach” is the difference between cold-water temperature and entering-air wet-bulb temperature. Wet bulb therefore acts as the reference lower boundary for the evaporative process, while actual tower performance depends on tower design, airflow, water flow, fill condition, load, and ambient conditions.
HVAC air-state work
Wet bulb is one coordinate of the broader psychrometric state. Engineers may use it with dry bulb and pressure to determine moisture-related properties and to evaluate air-conditioning processes, but equipment selection also requires load, airflow, manufacturer data, and the relevant design conditions.
Psychrometer measurements
A traditional psychrometer compares a dry thermometer with a wetted thermometer exposed to airflow. That wetted-sensor reading is an instrument measurement, while this calculator reports thermodynamic wet-bulb temperature from psychrometric equations. Under properly controlled measurement conditions they are closely related, but the distinction matters for high-accuracy work; poor wetting, radiation exposure, weak airflow, or reading before stabilization can bias the measured value.
Evaporative cooler effectiveness
PNNL uses this relationship to express how closely supply air approaches the entering wet bulb. The denominator is the wet-bulb depression. If dry bulb is 95°F and wet bulb is 70°F, the theoretical evaporative span is 25°F; a cooler with 80% effectiveness would have a simplified leaving-air estimate of \(95-0.80(25)=75^\circ F\), before accounting for equipment-specific limitations.
Common Wet Bulb Mistakes
Most wet-bulb errors come from mixing different air states, using the wrong pressure or temperature concept, or treating a psychrometric result as a safety or equipment rating it was never intended to be.
Entering 0.50 instead of 50%
The relative-humidity field is percent. Enter 50 for 50% RH. Entering 0.50 means one-half of one percent, a dramatically drier air state.
Using wet bulb above dry bulb
For this moist-air model, wet bulb cannot exceed dry bulb. The calculator blocks that combination. Recheck sensor labels, units, wetting, airflow, and whether both readings came from the same air sample.
Confusing dew point with wet bulb
Dew point is the condensation/saturation temperature for the existing vapor pressure. Wet bulb is the evaporative-saturation temperature. Use dew point, not wet bulb, when checking whether a surface is cold enough for condensation.
Ignoring elevation for precision work
Standard pressure is a useful baseline, but ASHRAE psychrometric relationships include total pressure. At high elevation, use elevation correction or measured station pressure rather than assuming sea-level pressure.
Using the Stull formula outside its stated scope
The Stull approximation is a standard-sea-level empirical fit with a published range of 5% to 99% RH and −20°C to 50°C, with exclusions for very cold, dry combinations. Do not extend an approximation beyond its stated domain just because it returns a number.
Treating wet bulb as WBGT
WBGT incorporates additional environmental heat-stress effects and uses a natural wet-bulb measurement as one component. Thermodynamic wet bulb from this calculator is not interchangeable with WBGT.
Assumptions and Limits
The calculator is a pressure-aware perfect-gas engineering psychrometric solver, but it is still a model. Its result is only as representative as the entered air condition and pressure basis.
Perfect-gas psychrometrics
The implementation uses ASHRAE-based perfect-gas moist-air relationships. ASHRAE notes that these are appropriate for most air-conditioning problems, while more detailed real-gas formulations may be used when greater thermodynamic precision is required.
Pressure must stay physically valid
The calculator requires total pressure to remain above water-vapor saturation pressure for the entered state. Extreme high-temperature/low-pressure combinations can fall outside that physical domain and are blocked rather than reported as normal results.
Below-freezing wet bulb
The calculator switches to an ASHRAE ice-bulb form below 0°C. Conditions close to freezing deserve extra care because actual wetted-sensor behavior depends on whether the wick is liquid or frozen and on measurement practice.
Field measurements can dominate numerical precision
Displayed digits indicate numerical resolution, not measurement uncertainty. A result shown to hundredths of a degree does not mean field sensors are accurate to hundredths; sensor calibration, airflow, radiation shielding, water purity and wick condition, spatial gradients, and transient operation can create larger uncertainty than numerical iteration.
Sources and Verification
The calculator method and this guide were checked against current ASHRAE psychrometric guidance, the PsychroLib implementation of ASHRAE relationships, peer-reviewed wet-bulb literature, and authoritative application guidance.
- ASHRAE Handbook—Fundamentals 2025, Chapter 1: Psychrometrics — supports moist-air properties, standard atmosphere, pressure dependence, perfect-gas relationships, and higher-precision real-gas options.
- PsychroLib Overview — documents wet-bulb, dew-point, RH, humidity-ratio, and pressure relationships based on ASHRAE Handbook equations.
- Stull (2011), Wet-Bulb Temperature from Relative Humidity and Air Temperature — supports the commonly cited explicit Stull approximation and its published standard-pressure validity range and error statement.
- Romps (2026), Wet-Bulb Temperature from Pressure, Relative Humidity, and Air Temperature — provides current peer-reviewed treatment of pressure-aware thermodynamic and psychrometric wet-bulb definitions and calculation methods.
- PNNL Building America: Evaporative Cooling Systems — supports wet-bulb depression, evaporative-cooler effectiveness, and the relationship between entering wet bulb and achievable evaporative cooling.
- Vecellio et al. (2022), Evaluating the 35°C Wet-Bulb Temperature Adaptability Threshold for Young, Healthy Subjects — supports the statement that uncompensable heat stress can occur below the often-cited theoretical 35°C wet-bulb limit and that critical conditions vary with the environment.
- OSHA Heat Hazard Recognition — supports using WBGT rather than ordinary wet bulb or heat index alone for more complete worksite heat-hazard assessment.
- Met Office: What Wet Bulb Temperatures Are and What They Are Used For — supports the evaporative measurement principle, saturation behavior, and traditional wet-bulb measurement context.
The 86°F, 50% RH worked example was recomputed from the same published psychrometric relationships and checked in reverse: the resulting wet bulb, paired with the original dry bulb and pressure, reproduces approximately 50% RH. Unit equivalence was also checked by repeating the same state as 30°C and 50% RH.
Wet Bulb Calculator FAQ
These questions address the distinctions and edge cases that most often change how a wet-bulb result should be used.
What is wet-bulb temperature?
Wet-bulb temperature is the temperature associated with evaporative saturation of moist air at a given pressure. A wetted, ventilated thermometer cools as water evaporates; drier air permits more evaporation and therefore a larger drop below dry-bulb temperature.
Can wet-bulb temperature be higher than dry-bulb temperature?
Not for the ordinary unsaturated moist-air state modeled here. The calculator enforces wet bulb ≤ dry bulb. If a field reading shows the opposite, check the sensors, units, wick condition, airflow, radiation exposure, and whether the readings were taken from the same air sample.
What happens to wet bulb at 100% relative humidity?
At saturation there is no remaining evaporative cooling potential, so wet-bulb temperature equals dry-bulb temperature. Dew point also equals dry bulb at the saturated state.
What is the difference between wet bulb and dew point?
Dew point is the temperature at which the existing water vapor reaches saturation during cooling without adding moisture. Wet bulb is the evaporative-saturation temperature. For ordinary unsaturated air, dew point is at or below wet bulb, and wet bulb is at or below dry bulb.
Does elevation affect wet-bulb temperature?
Yes. Wet-bulb psychrometric relationships depend on total atmospheric pressure, and standard atmospheric pressure decreases with elevation. The effect is especially worth accounting for at substantial elevation or when comparing engineering calculations across sites.
What is wet-bulb depression?
Wet-bulb depression is dry-bulb temperature minus wet-bulb temperature. A larger depression generally indicates greater evaporative-cooling potential; a depression near zero indicates air close to saturation.
Is wet bulb the same as WBGT?
No. This calculator returns thermodynamic wet-bulb temperature. WBGT is a heat-stress index that incorporates additional environmental effects and uses natural wet-bulb temperature as one component. OSHA recommends WBGT for more complete occupational heat assessment.
What wet bulb temperature is dangerous?
There is no single thermodynamic wet-bulb temperature that universally separates safe from dangerous conditions. The often-cited 35°C (95°F) value is a theoretical physiological limit under specific assumptions, while controlled experiments in young healthy adults found uncompensable heat stress at substantially lower wet-bulb conditions, including a mean critical value near 30.6°C in the warm-humid environments studied. Risk also depends on air temperature, radiant heat, airflow, activity, clothing, exposure duration, age, and health. For workplace decisions, use WBGT and applicable heat-safety guidance rather than this thermodynamic wet-bulb result alone.
Can wet-bulb temperature be below freezing?
Yes. Subfreezing wet-bulb conditions are possible. This calculator uses the ASHRAE ice-bulb branch below 0°C, but real wetted-sensor measurements near freezing require care because the phase and condition of the wick affect the measurement.