Wet Bulb Temperature Calculator

Why wet-bulb temperature matters for evaporative cooling

Wet-bulb temperature explains why the same dry-bulb air temperature can feel very different as humidity changes. Air temperature alone does not show how readily people, animals, plants, or equipment can lose heat through evaporation. In dry air, sweat or water on a surface evaporates more readily and carries heat away; in humid air, evaporation slows and that cooling pathway becomes less effective.

Wet-bulb temperature (often written as T_w) combines air temperature and humidity into a value that approximates the lowest temperature air can reach through evaporative cooling at constant pressure. When relative humidity nears 100%, wet-bulb temperature approaches dry-bulb temperature because a wetted surface has little remaining opportunity to cool by evaporation.

The wet-bulb concept is useful because it focuses on the moisture exchange that a conventional thermometer does not show. A wet-bulb thermometer is traditionally covered with a wetted wick and exposed to airflow. Evaporation from that wick removes heat, lowering its reading below the surrounding air temperature when the air can accept more water vapor. The amount of lowering depends strongly on humidity.

What this wet-bulb calculator estimates

This wet-bulb calculator estimates an evaporation-limited temperature from the two weather inputs below:

• Air temperature T in °C (dry-bulb temperature)
• Relative humidity RH in % (0–100)

Wet-bulb temperature can be obtained through full psychrometric relationships, but this page uses a well-known empirical approximation for a fast estimate under typical outdoor conditions. Treat the displayed value as an estimate, not as a laboratory-grade psychrometric measurement.

The calculator does not ask for elevation, station pressure, wind speed, solar exposure, clothing, activity level, or a measured wick temperature. Those omissions are deliberate: the displayed result is the formula’s humidity-and-temperature estimate, not a replacement for an instrumented site assessment. Enter the best available shaded air temperature and relative humidity, and keep both inputs in the units shown.

Wet-bulb temperature variables

This wet-bulb calculation uses the following symbols:

• T = dry-bulb air temperature (°C)
• RH = relative humidity (%)
• T_w = wet-bulb temperature (°C)

Relative humidity is a percentage, not a decimal fraction in this calculator. For example, an observation reported as 60% should be entered as 60. The approximation uses the temperature and humidity values directly, then reports the calculated wet-bulb temperature to one decimal place. A result should normally be considered alongside the original dry-bulb temperature rather than in isolation.

Wet-bulb estimation formula (empirical approximation)

The calculator applies the Stull (2011) approximation for wet-bulb temperature over typical meteorological ranges:

Note: atan is the arctangent function. The formula is useful here because it produces a quick wet-bulb estimate without requiring station pressure or elevation. It is an empirical relationship, so it is intended to approximate the output of more detailed psychrometric methods rather than reproduce every physical condition of an individual measurement.

Because the formula contains nonlinear terms, wet-bulb temperature does not change by one fixed amount for every percentage-point change in humidity. The effect of a humidity change also depends on dry-bulb temperature. That is why it is better to recalculate after changing either input than to infer a result from a simple rule of thumb.

How to interpret the wet-bulb temperature result

Interpret this wet-bulb result by comparing T_w with the dry-bulb temperature you entered:

• If T_w is much lower than T, the air is relatively dry and evaporation from sweat, evaporative coolers, or cooling towers can remove heat more effectively.
• If T_w is close to T, the air is humid and evaporation offers less cooling. The same air temperature may therefore feel more oppressive.

For heat stress, wet-bulb temperature is relevant because it relates to the body’s ability to cool itself through perspiration. It is not a stand-alone safety assessment: wind, sun exposure, clothing, workload, acclimatization, hydration, and medical factors also affect real-world risk.

A wet-bulb reading should not be treated as a personal prediction of comfort or illness. Air movement can aid sweat evaporation, while direct sunlight and radiant heat can increase heat load even if air temperature and humidity remain unchanged. Conversely, a person indoors, at rest, in shade, and with adequate cooling may experience conditions very differently from someone doing strenuous outdoor work. Use the estimate as context, then apply the relevant workplace, athletic, agricultural, or public-health guidance.

Worked example: estimating wet-bulb temperature at 32 °C and 60% RH

For a wet-bulb estimate with air temperature T = 32 °C and relative humidity RH = 60%, the formula used by this calculator gives a wet-bulb temperature of approximately 25 °C when rounded.

Interpretation: At 32 °C and 60% relative humidity, a wet-bulb value near 25 °C shows that humidity is restricting evaporation. That is more burdensome for sweating and other evaporative cooling than a 32 °C day with much lower humidity. The result is not obtained by adding the entered values; it comes from the empirical wet-bulb relationship shown above.

If either input changes, the estimate should be recalculated. Raising humidity at the same dry-bulb temperature generally moves wet-bulb temperature upward and narrows the gap between the two temperatures. Lowering humidity generally increases the evaporation potential and widens that gap. At the same relative humidity, a change in air temperature can also change the wet-bulb result and the size of the difference.

Wet-bulb temperature versus related weather measures

Wet-bulb temperature is distinct from several other temperature and moisture measures used in weather, comfort, and engineering:

Dry-bulb temperature is the ordinary air temperature reported by a shielded thermometer. Dew point instead describes the temperature at which air would become saturated if cooled, making it especially useful for condensation and moisture discussions. Heat index is a separate human-comfort index with its own assumptions. Wet-bulb temperature is most directly concerned with the temperature reached by evaporation under the applicable conditions.

Wet-bulb estimate assumptions, valid ranges, and limitations

This wet-bulb calculator is designed for a quick humidity-and-temperature estimate, so keep the following limits in mind:

• Units: Inputs are interpreted as °C and %RH. Relative humidity should be between 0 and 100.
• Approximation rather than a direct psychrometric solve: The Stull-style formula is empirical, so its accuracy varies by conditions. It is commonly reported to be within about 1 °C for many near-surface weather conditions, with larger errors possible at extremes.
• Typical operating range: The estimate is best suited to ordinary meteorological temperatures and humidities. High-precision engineering work should use a full psychrometric calculation with station pressure or altitude and validated measurements.
• Pressure and altitude are not inputs: True wet-bulb temperature depends on atmospheric pressure. This simplified calculation does not explicitly model pressure or elevation.
• Not a safety device: Heat illness risk depends on more than wet-bulb temperature. Follow applicable official guidance and measured on-site conditions for occupational or athletic decisions.

Check the source and timing of weather observations before relying on an estimate. A humidity value measured at a nearby station may not represent a shaded work area, greenhouse, field, roof, or enclosed space. Sensors can also differ in placement, ventilation, calibration, and reporting interval. When a decision depends on a narrow margin, direct local measurement and the appropriate professional method are more suitable than a simplified web estimate.

The calculator accepts valid humidity values from 0 through 100, but an accepted input is not a guarantee that an empirical formula is equally accurate at every extreme. If the result will support engineering design, controlled-environment work, or a safety-critical decision, confirm it with a pressure-aware psychrometric method and measurements appropriate to that setting.

Wet-bulb temperature references and method credibility

The wet-bulb approximation and the broader humidity context on this page are based on the following sources:

• Stull, R. (2011). Wet-Bulb Temperature from Relative Humidity and Air Temperature. (Commonly cited for the fast approximation used in many calculators.)
• NOAA/NWS psychrometric and heat-stress educational materials (general background on humidity and human comfort).

The cited approximation is useful for quick comparison and education because it requires only the two inputs available on this page. It should not be confused with a full psychrometric chart calculation, a sling psychrometer observation, or a heat-stress program that includes environmental and personal exposure factors. Those tools answer related but more specialized questions.