Wet Bulb Temperature Calculator

Why wet-bulb temperature matters

Air temperature alone (often called the dry-bulb temperature) doesn’t tell you how hard it is for people, animals, or equipment to shed heat. The missing piece is humidity. When the air is dry, sweat (or any water on a surface) can evaporate readily, carrying heat away and providing strong cooling. When the air is humid, evaporation slows down, and that cooling mechanism becomes less effective.

Wet-bulb temperature (often written as T_w) combines temperature and humidity into a single value that approximates the lowest temperature air can reach by evaporative cooling at constant pressure. As relative humidity approaches 100%, wet-bulb temperature approaches dry-bulb temperature because evaporation can no longer cool the wetted surface much.

What this calculator estimates

This calculator estimates wet-bulb temperature from:

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

Because wet-bulb temperature is defined through psychrometric relationships, there are multiple ways to compute it. Practical web calculators often use a well-known empirical approximation that is fast and reasonably accurate for typical outdoor weather conditions. The output here should be treated as an estimate, not a laboratory-grade psychrometric measurement.

Core idea and variables

We’ll use the following symbols:

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

Estimation formula (empirical approximation)

A commonly used approximation for wet-bulb temperature over typical meteorological ranges is the Stull (2011) formula. One way to write it is:

Note: atan is the arctangent function. This approximation is popular because it is computationally light while remaining fairly accurate for everyday ranges.

How to interpret the result

Wet-bulb temperature is often most useful in comparison with the dry-bulb temperature:

• If T_w is much lower than T, the air is relatively dry and evaporative cooling (sweat, swamp coolers, cooling towers) is more effective.
• If T_w is close to T, the air is humid and evaporation provides little cooling. People may feel “sticky” or overheat more easily at the same air temperature.

For heat stress, wet-bulb temperature is an important signal because it relates to the body’s ability to cool itself through perspiration. However, real-world risk also depends on wind, sun exposure, clothing, workload, acclimatization, hydration, and medical factors—so treat this as one input into decisions, not the only one.

Worked example

Example inputs: Air temperature T = 32 °C, Relative humidity RH = 60%.

Using the approximation above, the estimated wet-bulb temperature is approximately:

• T_w ≈ 25 °C (rounded)

Interpretation: Even though the air temperature is 32 °C, the wet-bulb temperature around 25 °C indicates evaporation is constrained by humidity. You can expect it to feel significantly more oppressive than a 32 °C day with low humidity, and activities that rely on evaporative cooling (including sweating) will be less effective.

Wet-bulb vs. related measures

Assumptions, valid ranges, and limitations

• Units: Inputs are interpreted as °C and %RH. Relative humidity should be between 0 and 100.
• Approximation (not a direct psychrometric solve): The Stull-style formula is empirical; accuracy varies with conditions. It is generally reported as being within about ~1 °C for common near-surface weather ranges, but errors can be larger at extremes.
• Typical operating range: Best suited for everyday meteorological conditions (roughly “normal” outdoor temperatures and humidities). If you need high precision for engineering work, use a full psychrometric calculation with station pressure (or altitude) and validated instrument data.
• Pressure/altitude not modeled: True wet-bulb temperature depends on atmospheric pressure. This simplified estimate does not explicitly take pressure or elevation as an input.
• Not a safety device: Heat illness risk depends on more than wet-bulb temperature. For occupational or athletic safety decisions, follow official guidance and measured on-site conditions.

References (method credibility)

• 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).