Wet Bulb Calculator

What the wet bulb calculator shows

Wet bulb temperature is the lowest temperature air can reach by evaporating water into it. A thermometer wrapped in a wet wick reads it directly, because evaporation cools the wick until the air can take no more moisture. The wet bulb calculator gives the same value from the dry bulb temperature and relative humidity, plus the wet bulb depression, the gap between the two, which shows how dry the air is and how much evaporative cooling is possible.

Wet bulb formula

The calculator uses the empirical formula published by Roland Stull in 2011, with T in °C and RH in percent:

It is valid from 5% to 99% relative humidity and from −20°C to 50°C, with errors mostly under 0.3°C. Fahrenheit inputs are converted to Celsius, solved, and converted back.

Wet bulb calculator example

Air at 20°C and 50% relative humidity:

That matches the worked example in Stull’s paper. On a hot day of 95°F at 40% humidity, the wet bulb calculator gives 76.1°F, a depression of 18.9°F.

How to use the wet bulb calculator

Choose Celsius or Fahrenheit, enter the air temperature from a normal thermometer, and enter the relative humidity in percent. Press Calculate to read the wet bulb temperature and the depression. Take both readings at the same spot and time, out of direct sun. The formula assumes sea level air pressure, which covers most uses with good accuracy.

Reading the wet bulb temperature

The wet bulb always sits between the dew point and the dry bulb, and it equals the dry bulb only at 100% humidity. A large wet bulb depression means dry air and strong evaporative cooling; a small one means humid air where sweat and swamp coolers do little. For people, a wet bulb near 35°C is the theoretical limit where the body can no longer shed heat by sweating, and serious heat stress starts far below that during work or exercise.

When to use a wet bulb temperature calculator

• Finding indoor wet bulb for an HVAC charge check
• Sizing evaporative coolers and cooling towers
• Judging heat stress for outdoor work and sport
• Checking spray and coating conditions
• Reading a psychrometric chart point without the chart

Common wet bulb calculator mistakes

• Mixing units. Pick the unit that matches your thermometer.
• Entering humidity as a fraction. Enter 50 for 50%, not 0.5.
• Measuring in sun. Direct sun raises the dry bulb reading.
• Confusing wet bulb with dew point. They are different values.

Tips for accurate wet bulb readings

• Let your thermometer and hygrometer settle for a few minutes.
• Measure HVAC wet bulb at the return grille.
• Recheck during the hottest part of the day for heat stress planning.

Related psychrometric tools

Wet bulb is one of several moist air values. The dew point calculator finds the condensation point, the humidity calculator works back to relative humidity, and the heat index calculator gives the felt temperature. This wet bulb calculator fills in the evaporation limit.

How to calculate wet bulb temperature from relative humidity

The exact wet bulb comes from the psychrometric equation, which balances the heat drawn from the air against the heat needed to evaporate water into it. Solving it needs vapor pressure tables and several steps of iteration, which is why technicians long used a sling psychrometer and a chart. Stull fitted a single equation to the exact solution, so you can calculate wet bulb from relative humidity and temperature in one step. Its accuracy is good from 5% to 99% humidity and from −20°C to 50°C; outside that window the wet bulb calculator flags the result. If you know the dew point instead of the humidity, convert it first with the humidity calculator, then enter the humidity here.

Wet bulb depression in HVAC and cooling

Evaporative coolers can cool supply air to within about 70 to 90% of the wet bulb depression, so in Phoenix at 40°C and 20% humidity, where the depression is about 17°C, a good cooler delivers air in the mid 20s. In humid Houston the same unit barely helps. Cooling towers work the same way: the water can never get colder than the entering wet bulb, and designers quote the approach, the gap between cold water and wet bulb, as a key rating. For air conditioner charging, indoor wet bulb sets the target superheat, and for comfort and condensation risk, pair this result with the dew point calculator to see how close surfaces are to sweating.

Wet bulb temperature chart

Wet bulb temperature in °C from the Stull formula for common air temperatures and humidity levels:

Related Calculators