Wet Bulb Calculator
Calculate wet-bulb temperature
Results
| Wet-bulb temperature | — |
|---|---|
| Dew point | — |
Psychrometric charts look straightforward until you are on a roof in July trying to read wet-bulb off a sling psychrometer while the HVAC tech waits for a number. Dry-bulb and relative humidity are easy to grab from a weather app, but wet-bulb sits in the middle of the chart—and that is the value you need for cooling-tower checks, evaporative cooler sizing, and heat-stress screening. This calculator skips the chart lookup: enter dry-bulb temperature and RH, pick °C or °F, and get wet-bulb plus dew point in one pass.
How to Use This Wet Bulb Calculator
- Select temperature units. Choose °C or °F at the top of the form. Inputs and both results use the same unit.
- Enter dry-bulb temperature. This is ordinary air temperature—the reading from a standard thermometer, not a wetted sensor.
- Enter relative humidity (%). Use a value from 0 to 100. Dew point requires RH above 0%; at exactly 0% the tool will flag an error because there is no moisture to condense.
- Click Calculate. Wet-bulb and dew-point temperatures appear in the results table, each shown to two decimal places.
For a dedicated dew-point readout with comfort labels and step-by-step math, try the Dew Point Calculator. On hot, humid days, pair this with the Heat Index Calculator to see how the same air mass feels to people working outdoors.
Wet Bulb Formulas and Practical Applications
Wet-bulb temperature is the lowest temperature unsaturated air can reach when water evaporates into it at constant pressure. Picture wrapping your thermometer bulb in a damp cloth and swinging it—evaporation pulls heat away until the reading stabilizes. That stabilized value is the wet-bulb. It is always at or below dry-bulb for unsaturated air and equals dry-bulb when RH hits 100%.
This tool estimates wet-bulb using the Stull (2011) approximation from dry-bulb temperature T (°C) and relative humidity RH (%):
Twb = T × arctan[0.151977 × √(RH + 8.313659)] + arctan(T + RH) − arctan(RH − 1.676331) + 0.00391838 × RH1.5 × arctan(0.023101 × RH) − 4.686035
Dew point—where condensation begins—is computed separately with a Magnus-type relation so you get both psychrometric anchors from the same two inputs:
γ = ln(RH / 100) + (17.62 × T) / (243.12 + T)
Tdp = (243.12 × γ) / (17.62 − γ)
In our testing, Stull tracks sling psychrometer readings well for typical outdoor and indoor ranges (roughly −20°C to 50°C dry-bulb). Extreme desert dryness or saturated fog-edge cases may drift from calibrated lab instruments—validate with a proper psychrometer when commissioning equipment.
Where wet-bulb shows up in real work
- HVAC and cooling towers: Wet-bulb sets the practical limit for evaporative cooling; compare entering air wet-bulb to leaving water temperature on tower performance logs.
- Heat stress: High wet-bulb means evaporative cooling (sweat) works poorly—the body struggles even when dry-bulb looks tolerable.
- Weather and fire weather: Wet-bulb depression (dry minus wet) signals how fast fuels can dry; low depression often means sticky, hard-to-burn conditions at the surface.
- Physics labs: Quick check when students have thermometer + hygrometer data but no psychrometric chart handy.
Frequently Asked Questions
What is wet-bulb temperature?
Wet-bulb temperature is the lowest temperature air can reach when water evaporates into it at constant pressure. It combines heat and humidity into one number used in HVAC, weather analysis, and heat-stress assessment.
Why is wet-bulb always lower than dry-bulb?
Evaporation consumes latent heat, cooling the wetted sensor below the surrounding dry air temperature. Only when the air is already saturated (RH = 100%) does wet-bulb equal dry-bulb—no further evaporation can occur.
Why does dew point require RH above 0%?
The Magnus dew-point formula uses the natural log of RH/100. At 0% humidity that logarithm is undefined, so the calculator returns an error. In practice, air always holds trace moisture; enter the smallest value your instrument reports.
How accurate is this wet-bulb estimate?
The Stull approximation is widely used for field estimates across normal temperature and humidity ranges. It is not a replacement for a calibrated sling psychrometer or ASHRAE-grade psychrometric chart when you are signing off on equipment commissioning or legal compliance documents.
Can I get wet-bulb from dew point instead of RH?
This form expects dry-bulb and RH. If you only have dew point, use a psychrometric chart or a tool that accepts dew point as input. You can still cross-check: the dew point shown here should match the standalone Dew Point Calculator for the same inputs.
Disclaimer. RapidRatio is informational only. It is not professional engineering, meteorological, or laboratory advice. Wet-bulb and dew-point outputs are estimates; verify measurements with calibrated instruments when precision matters.