Wet-bulb temperature is the lowest temperature air can reach purely through evaporative cooling — the reading you'd get from a thermometer with a wet cloth wick, swung through the air. It matters far more than plain air temperature for judging heat stress, because it captures how effectively sweat can actually cool the body.
How the Wet Bulb Temperature Calculator works
This calculator uses the Stull (2011) empirical approximation, a formula fit to psychrometric tables that estimates wet-bulb temperature directly from dry-bulb (ordinary) air temperature and relative humidity, without needing an iterative psychrometric-chart lookup. It is accurate to roughly ±1 degC across the range most weather and heat-stress applications care about.
Inputs and what they mean
The Wet Bulb tab takes the ordinary air temperature and the relative humidity. The Heat Safety tab reuses that same result to show where it falls on a five-tier heat-stress scale, plus a side-by-side comparison against a scenario with 20 more points of humidity. The Reverse tab flips the problem: given an air temperature and a measured wet-bulb reading — the classic sling-psychrometer use case — it numerically solves for the relative humidity that produced it.
Limits and edge cases
The Stull formula is validated for air temperatures from -20 degC to 50 degC and relative humidity from 5% to 99%; outside that range the estimate degrades. A wet-bulb reading can never exceed the dry-bulb air temperature, and the Reverse tab rejects inputs that would imply otherwise. This tool reports psychrometric wet-bulb temperature, not WBGT — occupational heat-safety limits (e.g. from OSHA or sports-medicine bodies) are usually expressed in WBGT, which reads a few degrees lower because it also accounts for radiant heat and wind.