WBGT vs. Wet-Bulb Temperature — Two Different Things With the Same Name

If you searched for a "wet bulb calculator" looking for heat-stress or workplace-safety guidance, this page is for you — what you likely want is WBGT, which is a related but genuinely different number from the thermodynamic wet-bulb temperature this calculator computes.

What this calculator computes

This site's wet-bulb output is the thermodynamic (psychrometric) wet-bulb temperature — a specific, well-defined property of the air's current state, solved from the psychrometer equation (see the wet-bulb page). It depends only on dry-bulb temperature, humidity, and barometric pressure — nothing about wind speed, sun exposure, or physical surroundings enters into it.

What WBGT actually is

Wet-Bulb Globe Temperature is a composite heat-stress index, not a pure air property. It's built from three separate physical measurements taken simultaneously at the actual work site:

Natural wet-bulb temperature (T_nwb) — a wet-wick thermometer exposed to whatever natural airflow exists at the location (not force-ventilated like a sling psychrometer), so it responds to real ambient wind speed, not just humidity.
Globe temperature (T_g) — read from a black globe thermometer, which captures radiant heat load from sunlight or hot equipment.
Dry-bulb temperature (T_db) — ordinary air temperature.

Outdoors (with solar load): WBGT = 0.7 × T_nwb + 0.2 × T_g + 0.1 × T_db. Indoors or with no solar load: WBGT = 0.7 × T_nwb + 0.3 × T_g.

Why they're not interchangeable

Natural wet-bulb temperature (the input to WBGT) is not the same number as the thermodynamic wet-bulb this calculator computes, even at identical air conditions — because natural wet-bulb depends on actual local wind speed at the sensor (less airflow means less evaporative cooling, pushing the reading higher, closer to dry-bulb), while the thermodynamic value assumes the idealized forced-airflow equilibrium the psychrometer equation describes. WBGT also folds in radiant heat through the globe thermometer term, which has no equivalent at all in a pure air-property calculation.

Which one do you actually need?

If the goal is OSHA/NIOSH heat-stress compliance, work/rest cycle planning, or athletic heat-safety guidelines, you need WBGT measured on-site with the proper instrument cluster (or a calibrated WBGT meter) — this calculator's output is not a substitute, since it's missing the wind-speed and radiant-heat components entirely. If the goal is HVAC design or analysis — coil selection, duct condensation checks, evaporative cooler performance — the thermodynamic wet-bulb this calculator computes is exactly the right number, and WBGT would be the wrong one to use instead.

FAQ

Can I approximate WBGT using this calculator's wet-bulb output?: Only roughly, and not for compliance purposes — this calculator's thermodynamic wet-bulb ignores wind speed and radiant heat, both of which materially affect real WBGT readings. For any safety-relevant use, use a proper WBGT meter or on-site measurement.
Why do both use the term "wet-bulb" if they're different?: Because WBGT's natural-wet-bulb component is still conceptually a wet-wick thermometer reading, just taken under real ambient (not forced) airflow — the shared name reflects a shared measurement technique, not an identical final number.
Is this calculator useful at all for outdoor heat-safety work?: It can give a general sense of how humid conditions are, but it should not be used as or confused with an actual WBGT heat-stress reading for compliance or safety decisions.

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