WBGT Heat Stress Index Calculator

How the WBGT index is built

WBGT combines three separate thermometer readings into one number because no single reading captures heat stress on a working body. Dry-bulb temperature is ordinary air temperature. Natural wet-bulb temperature is measured with a wetted sensor left in open air (not spun, unlike a psychrometric wet bulb) and falls as evaporative cooling from humidity and airflow becomes more effective. Globe temperature is read from a matte black sphere with a thermometer at its center, which absorbs radiant heat the way exposed skin or clothing does.

The three readings are combined with fixed weightings rather than averaged evenly, because they do not contribute equally to heat stress. With direct solar load (outdoor, sun on the work area): WBGT = 0.7 × wet-bulb + 0.2 × globe + 0.1 × dry-bulb. Without solar load (indoor, or outdoor in full shade): WBGT = 0.7 × wet-bulb + 0.3 × globe, with dry-bulb dropped from the sum entirely. In both cases the wet-bulb term dominates — humidity and evaporative cooling capacity carry more weight in the index than either air temperature or radiant heat alone.

This calculator also reports a threshold value and a safe/caution/danger status, selected from a lookup table by workload intensity and whether the worker is heat-acclimatized. Heavier workloads and unacclimatized workers get a lower (more restrictive) threshold. The status compares the calculated WBGT against that threshold: below the threshold by more than 2°C is reported as safe, within 2°C of the threshold as caution, and above the threshold as danger.

When to Use This

• Screening whether measured site conditions (temperature, humidity, radiant heat) warrant a closer look at work/rest scheduling for outdoor or hot-process work.
• Comparing how much a given set of thermometer readings changes under indoor versus direct-sun conditions, since the two use different formula weightings.
• Seeing how the same environmental readings shift from a lower to a higher status band as workload intensity increases.

Common Mistakes

• Using a psychrometric (sling/spun) wet-bulb reading in place of a natural wet-bulb reading — natural wet-bulb is measured in still, unforced air and reads warmer than a spun wet-bulb in the same conditions, since it is not artificially ventilated.
• Selecting the outdoor/solar-load formula for shaded outdoor work, or vice versa — the two formulas weight globe and dry-bulb temperature differently, so picking the wrong one changes the index even with identical thermometer readings.
• Treating the reported threshold and status as a fixed, universally-sourced safety limit rather than a screening indicator — the underlying workload/acclimatization limit table is stated from published heat-stress guidance whose exact source and specific figures are not independently verifiable here, and multiple published schemes state comparable limits a degree or two apart. Check the applicable standard or local occupational health guidance for the site before treating any status as a stop-work decision.

FAQ

What is a globe thermometer reading and why does WBGT need it separately from air temperature?

A globe thermometer is a matte black hollow sphere with a thermometer sensor at its center. It absorbs and re-radiates heat the way a person standing in the sun does, so its reading rises above plain air temperature when there is radiant heat (direct sun, nearby hot equipment) even if the air temperature itself is unremarkable. Ordinary dry-bulb air temperature does not capture that radiant component, which is why WBGT reads it as a separate input.

Why does the calculation change between indoor and outdoor?

The indoor/outdoor selection controls whether the formula accounts for direct solar radiant load. With solar load, the index blends wet-bulb, globe, and dry-bulb readings (0.7 / 0.2 / 0.1). Without it — indoor, or outdoor fully shaded from the sun — dry-bulb temperature is left out and the weighting shifts to wet-bulb and globe only (0.7 / 0.3), since there is no direct solar component for the globe reading to add on top of.

Where do the threshold value and status band come from, and how authoritative are they?

The threshold is looked up by workload intensity and acclimatization status, and the status compares the calculated WBGT to that threshold with a 2°C caution margin below it (safe below threshold−2, caution between threshold−2 and threshold, danger above threshold). Unlike the index formula itself, the specific threshold figures come from a published heat-stress limit table whose original source and exact per-workload values could not be independently confirmed here — comparable published schemes differ by a degree or two and are not all stated on the same basis. Treat the status as a screening indicator to prompt a closer look, not as a verified regulatory limit for the specific site or jurisdiction.