Heat Stress Index Calculator

Outdoors in direct sun, WBGT = 0.7×Tnwb + 0.2×Tg + 0.1×Tdb, adding a dry-bulb air-temperature term to capture radiant solar heat load; indoors or in the shade, that solar contribution doesn't apply, so the formula drops to 0.7×Tnwb + 0.3×Tg with the globe temperature's weight increased instead. Entering the wrong setting for your conditions will meaningfully shift the calculated WBGT.

The calculator uses a separate limits table for unacclimatised workers that's roughly 2-3°C lower at every screening threshold than the acclimatised table, reflecting that someone with less than about a week of gradual heat exposure has a reduced physiological capacity to dissipate heat. At an identical WBGT reading, an unacclimatised worker might be pushed into a 50/50 work/rest regime while an acclimatised worker at the same task stays on continuous work.

WBGT is the metric ACGIH and OSHA actually build work/rest guidance around, computed from wet bulb, globe, and (outdoors) dry bulb readings that capture humidity, radiant heat, and solar load together. The heat index shown alongside it is the separate NWS/Steadman formula used for weather forecasts, computed only from dry-bulb temperature and an estimated humidity — it ignores radiant and solar load entirely and will understate risk for anyone working in direct sun, so always use the WBGT-based recommendation for work decisions, never the heat index.

No — the ACGIH screening tables built into this calculator assume standard single-layer work clothing. Heavier PPE or impermeable suits reduce the body's ability to shed heat and require additive clothing-adjustment corrections to the WBGT limits that aren't modeled here, so real-world limits for workers in heavy PPE should be set more conservatively than what's shown.