WBGT Heat Stress Calculator
Wet bulb globe temperature (WBGT) is the standard index for heat stress on people working or exercising in hot environments. Unlike the heat index it accounts for humidity, radiant heat from the sun, and air movement together, by combining three thermometer readings: the natural wet-bulb temperature (humidity and wind), the globe temperature (radiant heat), and the dry-bulb air temperature. This tool computes WBGT for outdoor conditions in sunlight and for indoor or shaded conditions, using the standard weightings. Enter the three temperatures in degrees Celsius. WBGT underpins occupational and athletic heat-safety guidelines from bodies such as OSHA and the military.
WBGT formula
Outdoor (with solar load): WBGT = 0.7 * Tnwb + 0.2 * Tg + 0.1 * Ta
Indoor or shade: WBGT = 0.7 * Tnwb + 0.3 * Tg
Tnwb = natural wet-bulb, Tg = globe, Ta = dry-bulb
The natural wet-bulb temperature dominates because evaporative cooling, governed by humidity and wind, is the largest factor in human heat balance, so it carries a weight of 0.7. The globe temperature captures radiant heat from the sun or hot surfaces. Outdoors in sunlight the dry-bulb air temperature also enters with a weight of 0.1; indoors or in shade, without direct solar load, the globe term takes the full remaining 0.3.
Worked example
Readings are a natural wet-bulb of 25 degrees Celsius, a globe of 35, and a dry-bulb of 30. Outdoors: WBGT = 0.7 * 25 + 0.2 * 35 + 0.1 * 30 = 17.5 + 7.0 + 3.0 = 27.50 degrees Celsius. Indoors or in shade: WBGT = 0.7 * 25 + 0.3 * 35 = 17.5 + 10.5 = 28.00 degrees Celsius. A WBGT near 28 calls for work-rest cycles and increased fluids under most heat-stress guidelines.
Frequently asked questions
What is wet bulb globe temperature?
Wet bulb globe temperature (WBGT) is a composite heat-stress index that combines the natural wet-bulb temperature, the globe temperature, and the dry-bulb air temperature into one value. By including humidity, radiant heat, and air temperature, it reflects the total heat load on the body better than air temperature or the heat index alone, which is why it is used for work and sport safety.
What is the difference between the indoor and outdoor formula?
Outdoors in direct sun there is a solar radiant load, so the formula weights the natural wet-bulb at 0.7, the globe at 0.2, and the dry-bulb at 0.1. Indoors or in full shade there is no solar load, so the dry-bulb term is dropped and the globe temperature takes the remaining 0.3 weight. Use the outdoor value when working in sunlight.
What WBGT level is dangerous?
Thresholds depend on workload and acclimatisation, so consult the specific guideline you work under. As a general guide, many occupational standards begin recommending work-rest cycles around a WBGT of 28 degrees Celsius for moderate work, with stricter limits for heavy work or unacclimatised people. Athletic bodies set their own activity-modification thresholds.
Why is the natural wet-bulb temperature weighted most heavily?
Because evaporation of sweat is the body's main way of shedding heat in hot conditions, and its effectiveness depends on humidity and air movement, both of which the natural wet-bulb thermometer captures. When the wet-bulb temperature is high, sweat evaporates poorly and heat stress rises sharply, so it carries the largest weight in the index.
Official sources
- U.S. OSHA and NIOSH: heat exposure and WBGT.
- U.S. National Weather Service: wet bulb globe temperature.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.