WBGT combines air temperature, humidity, wind, and solar load into a single number that predicts heat stress on a working body. It is the metric used in every national occupational heat-safety code. Enter the conditions and get the WBGT with the ISO 7243 work/rest cycle for a moderate metabolic rate.
Conditions
Results
| WBGT (°C) | Recommended work regime |
|---|---|
| < 25 | No restriction on work |
| 25 – 28 | Continuous work with normal breaks |
| 28 – 30 | Limit work: 45 min work / 15 min rest per hour |
| 30 – 32 | 30 min work / 30 min rest per hour |
| > 32 | Cease work; emergency-only exposure |
About this calculator
The Wet Bulb Globe Temperature (WBGT) is a composite index that combines dry-bulb air temperature (Ta), natural wet-bulb temperature (Tnwb), and black-globe temperature (Tg) into a single number.
Outdoor with solar load: WBGT = 0.7 × Tnwb + 0.2 × Tg + 0.1 × Ta
Indoor / no solar load: WBGT = 0.7 × Tnwb + 0.3 × Tg
Wet-bulb temperature is estimated from Ta and relative humidity using the Stull (2011) polynomial approximation of the psychrometric wet-bulb. In still air, the natural wet-bulb reading is a few tenths of a degree warmer; with any breeze the two converge, so for field purposes the Stull value is used as Tnwb.
If no globe thermometer is available, Tg is estimated as Ta plus a solar offset (0 to about 15 °C) based on the sun setting and wind speed. This is a rule-of-thumb estimate. For occupational compliance work, measure Tg directly with a 150 mm black-globe thermometer.
Work regime bands in the results table are the ISO 7243:2017 reference limits for a moderate metabolic rate (approximately 200 W/m², e.g. walking with a light load, hand work). Heavier work needs lower WBGT thresholds; lighter work tolerates higher ones. See ISO 7243 Table A.1 for the full lookup.
Standard. ISO 7243:2017 Ergonomics of the thermal environment — Assessment of heat stress using the WBGT (wet bulb globe temperature) index.
Original publication. Yaglou CP, Minard D. Control of heat casualties at military training centers. AMA Archives of Industrial Health 16(4):302–316 (1957).
Wet-bulb approximation. Stull R. Wet-bulb temperature from relative humidity and air temperature. Journal of Applied Meteorology and Climatology 50(11):2267–2269 (2011).