Wet Bulb Globe Temperature Calculator
Wet Bulb Globe Temperature (WBGT) is the internationally recognised heat stress index used by occupational health authorities, military services and sports medicine bodies worldwide. Unlike simple air temperature or heat index, WBGT integrates three measurements: natural wet bulb temperature (humidity and evaporation), black globe temperature (radiant solar heat) and dry bulb temperature (air temperature). The outdoor formula applies a 70/20/10 weighting to these three readings. Enter your readings in degrees Celsius below and the calculator returns the WBGT value along with an OSHA risk category.
WBGT outdoor formula (ISO 7243)
WBGT = 0.7 × Tw + 0.2 × Tg + 0.1 × Td
Where Tw is natural wet bulb temperature, Tg is black globe temperature and Td is dry bulb temperature, all in degrees Celsius.
This formula is defined in ISO 7243:2017 (Hot environments: estimation of heat stress on working man, based on the WBGT index) and is used by OSHA for occupational heat stress assessment.
OSHA WBGT risk categories
- Below 26 deg C: Low risk. Normal work and activity are generally safe for acclimatised workers.
- 26 to 28 deg C: Moderate. Monitor conditions; ensure adequate hydration and shade.
- 28 to 30 deg C: High. Reduce workload intensity; mandate rest breaks of at least 15 minutes per hour.
- 30 to 32 deg C: Very high. Strenuous work only for acclimatised workers; 30+ minutes rest per hour.
- Above 32 deg C: Extreme. Limit or stop outdoor strenuous work; risk of heat stroke is significant.
These thresholds align with OSHA Technical Manual Section III, Chapter 4, and the American Conference of Governmental Industrial Hygienists (ACGIH) Threshold Limit Values for heat stress.
Wet bulb globe temperature: frequently asked questions
What is Wet Bulb Globe Temperature (WBGT)?
WBGT is a composite heat stress index that accounts for temperature, humidity, wind speed and solar radiation. It combines dry bulb temperature, natural wet bulb temperature and black globe temperature. It is the standard metric used by OSHA, the military and sports organisations to assess heat illness risk during outdoor work and exercise.
What is the WBGT formula for outdoors?
The outdoor WBGT formula is: WBGT = 0.7 x Tw + 0.2 x Tg + 0.1 x Td, where Tw is the natural wet bulb temperature, Tg is the black globe temperature and Td is the dry bulb (air) temperature. The formula weights natural wet bulb most heavily (70%) because humidity and evaporative cooling have the greatest influence on heat stress.
What is a dangerous WBGT level?
The American College of Sports Medicine and OSHA use the following thresholds: below 28 degrees C is generally safe for most workers; 28 to 32 degrees C warrants caution and reduced workloads; 32 to 35 degrees C is high risk requiring frequent rest breaks; above 35 degrees C is extreme risk and most strenuous outdoor work should stop.
How does WBGT differ from heat index?
Heat index (apparent temperature) only combines air temperature and relative humidity in the shade. WBGT adds black globe temperature to account for radiant heat from the sun, making it a far more accurate measure of heat stress during outdoor activities. WBGT is the preferred metric for occupational health and military standards.
Can I use WBGT indoors?
Yes. For indoor environments without solar load, the simplified indoor formula is: WBGT = 0.7 x Tw + 0.3 x Tg, omitting the dry bulb term. The outdoor formula used in this calculator (with the 0.1 dry bulb weighting) is appropriate when there is direct sunlight.
Official sources
- OSHA Technical Manual, Section III, Chapter 4 (Heat Stress): osha.gov.
- ISO 7243:2017 Hot environments, estimation of heat stress: iso.org.
- ACGIH 2024 TLVs and BEIs: acgih.org.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.