Free WBGT calculator: estimate Wet Bulb Globe Temperature for sun or shade from temperature and humidity, with the flag condition and work-rest guidance.
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Wet Bulb Globe Temperature (WBGT) is the standard heat-stress index used by the military, OSHA, and sports organizations. This calculator estimates WBGT from air temperature and humidity for both shade and full sun, then maps it to the flag-condition system with work-rest guidance. Because direct sun and wind dominate real WBGT, we show a shade estimate and a conservative full-sun estimate rather than a single false-precision number.
WBGT (Wet Bulb Globe Temperature) is a composite heat-stress index. In full sun, radiant heat on a black globe and a wetted-wick (natural wet-bulb) thermometer dominate the reading — which is why WBGT is far higher in sun than in shade at the same air temperature. In shade the globe term falls to the air temperature, so WBGT is about 0.7 × wet-bulb + 0.3 × air. It is more relevant than heat index for outdoor work and sport because it captures sun and humidity load together.
WBGT Formulas
Monitor conditions for outdoor workers to prevent heat illness.
Football, soccer, and other sports use WBGT for practice safety protocols.
Determine flag conditions and mandatory work-rest cycles for troops.
Factories and warehouses with heat exposure use WBGT for safety compliance.
OSHA and NIOSH recommend WBGT for assessing occupational heat exposure.
Sports organizations use WBGT to determine safe practice conditions.
Armed forces worldwide use the flag system to regulate training intensity.
White (78-82°F): Unrestricted activity. Green (82-85°F): Use discretion. Yellow (85-88°F): Limit strenuous activity. Red (88-90°F): Suspend intense activity. Black (>90°F): Cancel outdoor exertion.
WBGT is built for direct-sun activity and factors in radiant (solar) heat, while heat index is a shade measure of temperature and humidity only. For the same conditions, WBGT in full sun is markedly higher, which is why it is the standard for outdoor work and sports.
Not exactly — direct sun can raise WBGT by 3–6°C (a black globe reads 15–30°C above air) and wind lowers it. That is why this tool shows a shade estimate (ISO 7243) and a conservative full-sun estimate side by side. For safety-critical decisions, measure with a WBGT meter.
Military training often occurs in extreme heat. WBGT and the flag system give standardized guidelines for work-rest cycles, hydration, and activity limits to prevent heat casualties.
The flags here follow the US military/occupational system. For continuous endurance exercise, ACSM uses stricter cutoffs — its Black flag begins near 28°C (82°F), roughly 6°C lower. If you are planning a race or hard workout, treat each flag as one step more severe.