WBGT Heat Stress Calculator

Q: What is WBGT?

Wet Bulb Globe Temperature — a composite heat-stress index that combines air temperature, humidity, wind, and solar radiation. It is the standard used by the U.S. military, OSHA, the ACSM, and most national athletic associations to set safe exercise conditions.

Q: How is WBGT different from "feels like" or the heat index?

The heat index (apparent temperature) is based only on temperature and humidity and was designed for people standing still in shade. WBGT adds solar radiation and wind, so it is more accurate for outdoor exertion. For exercise, WBGT is the better metric.

Q: What WBGT is too hot to run?

ACSM guidelines call ≥32.2 °C (90 °F) WBGT "black flag" — outdoor exertion should be cancelled or limited to short, low-intensity exposure. Many marathon medical directors use 28-29 °C as the threshold to cancel long-distance races. Individual tolerance varies.

Q: Why does humidity matter so much?

Sweat evaporation is the body's primary cooling mechanism, and high humidity blocks evaporation. At 90% humidity, sweat dripping off you is not cooling you. WBGT weights wet-bulb temperature at 70% because that captures the evaporative cooling problem better than dry-bulb temperature alone.

Q: How long does heat acclimatization take?

10-14 days of progressively longer heat exposure typically produces meaningful adaptation — earlier sweating, lower core temperature for the same work, expanded plasma volume. The first 5 days of a heat acclimatization plan should be conservative and well-hydrated.

Q: Should I treat shade and indoor environments the same?

For this calculator, yes — the "shade" mode removes the solar radiation load and applies the indoor-style 0.7·Tw + 0.3·Tdb formula. Indoor gyms without solar load but with high humidity (e.g., wrestling rooms, indoor courts) can still exceed safe WBGT.

Q: What are the signs of heat illness I should watch for?

Heat exhaustion: heavy sweating, weakness, dizziness, nausea, cool clammy skin. Heat stroke: hot dry or sweaty skin, confusion, very high core temperature, possible loss of consciousness. Heat stroke is a medical emergency — cool aggressively (cold water immersion is most effective) and call for help immediately.

Wet Bulb Globe Temperature (WBGT) is the international standard heat-stress index for athletics, military training, and outdoor work. It combines temperature, humidity, wind, and solar radiation into a single number that better predicts heat-illness risk than plain air temperature does. This calculator estimates WBGT from common weather inputs using the Stull (2011) wet-bulb equation and the Lemke & Kjellstrom globe-temperature regression, then maps the result to the ACSM/NATA activity bands for sports and outdoor exercise. This is an educational estimator. Suspected heat illness is a medical emergency — get into shade, cool the body aggressively, and call for help.

Reviewed by GetHealthyCalculators Editorial Team · Updated May 14, 2026

Quick Answer

WBGT combines temperature, humidity, wind, and sun into a single heat-stress index. Below 26.7°C (80°F) is low risk for outdoor exercise; 26.7-29.4°C is moderate; 29.5-31°C is high (limit intense work to ~45 min); 31.1-32.1°C is very high; ≥32.2°C (90°F) is extreme — cancel outdoor exertion.

These results are estimates based on general formulas and are not a substitute for professional medical advice. Consult a healthcare provider before making health decisions.

How the Formula Works

Estimate natural wet-bulb temperature from dry-bulb temperature and humidity using Stull (2011).
Tw ≈ T·atan(0.151977·√(RH+8.31))+ atan(T+RH) − atan(RH−1.68) + …
Estimate black-globe temperature from dry-bulb, solar radiation, and wind.
Tg ≈ Tdb + 0.0156·solar(W/m²) − 0.4·wind(m/s)
In sun: combine 70% wet-bulb, 20% globe, 10% dry-bulb. In shade: 70% wet-bulb and 30% dry-bulb (no solar load).
WBGT_outdoor = 0.7·Tw + 0.2·Tg + 0.1·Tdb
Map the WBGT value to the ACSM / NATA outdoor-exercise risk band.
<26.7°C low → ≥32.2°C extreme

Methodology & Sources

Reviewed and updated May 14, 2026 · Prepared by GetHealthyCalculators Editorial Team

Wet-bulb temperature is estimated with the Stull (2011) regression, which is validated for typical exercise conditions (0-50°C, 5-99% RH). Globe temperature is estimated with the Lemke & Kjellstrom (2012) simplified outdoor regression — accuracy in field comparisons is around ±2°C. Risk bands follow the American College of Sports Medicine position stand on exertional heat illness (2007) and the National Athletic Trainers' Association position statement on exertional heat illnesses (2015).

References

Stull R. Wet-Bulb Temperature from Relative Humidity and Air Temperature (2011) · Journal of Applied Meteorology and Climatology
ACSM Position Stand. Exertional Heat Illness during Training and Competition (2007) · Medicine & Science in Sports & Exercise
NATA Position Statement. Exertional Heat Illnesses (2015) · Journal of Athletic Training
Lemke B, Kjellstrom T. Calculating Workplace WBGT from Meteorological Data (2012) · Industrial Health

Limitations

The globe-temperature estimate from temperature, solar, and wind has a field accuracy of about ±2°C — a dedicated WBGT sensor is more accurate.
Stull's wet-bulb formula is validated at sea-level pressure (1013 hPa); higher elevations introduce small errors.
WBGT does not predict the risk for any one individual — fitness, acclimatization, hydration status, and medical conditions matter as much as the environmental score.
The ACSM bands are for healthy, acclimatized athletes. Children, older adults, and anyone on heat-sensitizing medication should treat conditions as one band more severe.
Microclimate matters — turf radiates more heat than grass, and direct mid-day sun adds load that this calculator only approximates from typical solar values.
Indoor / fully shaded environments have no solar load; the calculator applies the "shade" formula automatically when shade is selected.

Frequently Asked Questions

What is WBGT?

Wet Bulb Globe Temperature — a composite heat-stress index that combines air temperature, humidity, wind, and solar radiation. It is the standard used by the U.S. military, OSHA, the ACSM, and most national athletic associations to set safe exercise conditions.

How is WBGT different from "feels like" or the heat index?

The heat index (apparent temperature) is based only on temperature and humidity and was designed for people standing still in shade. WBGT adds solar radiation and wind, so it is more accurate for outdoor exertion. For exercise, WBGT is the better metric.

What WBGT is too hot to run?

ACSM guidelines call ≥32.2 °C (90 °F) WBGT "black flag" — outdoor exertion should be cancelled or limited to short, low-intensity exposure. Many marathon medical directors use 28-29 °C as the threshold to cancel long-distance races. Individual tolerance varies.

Why does humidity matter so much?

Sweat evaporation is the body's primary cooling mechanism, and high humidity blocks evaporation. At 90% humidity, sweat dripping off you is not cooling you. WBGT weights wet-bulb temperature at 70% because that captures the evaporative cooling problem better than dry-bulb temperature alone.

How long does heat acclimatization take?

10-14 days of progressively longer heat exposure typically produces meaningful adaptation — earlier sweating, lower core temperature for the same work, expanded plasma volume. The first 5 days of a heat acclimatization plan should be conservative and well-hydrated.

Should I treat shade and indoor environments the same?

For this calculator, yes — the "shade" mode removes the solar radiation load and applies the indoor-style 0.7·Tw + 0.3·Tdb formula. Indoor gyms without solar load but with high humidity (e.g., wrestling rooms, indoor courts) can still exceed safe WBGT.

What are the signs of heat illness I should watch for?

Heat exhaustion: heavy sweating, weakness, dizziness, nausea, cool clammy skin. Heat stroke: hot dry or sweaty skin, confusion, very high core temperature, possible loss of consciousness. Heat stroke is a medical emergency — cool aggressively (cold water immersion is most effective) and call for help immediately.

Plan your fluid replacement for this WBGT range

Exercise Hydration Calculator

WBGT Heat Stress Calculator

How the Formula Works

Methodology & Sources

References

Limitations

Frequently Asked Questions

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