Free tool · Training

WBGT calculator: heat stress for training and racing

Short answer

Wet bulb globe temperature (WBGT) is the heat-stress measure sports bodies use to modify or cancel exercise. At 30 °C (86 °F) and 60% humidity in sun and light wind, the Bureau of Meteorology approximation gives a WBGT of about 30.9 °C, in the ACSM band "Cancel or stop practice and competition".

Last reviewed 28 October 20263 published sourcesNot medical advice

Reps keeps track of this for you on iPhone, iPad and Apple Watch.

Get Reps free

WBGT calculator

Use the shade air temperature and relative humidity. The estimate assumes full sun and a light wind.

Enter the temperature and humidity to estimate the WBGT.

Runs entirely in your browser. Nothing you type is saved or sent anywhere.

ACSM WBGT bands for continuous activity and competition
WBGT °C	WBGT °F	Guidance
10.0 or less	50.0 or less	Generally safe; exertional heat stroke can occur associated with individual factors
10.1–18.3	50.1–65.0	Generally safe; exertional heat stroke can occur
18.4–22.2	65.1–72.0	Risk of exertional heat stroke and heat illness begins to rise; high-risk individuals should be monitored or not compete
22.3–25.6	72.1–78.0	Risk for all competitors is increased
25.7–27.8	78.1–82.0	Risk for unfit, non-acclimatized individuals is high
27.9–30.0	82.1–86.0	Cancel level for exertional heat stroke risk
30.1–32.2	86.1–90.0	Cancel or stop practice and competition
32.3 or more	90.1 or more	Cancel exercise

From Armstrong et al. (2007), as reproduced by Kakamu et al. (2017). For healthy adults in shorts, T-shirt, socks and sneakers.

Estimated WBGT (°C) by temperature and humidity, in sun with a light wind
Air temperature	20% RH	40% RH	60% RH	80% RH
20 °C (68 °F)	17.1	18.9	20.8	22.6
25 °C (77 °F)	20.6	23.1	25.6	28.0
28 °C (82 °F)	22.8	25.7	28.7	31.7
30 °C (86 °F)	24.3	27.6	30.9	34.2
32 °C (90 °F)	25.8	29.5	33.3	37.0
35 °C (95 °F)	28.2	32.6	37.0	Above 40

Worked out with the Bureau of Meteorology approximation. The Bureau's own table stops at 40 °C and lists anything higher as above 40.

The Bureau of Meteorology approximation

WBGT = 0.567 × Ta + 0.393 × e + 3.94, where Ta is the air temperature in °C and e the water vapour pressure in hPa, worked out from humidity as e = rh/100 × 6.105 × exp(17.27 × Ta / (237.7 + Ta)) (Bureau of Meteorology).

The formula assumes a moderately high radiation level in light wind. It overestimates heat stress in cloudy or windy conditions, at night and early in the morning, and slightly underestimates it in clear full sun with low humidity (Bureau of Meteorology). A measured WBGT from an event's own instruments always wins.

This calculator uses the same formula as the Bureau's published table, which covers air temperatures of 20–50 °C, and matches every cell of it to within 1 °C. The table rounds to whole degrees, and 12 of its 432 cells differ by 1 °C where a value sits on a rounding edge. Like the table, the calculator reports any WBGT above 40 °C as above 40 (Bureau of Meteorology).

The guidance bands come from the American College of Sports Medicine's position stand on exertional heat illness (Armstrong et al., 2007 (ACSM)), as tabulated in a peer-reviewed review of heat-illness prevention (Kakamu et al., 2017). Training guidance in the same table is stricter for people not yet acclimatized to the heat, meaning fewer than about three weeks of training in it (Kakamu et al., 2017).

For a feels-like temperature in the shade, use the heat index calculator. The dew point calculator shows how muggy the air is.

Know the weather before you go

Reps looks at the weather before your run, so you know what you are heading into before you lace up.

Download Reps on the App Store Running in Reps

Free to download · iPhone, iPad and Apple Watch

Common questions

What is WBGT?

Wet bulb globe temperature combines air temperature, humidity, sun and wind into one heat-stress number. It is measured with special instruments; this calculator estimates it from temperature and humidity alone, assuming full sun and a light wind.

What WBGT is too hot to exercise?

The ACSM's 2007 position stand marks 27.9–30.0 °C (82.1–86.0 °F) as the cancel level for exertional heat stroke risk, 30.1–32.2 °C as cancel or stop practice and competition, and 32.3 °C (90.1 °F) or more as cancel exercise. Risk for unfit, non-acclimatized people is already high from 25.7 °C.

How is WBGT different from the heat index?

The heat index is a feels-like temperature in the shade from temperature and humidity. WBGT is built for exertion in the sun and is what the ACSM bands use. The two numbers are on different scales, so don't compare them directly.

Sources

Figures on this page were checked against these publications on 28 October 2026.

Australian Government Bureau of Meteorology. About the WBGT and Apparent Temperature indices (thermal stress). www.bom.gov.au/info/thermal_stress/
Armstrong LE, Casa DJ, Millard-Stafford M, Moran DS, Pyne SW, Roberts WO. American College of Sports Medicine position stand. Exertional heat illness during training and competition. Medicine & Science in Sports & Exercise. 2007;39(3):556–572. doi.org/10.1249/MSS.0b013e31802fa199
Kakamu T, Wada K, Smith DR, Endo S, Fukushima T. Preventing heat illness in the anticipated hot climate of the Tokyo 2020 Summer Olympic Games. Environmental Health and Preventive Medicine. 2017;22:68. doi.org/10.1186/s12199-017-0675-y

More training tools

See every free tool →