WBGT Heat Stress Calculator (estimated)

Understanding the WBGT heat stress calculator — concept and principle

The wet-bulb globe temperature (WBGT) is the heat stress index used in occupational safety, military training and sport to decide how hard and how long people can work or exercise in the heat. A true WBGT reading needs three instruments; this calculator gives a quick estimate from air temperature and relative humidity alone, using the approximation published by the Australian Bureau of Meteorology (BoM).

Air temperature supplies most of the number, humidity about a third.

How it is calculated

Saturation vapour pressure: es = 6.105 × exp(17.27 × T ÷ (237.7 + T)) hPa.
Actual vapour pressure: e = RH ÷ 100 × es.
Estimated WBGT ≈ 0.567 × T + 0.393 × e + 3.94 (°C).

The principle

The full index, defined in ISO 7243, weights three readings: the natural wet-bulb temperature (Tnw), which shows how well sweat can evaporate; the black-globe temperature (Tg), which captures radiant heat from the sun and hot surfaces; and the dry air temperature (Ta). Outdoors in sun, WBGT = 0.7 Tnw + 0.2 Tg + 0.1 Ta; indoors or without sun, WBGT = 0.7 Tnw + 0.3 Tg. Evaporation dominates the weighting because sweating is the body's main way to shed heat. The BoM formula replaces the measurements with a statistical fit to temperature and vapour pressure e, which rises steeply with temperature.

Worked example

Inputs: T = 31 °C, RH = 60%.
es = 6.105 × exp(17.27 × 31 ÷ 268.7) = 44.77 hPa.
e = 0.60 × 44.77 = 26.86240136 hPa.
0.567 × 31 = 17.577 and 0.393 × 26.862 = 10.557.
WBGT ≈ 17.577 + 10.557 + 3.94 = 32.07392373 °C.

Each point is the calculator re-run with only “Air temperature (°C)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Air temperature (°C)	WBGT (°C, approx.: light wind, fairly sunny)
7.75	10.81765642
15.5	16.8720154
23.2	23.82907513
31	32.07392373
38.8	42.11224946
46.5	54.59405763
62	90.36077369

How to read the result

WBGT values are lower than air temperature in dry heat and can approach or exceed it in humid sun. Many occupational and sports guidelines start limiting strenuous activity from the high 20s °C, and the allowable work rate falls as WBGT rises. In US military training, the most severe heat category ("black flag") begins at 32.2 °C (90 °F), when strenuous training is suspended; the example is just below that.

Common mistakes

Treating this estimate as a measured WBGT; it cannot see the sun, wind or hot surfaces.
Confusing WBGT with the heat index; WBGT numbers are lower for the same conditions, so thresholds are not interchangeable.
Ignoring workload and clothing; heavy work or protective clothing makes the same WBGT far more dangerous.

Keep in mind: This is an approximation, not a diagnosis or a safety certification. The BoM formula assumes light wind and fairly sunny conditions, so actual WBGT is higher in strong sun and still air, lower in shade or wind. For workplace decisions use a measured WBGT and the rules of your national safety authority; stop activity and seek medical help for signs of heat illness.

Frequently asked questions

What instrument measures WBGT?

A WBGT meter combines a natural wet-bulb thermometer, a 150 mm black globe thermometer and a shaded dry-bulb thermometer.

Can WBGT be higher than air temperature?

Yes, in strong sun and high humidity, as the globe temperature can be far above the air temperature.

Sources: ISO 7243:2017, Ergonomics of the thermal environment — Assessment of heat stress using the WBGT index · Australian Bureau of Meteorology, About the WBGT and Apparent Temperature indices · Yaglou C. P. and Minard D., Control of heat casualties at military training centers, AMA Archives of Industrial Health 16, 302–316 (1957)

Checked 8 Nov 2026. Standard textbook formulas in SI units, computed in your browser. For learning and estimates — real designs must follow your local codes and be checked by a qualified engineer.

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