What is the WBGT (Wet Bulb Globe Temperature)?

WBGT is a composite heat stress index that combines humidity, radiant heat (sunlight) and air temperature into a single number. Outdoors it is WBGT = 0.7*T_w + 0.2*T_g + 0.1*T_a; indoors (no sunlight) it is WBGT = 0.7*T_w + 0.3*T_g. T_w is the natural wet-bulb temperature, T_g is the globe temperature, and T_a is the dry-bulb air temperature. It is the basis for activity and work limit decisions in JIS Z 8504 and the Japanese MHLW guidelines.

Why use WBGT instead of plain air temperature?

Heat illness risk depends not just on air temperature but strongly on humidity and radiant heat. For example, 32 C with 65% relative humidity feels very different from 32 C with 40% RH because evaporative cooling from sweat works very differently. WBGT gives a 70% weight to wet-bulb temperature so humidity dominates the index, and uses globe temperature so radiant solar heating is included as well.

How do the WBGT risk categories work?

Per the Japanese MHLW guidelines, WBGT below 25 is essentially safe, 25-28 is caution, 28-31 is warning, 31-35 is strong warning, and 35 and above is danger. This tool colour-codes the category for the current value and overlays the bands on the chart so you can read the risk at a glance.

How is the natural wet-bulb temperature T_w computed?

We use the Stull (2011) approximation, which estimates the natural wet-bulb temperature directly from the dry-bulb temperature and relative humidity. A real WBGT meter uses a gauze-wrapped wet-bulb thermometer under natural ventilation; the Stull formula reproduces that measurement with about 0.3-0.5 C accuracy over typical meteorological ranges, which is good enough for occupational health monitoring when you have weather data but no instrument.

WBGT Heat Index Simulator — Free Online Calculator

Parameters

Dry-bulb temperature T_a

°C

Relative humidity RH

Solar radiation

W/m²

Setting (0 = outdoor / 1 = indoor)

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Outdoor uses the solar-inclusive formula WBGT=0.7T_w+0.2T_g+0.1T_a; indoor uses the no-sun formula WBGT=0.7T_w+0.3T_g.

Results

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Natural wet-bulb T_w (Stull)

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Globe temperature T_g (est.)

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WBGT index

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Risk category

WBGT vs. dry-bulb temperature with risk bands

x = dry-bulb T_a (°C) / y = WBGT (°C) / curves = iso-RH at 30/60/90% / bands = risk categories / yellow dot = current (T_a, WBGT)

Theory & Key Formulas

WBGT (Wet Bulb Globe Temperature) is a composite heat stress index that combines humidity, radiant heat and air temperature into a single number. The formula differs between outdoor (with sun) and indoor (no sun) settings.

Outdoor:

$$\text{WBGT} = 0.7\,T_w + 0.2\,T_g + 0.1\,T_a$$

Indoor (no sun):

$$\text{WBGT} = 0.7\,T_w + 0.3\,T_g$$

Natural wet-bulb (Stull 2011 approximation):

$$T_w = T_a\arctan\!\bigl[0.151977\sqrt{\text{RH}+8.313659}\bigr] + \arctan(T_a+\text{RH}) - \arctan(\text{RH}-1.676331) + 0.00391838\,\text{RH}^{1.5}\arctan(0.023101\,\text{RH}) - 4.686035$$

Simple globe-temperature estimate:

$$T_g \approx T_a + 0.005\,S_{\text{solar}}$$

T_a is the dry-bulb temperature [°C], RH is relative humidity [%], S_solar is solar irradiance [W/m²]. The Stull formula reproduces the natural wet-bulb temperature from just air temperature and humidity, with about 0.3 to 0.5 C accuracy.

What is the WBGT heat index simulator?

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In Japan in summer you keep hearing "today's WBGT is over 28, so outdoor activities are cancelled." Why don't they just look at the air temperature?

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Roughly speaking, the human body is hammered not just by temperature but by humidity and radiant heat too. Air temperature alone misses dangerous days. WBGT puts a 70% weight on the wet-bulb temperature so it captures humidity strongly, and it adds the globe temperature to capture radiant solar heat. Drag the humidity slider above from 30 to 90 at a fixed 32 C and watch how much the WBGT moves.

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Yes, when I raise humidity the WBGT shoots up and the risk colour changes. Why does humidity matter so much?

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The main way humans cool down is by evaporating sweat. When humidity is high, sweat does not evaporate and the body cannot dump heat. That is why the wet-bulb term carries a coefficient of 0.7 in the WBGT formula. At 32 C and 65% RH outdoors, WBGT is about 29 C, which falls into the "warning" band; sports and outdoor work guidelines recommend extra breaks at that point.

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When I switch the indoor slider to 1, changing the solar radiation no longer moves WBGT. Why?

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In indoor mode the tool assumes no direct sun, so the globe temperature is equal to the air temperature. The formula also switches to WBGT = 0.7 T_w + 0.3 T_g and the explicit air-temperature term disappears. In a real gym or factory without direct sunlight, globe and air temperatures are essentially equal. Outdoors under strong sun, by contrast, the globe temperature usually runs 5 to 10 C above air temperature.

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You compute the natural wet-bulb temperature with the Stull approximation from air temperature and humidity. How well does that match a real wet-bulb thermometer?

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Stull (2011) is accurate to about plus/minus 0.3 to 0.5 C against measured natural wet-bulb across temperatures 5 to 50 C and humidities 5 to 99%. Because it only needs T and RH, which come from any weather station, it is widely used for real-time heat stress monitoring in occupational health and disaster prevention. When you do not have a WBGT meter on site, the Stull approximation is a very useful substitute.

FAQ

We use the thresholds from the Japanese MHLW "Heat illness prevention at work" guidelines and JIS Z 8504: below 25 almost safe, 25 to 28 caution, 28 to 31 warning, 31 to 35 strong warning, 35 and above danger. The Japan Sport Association's sports activity guidelines use essentially the same thresholds: strenuous exercise stopped at WBGT 28, exercise in general stopped at WBGT 31.

A globe thermometer is a 15 cm hollow copper sphere painted matte black; its surface temperature is set by the balance between solar absorption and convective cooling. To first order the rise above air temperature is proportional to solar irradiance. Under typical outdoor wind conditions, T_g approximately equals T_a + 0.005 * S (with S in W/m^2) is a widely used field rule of thumb. This tool uses that simple form; differences of a few degrees from a real globe thermometer are expected depending on wind and globe properties.

Outdoor mode uses the solar-inclusive formula WBGT = 0.7 T_w + 0.2 T_g + 0.1 T_a and estimates T_g as T_a + 0.005 * S. Indoor mode assumes no direct sunlight, sets T_g = T_a, and switches to WBGT = 0.7 T_w + 0.3 T_g. The globe coefficient changes from 0.2 to 0.3 and the explicit air-temperature term disappears, matching the two formulas defined in JIS Z 8504.

For outdoor sport the standard guidance is to stop strenuous exercise above WBGT 28 and to stop exercise in general above WBGT 31. For occupational work the thresholds depend on workload: heavier work requires lower WBGT thresholds and longer breaks. This tool gives a rough estimate; for actual stop decisions, use a measured WBGT on site together with the workload, clothing and acclimatization of the people involved. Use lower thresholds for elderly people, infants, and those with chronic conditions.

Real-world applications

Occupational health and worksite management: Construction sites, steel mills and outdoor maintenance work in Japan are required to manage WBGT during the summer months. The MHLW guidelines define WBGT thresholds for each workload class; above the threshold, employers must extend breaks, deploy cooling vests or air-conditioned suits, or shift the work to cooler hours. This simulator lets you plug in expected temperature, humidity and solar radiation to get a first risk-category estimate before going on site.

School physical education and sports: The Japan Sport Association's heat-illness prevention guidelines are written in WBGT and are used in schools to decide whether to cancel PE classes or club activities. Direct sun on a ground vs. a humid gym call for different treatments; switching between indoor and outdoor mode in this tool gives a more realistic risk picture for each setting.

Event and entertainment safety: Organizers of summer festivals and outdoor sport events increasingly manage heat-illness risk using WBGT. Combining the Ministry of the Environment's WBGT forecasts with scenario calculations from this tool helps inform decisions on adding hydration stations, deploying misting showers, and ultimately on postponing the event when the forecast WBGT is too high.

Building and HVAC design: For gymnasiums, sports facilities and factory work areas, designers must estimate the indoor WBGT under expected weather and solar conditions and size the HVAC accordingly. Indoor mode in this tool isolates the humidity contribution from solar effects, which is convenient for HVAC target-setting calculations.

Common misconceptions and cautions

The most common misconception is to assume that "if the air temperature is not high, you will not get heat illness." Because the wet-bulb term carries a weight of 0.7 in WBGT, a hot-and-humid day at 28 C with 90% RH can easily push WBGT above 28 and into the "warning" category, even though the apparent temperature is moderate. The end of the rainy season in Japan is a classic example. Trust the computed WBGT rather than your subjective sense of warmth.

The second common error is to assume that being indoors is automatically safe. Indoor mode removes the solar term, but the humidity term is unchanged. In large spaces with poor ventilation such as gyms or factories, the indoor air can carry the same humidity as outside and the temperature can still climb above 30 C, pushing the indoor WBGT into the "strong warning" range. Switching this tool to indoor mode while plugging in the actual indoor temperature and humidity makes such indoor heat stress visible.

Finally, remember that WBGT is an average-population index and does not account for individual risk. Elderly people, infants, those with chronic illness and people who are not yet acclimatized are at much greater risk at the same WBGT. The risk category from this tool is only a general guideline; on site, the rule of thumb is "design for the most vulnerable person present." For rigorous decisions, complement it with on-site measurement using a calibrated WBGT meter, because both Stull's approximation and the simple globe-temperature estimate carry uncertainty.

WBGT (Wet Bulb Globe Temperature) Heat Index Simulator

What is the WBGT heat index simulator?

FAQ

Real-world applications

Common misconceptions and cautions

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