What this calculator does
Wet Bulb Globe Temperature, or WBGT, is the heat-stress index used by sporting bodies, the military and workplace safety regulators to decide when conditions are too dangerous for strenuous activity. Unlike a simple air temperature reading, WBGT combines three separate measurements: humidity, radiant heat from the sun, and air temperature, into one number that better reflects how hot the body actually feels under exertion.
This is a different figure from plain wet-bulb temperature or the heat index used in weather forecasts. Wet-bulb temperature alone only captures the cooling effect of evaporation, and heat index is built from temperature and humidity without accounting for direct sun exposure. WBGT adds a black globe thermometer reading specifically to capture that radiant heat load, which is why it is the standard used for outdoor sport and outdoor work rather than a shaded weather-station reading.
The formula
The outdoor formula weights the three readings unevenly, reflecting how much each contributes to the body’s actual heat load: WBGT = 0.7 x natural wet-bulb temperature + 0.2 x black globe temperature + 0.1 x dry-bulb (ordinary air) temperature. Humidity, carried through the wet-bulb reading, dominates the result because evaporative cooling is the body’s main defence against overheating.
| Term | Meaning |
|---|---|
| Tw (wet-bulb) | Temperature measured with a thermometer kept wet and exposed to airflow, reflecting cooling by evaporation. |
| Tg (globe) | Temperature measured inside a matte black copper sphere, reflecting radiant heat from the sun. |
| Td (dry-bulb) | Ordinary air temperature, measured with a standard thermometer shielded from direct sun. |
The inputs explained
| Field | What to enter |
|---|---|
| Natural wet-bulb temperature (°C) | The natural wet-bulb reading, from a thermometer with a wet wick exposed to natural airflow. |
| Black globe temperature (°C) | The black globe temperature, from a black globe thermometer left to reach equilibrium in the sun. |
| Dry-bulb (air) temperature (°C) | The ordinary shaded air temperature. |
When to use it
Deciding whether outdoor sport should go ahead
Many sporting codes set specific WBGT thresholds for modifying or cancelling outdoor training and matches. Working out the current WBGT from on-site readings is how those thresholds are actually applied.
Setting work and rest cycles on an outdoor job site
Occupational health guidelines often specify work-to-rest ratios that step down as WBGT rises. Calculating the current WBGT is the first step in applying that guidance.
Comparing conditions across a hot afternoon
Recording wet-bulb, globe and dry-bulb readings at intervals through a hot day and recalculating WBGT each time tracks how the real heat-stress risk changes, rather than relying on air temperature alone.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does WBGT change as globe temperature rises on a humid day?
A fixed humid, warm day with a range of globe (radiant heat) readings.
| Globe temperature | WBGT | Heat-stress category |
|---|---|---|
| 28°C | 25.4 °C | High: schedule breaks and reduce intense activity |
| 32°C | 26.2 °C | High: schedule breaks and reduce intense activity |
| 36°C | 27.0 °C | High: schedule breaks and reduce intense activity |
| 40°C | 27.8 °C | High: schedule breaks and reduce intense activity |
| 44°C | 28.6 °C | Very high: limit strenuous activity, especially for the unacclimatised |
| 48°C | 29.4 °C | Very high: limit strenuous activity, especially for the unacclimatised |
Questions
How is WBGT different from the heat index?
Heat index is built from air temperature and humidity alone and assumes shade. WBGT adds a black globe reading specifically to capture direct sun exposure, which is why it is used outdoors for sport and physical work, where heat index would understate the risk.
Why does wet-bulb temperature carry the largest weight in the formula?
Evaporative cooling, through sweating, is the body’s primary defence against heat during exertion. High humidity blunts that cooling, so the wet-bulb reading, which reflects humidity, is weighted at 0.7 against 0.2 for radiant heat and 0.1 for plain air temperature.
Is this the same formula used indoors?
No. This is the outdoor formula. An indoor or no-direct-sun formula drops the globe term and reweights wet-bulb and dry-bulb temperature differently, so it is not calculated here.
What do the WBGT risk categories actually mean?
They are commonly used general bands, roughly: below 18°C low concern, 18 to 23°C moderate, 23 to 28°C high, 28 to 30°C very high, and above 30°C extreme, at which point strenuous outdoor activity is often curtailed. Specific sporting codes and workplaces may set their own thresholds.
For a simpler read on evaporative cooling alone, see the wet bulb temperature calculator, or for the shaded, humidity-and-temperature-only figure used in weather forecasts, see the heat index calculator.