WBGT Calculator
Wet bulb globe temperature from air temp, humidity, and globe reading, with TB MED 507 heat flags.
What it does
Wet bulb globe temperature (WBGT) is the heat-stress index behind military flag conditions, workplace guidelines, and sports safety rules. It combines air temperature, humidity, radiant heat, and wind into one number, and it weighs humidity far harder than a plain thermometer does.
The classic form is WBGT = 0.7 Tw + 0.2 Tg + 0.1 Td outdoors in sun, and WBGT = 0.7 Tw + 0.3 Td in shade or indoors, where Tw is the natural wet bulb temperature, Tg the black globe temperature, and Td the dry bulb air temperature. This calculator computes the wet bulb with Stull's 2011 fit over liquid water, a widely cited approximation that performs well across normal weather.
The flag categories follow U.S. Army TB MED 507, Heat Stress Control and Heat Casualty Management: white at 78 °F, green at 82, yellow at 85, red at 88, and black at 90 °F and up, each with tighter work and rest cycles.
Shade mode drops the globe term because radiant load is largely gone. Sun mode needs a 150 mm matte black globe reading, which a car thermometer or weather app cannot counterfeit.
How to use
- Pick Fahrenheit or Celsius.
- Enter the air temperature.
- Enter the relative humidity.
- Choose full sun or shade.
- In sun mode, add a black globe reading.
- Read the WBGT, then the heat category.
Formula
- Tw
- Natural wet bulb temperature, from the Stull (2011) fit over liquid water.
- Tg
- Black globe temperature, a 150 mm matte black sphere in the sun.
- Td
- Dry bulb air temperature.
- RH
- Relative humidity in percent, 5 to 100.
Stull's wet-bulb estimate stays within a few tenths of a degree in normal conditions and degrades in strong wind or very dry air. Flag bands are defined in °F, so Celsius entries judge on the same scale through an internal conversion. The tool rejects globe readings more than 40 °C below or 50 °C above the air temperature, since those are data entry errors rather than weather.
Worked example
Example: 95 °F, 50 % humidity, full sun
Air 95 °F, relative humidity 50 %, black globe 120 °F in full sun.
- Stull wet bulb: 79.9 °F (26.6 °C).
- WBGT = 0.7 x 79.9 + 0.2 x 120 + 0.1 x 95.
- Terms: 55.9 + 24.0 + 9.5 = 89.4 °F (31.9 °C).
- 89.4 °F clears the 88 °F red line: heat category 4.
- Shade check: 0.7 x 79.9 + 0.3 x 95 = 84.4 °F, category 2.
- Tool shows 89.4 °F with heat category 4 (red flag).
Reference table
| Air temp | Humidity | Wet bulb | Shade WBGT | Category |
|---|---|---|---|---|
| 80 °F | 40 % | 63.9 °F | 68.8 °F | Below flag range |
| 85 °F | 50 % | 71.3 °F | 75.4 °F | Below flag range |
| 90 °F | 60 % | 78.8 °F | 82.2 °F | Category 2 green |
| 95 °F | 50 % | 79.9 °F | 84.4 °F | Category 2 green |
| 100 °F | 30 % | 75.6 °F | 82.9 °F | Category 2 green |
Limits
- Stull's fit assumes liquid-water wet bulbs and light wind; high wind or icy surfaces shift real values.
- Sun mode needs a real 150 mm black globe reading; app temperatures are not globe temperatures.
- Work and rest decisions also depend on workload, clothing, acclimatization, and individual risk.
- Flag bands default to TB MED 507; other organizations may set different cutoffs.
Common mistakes
- Feeding an ordinary air temperature into the globe field.
- Treating shade WBGT as safe: 84.4 °F still sits in a flag category.
- Comparing Celsius readings against Fahrenheit thresholds.
- Using dew point where relative humidity belongs.
- Rounding a red-flag day down to green; the bands are step functions.
Try it yourself
Quick checks. Answers sit tucked under each question.
90 °F, 60 %, full sun, globe 118 °F?
WBGT 87.8 °F, heat category 3 (yellow flag).
100 °F, 40 %, full sun, globe 130 °F?
92.1 °F, category 5 (black flag).
95 °F and 50 % in shade?
84.4 °F, category 2 (green flag).
Questions
What is WBGT?
Wet bulb globe temperature: a heat-stress index weighting wet bulb 0.7, globe 0.2, and air temperature 0.1 in sun. It drives military flag conditions and workplace and sports heat rules worldwide.
Why is shade WBGT lower?
Shade removes radiant load, so the globe term disappears and air temperature carries 0.3 of the score. On the example day that is a 5 °F drop, 89.4 to 84.4.
Where does the wet bulb number come from?
Stull's 2011 fit over liquid water uses air temperature and relative humidity; it matches measured wet bulbs within a few tenths of a degree in normal conditions.
What do the flag colors mean?
TB MED 507 bands: white 78 to 81.9 °F, green 82 to 84.9, yellow 85 to 87.9, red 88 to 89.9, black 90 and up, each with tighter work to rest cycles.
Do I need a black globe?
For sun mode, yes: a 150 mm matte black sphere with a thermometer at its center. Shade mode needs only air temperature and humidity.
Does humidity or heat matter more?
Humidity, because wet bulb carries 0.7 of the score. 90 °F at 60 % and 100 °F at 30 % land within a degree of each other in shade, 82.2 versus 82.9 °F.
Sources
- U.S. Army TB MED 507: Heat Stress Control and Heat Casualty Management (PDF)
- National Weather Service: Wet Bulb Globe Temperature
- Stull 2011: Wet-Bulb Temperature from Relative Humidity and Air Temperature, Journal of Applied Meteorology and Climatology