Tool
WBGT Calculator
WBGT — wet bulb globe temperature — is the heat index that actually governs outdoor work, sport and military training. It combines air temperature, humidity and radiant heat from the sun into one number, and that number is what the flag colours are based on. Enter your conditions below to get it, or read the current reading for your city.
Enter dry bulb temperature and humidity — the WBGT and its flag band appear here.
Both directions follow ISO 7243. Where no globe thermometer is available, the shade figure uses the Stull (2011) wet bulb weighted 0.7 against dry bulb, and the full-sun figure uses the vapour-pressure approximation used by national weather services.
Current WBGT & Wet Bulb — My Location
Type a city to see right now: dry bulb, wet bulb and WBGT, with the flag colour those conditions call for.
Wet bulb from the Stull (2011) approximation. WBGT from the shade form 0.7·Tw + 0.3·Ta, or the full-sun vapour-pressure form 0.567·Ta + 0.393·e + 3.94 — useful when no globe thermometer is on hand, but an on-site instrument always beats an estimate. Live readings from Open-Meteo via the same weather API as Dew Point Today.
What Is WBGT?
Wet bulb globe temperature is a composite index, not a thermometer reading. It was developed by the US Marine Corps in the 1950s after heat casualties during training, and it answers a single question: how much heat stress will a person under this sky actually carry?
Three measurements feed it. The dry bulb is ordinary air temperature. The natural wet bulb is a water-soaked wick exposed to the air, which shows how fast evaporation can cool a surface — the body's main cooling channel. The globe temperature is a blackened sphere that absorbs direct sun and reflected ground heat, standing in for the radiant load on a person.
Because the globe term is in the formula, WBGT is not interchangeable with the heat index. Heat index is a shade calculation published for the general public; WBGT is the index written into exposure limits, which is why employers, coaches and safety officers work in WBGT.
How Is WBGT Calculated?
With instruments on site, WBGT follows the ISO 7243 definitions:
- Outdoors, in full sun: WBGT = 0.7 × natural wet bulb + 0.2 × globe temperature + 0.1 × dry bulb
- Indoors or in shade: WBGT = 0.7 × natural wet bulb + 0.3 × globe temperature
Almost nobody carries a globe thermometer. That is why the calculator above works from the inputs you do have: in shade, the wet bulb is weighted 0.7 against dry bulb at 0.3; in full sun, the estimate runs from dry bulb and vapour pressure (0.567 × Ta + 0.393 × e + 3.94), the form national weather services use for a cloudless day at low elevation. Both are estimates — strong wind, heavy cloud or altitude will pull the real reading below them.
For the wet bulb component itself, this site uses the Stull (2011) approximation, accurate to about ±0.3°C in normal weather. The full derivation is on the wet bulb calculator page.
WBGT Index Chart: Flag Colours and What to Do
Thresholds are in °F because the flag convention comes from US sport and military heat policy. Agencies differ by a degree or two at the edges; the bands below are the common set.
| Flag | WBGT (°F) | Risk | Guidance |
|---|---|---|---|
| White | below 80 | Low | Normal activity. |
| Green | 80 – 84.9 | Moderate | Scheduled water breaks on longer sessions. |
| Yellow | 85 – 87.9 | High | Shorten hard work and training; move out of midday. |
| Red | 88 – 89.9 | Very high | Short bouts only, frequent rest in shade or air conditioning. |
| Black | 90 and above | Extreme | Postpone outdoor work and training. |
WBGT vs Heat Index vs Wet Bulb
Three indices get quoted for the same hot afternoon, and they are not interchangeable — the numbers they produce often differ by 5 to 15 degrees.
| Index | Inputs | Sun and wind | Used for |
|---|---|---|---|
| WBGT | Dry bulb, humidity, radiant load | Yes — globe term | Exposure limits: OSHA, sport, military |
| Heat index | Temperature, humidity | No — assumes shade | Public comfort and heat alerts |
| Wet bulb | Temperature, humidity | Partly — evaporative only | Survivability limit, one WBGT input |
The practical consequence: heat index of 100°F sounds alarming but may sit in the yellow flag band, while a humid 88°F morning can already be red. If you are deciding whether to run, pour concrete or hold practice, use WBGT.
OSHA WBGT Calculator: What Employers Need
OSHA enforces heat hazards through the General Duty Clause rather than a fixed WBGT limit: employers must provide water, rest and shade, and must act before workers get sick. In practice, safety plans are written against the WBGT flag bands above, with work-to-rest ratios tightened as the flag moves from green to yellow to red.
For a site-level number you need three readings — the calculator above takes them in its measured mode. For planning tomorrow, the live reading gives a per-city estimate good enough to schedule around, and the same numbers are updated through the day.
Take the WBGT Reading With You
WBGT changes hour by hour, and the flag band can cross a threshold while you are still on site. The TraceWeather app keeps the current WBGT, wet bulb and flag colour for your saved locations, so you get the number without opening a browser.
WBGT Questions, Answered
How is WBGT calculated?
Outdoors in full sun, WBGT = 0.7 × natural wet bulb + 0.2 × globe temperature + 0.1 × dry bulb. Indoors or in shade the solar term is folded into the globe reading, so WBGT = 0.7 × natural wet bulb + 0.3 × globe temperature. When no globe thermometer is available, the shade figure can be approximated as 0.7 × wet bulb + 0.3 × dry bulb, and the full-sun figure from dry bulb and vapour pressure.
Is WBGT the same as temperature?
No. Air temperature measures how hot the air is; WBGT measures how hard it is for the body to shed heat. A 90°F day at 30% humidity sits in a lower flag band than an 85°F day at 80% humidity, because humidity blocks evaporation and the sun adds radiant load that a plain thermometer never sees.
What is the difference between WBGT and heat index?
Both combine temperature and humidity, but they answer different questions. Heat index is calculated for a person in shade and ignores solar radiation and wind — it is a comfort and public-health measure. WBGT adds the radiant load (globe temperature) and is the index used for occupational and athletic exposure limits, which is why OSHA and military heat policy are written in WBGT rather than heat index.
What is the difference between wet bulb and WBGT?
Wet bulb temperature is one of WBGT's inputs, not a substitute for it. It captures evaporative cooling alone, so it ignores how much radiant heat the sun is adding. On a still, sunny 95°F afternoon the wet bulb reading and the WBGT can be several degrees apart, which is enough to move you across a flag threshold.
What WBGT is unsafe?
In the US flag convention used for sport and outdoor work, 82°F and above is high risk, 85°F and above very high risk, and 90°F and above extreme risk — outdoor work and training should be rescheduled. Separately, the 35°C (95°F) wet bulb limit is the survivability threshold, where sweat can no longer cool the body at all even in shade with unlimited water.
What is the wet bulb globe temperature today?
Enter your city in the live reading above and the page works it out from current temperature and humidity, showing dry bulb, wet bulb, WBGT and the flag colour that applies right now.
Related tools
- Wet Bulb Calculator — dry bulb and humidity to wet bulb, and the 35°C survivability limit
- Dew Point Calculator — temperature and humidity to dew point
- Dew Point Today — live readings by city