Enter what you know and this calculator returns the wet bulb globe temperature along with the flag level it falls in. Two modes: from weather, if you have temperature and humidity, and from instruments, if you have three thermometer readings and want the exact value.
Method. Outdoors with a solar load, the reference
weighting is WBGT = 0.7·Tnwb + 0.2·Tg + 0.1·Ta. With no solar load the dry bulb
term drops out and the globe term carries 0.3: WBGT = 0.7·Tnwb + 0.3·Tg. This is an
exact calculation from your three readings, not an estimate.
Method and limits. Shade WBGT is computed from temperature and vapour pressure, then adjusted for sun exposure and wind. It is an estimate, typically within about 2°F of a measured reading in moderate conditions and less certain in calm, intensely sunny air or over hot artificial turf, which can run several degrees hotter than this figure. Where a policy requires on-site measurement, measure on site. The WBGT app runs the full Liljegren physical model on live weather data instead of this simplification.
Which mode should you use?
From instruments is the exact calculation. If you are holding a psychrometer, a black globe and a dry bulb thermometer, or a device that reports all three channels separately, this mode applies the reference weighting and the answer is not an approximation. It is also the mode to use for coursework and certification exams, which test the weighting itself.
From weather is an estimate, and worth understanding before you lean on it. It computes a shade WBGT from air temperature and vapour pressure, then adds a sun and wind adjustment. In moderate conditions it usually lands within about 2°F of a measured reading. It is least reliable in calm, intense sun, and it does not know what you are standing on, which is the part that catches people out.
What a calculator cannot see
A weather-based estimate describes the air at a weather station in a grass field. Your actual environment may be considerably hotter, and these differences are routinely larger than the error in the calculation itself.
Surface under you
Synthetic turf can run 40 to 60°F above air temperature in full sun and radiates into everyone on it. Asphalt behaves the same way.
Enclosure and reflection
A stadium bowl, a trench or a courtyard cuts wind and bounces radiant heat back. Readings there beat an open-field forecast.
Local wind
Wind at a weather station two miles away is not wind at your site. Sheltered ground loses most of the cooling the number assumes.
Timing and thermal lag
WBGT usually peaks after air temperature does, because surfaces keep releasing stored heat into the evening.
How to take a reading that means something
If your policy calls for on-site measurement, where and how you measure decides whether the number is usable.
- Measure at the activity site, not the parking lot, the press box or the building entrance.
- Put the instrument in full sun if the activity is in full sun, at roughly chest height on a tripod.
- Keep it clear of walls, vehicles, generators and anything else radiating or exhausting heat.
- Wet the wick with distilled water and let the device equilibrate, usually 10 to 20 minutes.
- Take the first reading about 15 minutes before activity starts, then repeat on a set interval, commonly every 30 minutes.
- Log every reading with the time. A number nobody wrote down will not help you later.
WBGT flag conditions
A number only helps if it tells you what to do. These are the five flag categories the U.S. military established and athletics and industry adopted. Thresholds shift by guideline and by how acclimatized and how heavily dressed people are, so treat these as the common baseline rather than a universal law.
- Normal activity. Heat stress is low for most people.
- Water available and encouraged throughout.
- Normal activity for acclimatized people.
- Watch anyone unacclimatized, unfit, or in heavy gear.
- Reduce intense exertion and total time in the heat.
- Add rest breaks in shade; increase fluid intake.
- Cut hard training sharply. Unacclimatized people should stop strenuous work.
- Frequent breaks, minimal equipment, close supervision.
- Cancel or postpone outdoor exertion.
- Essential work only, with short cycles and active cooling on hand.
A cold-water immersion tub or equivalent rapid-cooling method should be ready and staffed from roughly 80°F WBGT upward, not improvised after someone goes down.
Worked examples
Four afternoons that all feel hot and read very differently.
| Conditions | Estimated WBGT | Flag |
|---|---|---|
| 100°F, 20% humidity, full sun, moderate wind | 83°F | Green |
| 95°F, 30% humidity, full sun, light wind | 84°F | Green |
| 85°F, 90% humidity, full sun, light wind | 89°F | Red |
| 90°F, 70% humidity, full sun, light wind | 90°F | Red |
| 90°F, 70% humidity, full sun, calm | 91°F | Black |
Read the first and third rows together. The 100°F afternoon is the coolest situation on the list, and the 85°F afternoon is a red flag condition, because dry air lets sweat evaporate and 90% humidity does not. Air temperature ranked these backwards.
Then read the last two rows. Nothing changed but the wind dropping, and the call moved from modify practice to cancel it.
Skip the inputs with the WBGT app
A calculator needs you to know the conditions. The app reads them for your location and runs the full Liljegren physical model on live data, then forecasts the number forward so you can plan a practice or a shift before the day starts.
- Live WBGT for your location, no inputs to type
- Hourly and 14-day WBGT forecasts
- Flag level and guidance for your selected mode
- Any city worldwide, in Fahrenheit or Celsius
WBGT is estimated from weather data with the peer-reviewed Liljegren model. Where a policy requires on-site measurement, use the app for planning and as a cross-check alongside your instrument.
Common questions
How do you calculate WBGT?
Outdoors with a solar load, WBGT = 0.7 x natural wet bulb + 0.2 x black globe + 0.1 x dry bulb. Indoors or with no solar load, WBGT = 0.7 x natural wet bulb + 0.3 x black globe. The dry bulb term exists only to account for sunlight, so it drops out in shade.
Can you calculate WBGT from temperature and humidity alone?
Only as an estimate, and only with assumptions about sun and wind, which are the two inputs doing the most work. Temperature and humidity get you a shade value; whether the activity is in full sun can move the real figure 5 to 8°F. This calculator asks for sun and wind for that reason.
Is this WBGT calculator accurate?
Instrument mode is exact, since it is the reference weighting applied to your readings. Weather mode is an estimate that typically lands within about 2°F in moderate conditions, with more error in calm intense sun and over hot artificial surfaces. For anything a policy governs, measure on site.
What is the difference between WBGT and the wet bulb temperature it contains?
The natural wet bulb reading is one input, weighted 0.7. WBGT is the composite. Natural wet bulb is also measured unaspirated and unshielded, so it is not the same as the psychrometric wet bulb temperature you get from a sling psychrometer or a humidity chart.
What WBGT is too hot to practice?
Most athletic guidelines suspend outdoor practice at or above 90°F WBGT and require significant modification from 88°F. Some state associations and the Texas UIL set their own numbers by region, so check the rule that applies to you.
Does the calculator work in Celsius?
Yes. The °F and °C toggle converts the inputs and the result together. Keep your chart and your reading in the same unit; ACGIH and ISO thresholds are in Celsius while U.S. athletic guidance is in Fahrenheit.
Related WBGT guides
Know the real heat number
Real-time WBGT, flag levels and forecasts. Free on iPhone.
