WBGT Calculator (Wet-Bulb Globe Temperature)
Heat stress index from the natural wet-bulb, black globe and air temperatures, outdoors or indoors, with the US Army heat category flag.
Results
What this tool does
The wet-bulb globe temperature was developed for the US Marine Corps in the 1950s after recruits collapsed in training, and it is now the index in ISO 7243 and in most work-in-heat and sports guidelines. Unlike the heat index it counts the sun and the wind: the wet bulb, wrapped in a wet wick and exposed to the air, carries 70 % of the weight because sweat cooling is what the body depends on, and the black globe measures radiant heat. It needs those three sensor readings, not the forecast air temperature and humidity. The flags follow the US Army's five categories, from 25.6 °C upward; work-rest and drinking schedules for each are set in the relevant guidance, not here.
Formula
outdoors: WBGT = 0.7 Tnwb + 0.2 Tg + 0.1 Ta · indoors or shade: WBGT = 0.7 Tnwb + 0.3 Tg
Variables
| Symbol | Meaning | Unit |
|---|---|---|
tw | Natural wet-bulb temperature | °C |
tg | Black globe temperature | °C |
td | Air (dry-bulb) temperature | °C |
sl | Where | — |
WB | Wet-bulb globe temperature (WBGT) | °C |
WF | Same, in Fahrenheit | °F |
CT | US Army heat category | — |
Worked example
- Natural wet-bulb temperature24 °C
- Black globe temperature40 °C
- Air (dry-bulb) temperature30 °C
- Where1
- Wet-bulb globe temperature (WBGT)27.8 °C
- Same, in Fahrenheit82.0 °F
- US Army heat categoryHeat category 2 (green flag)
Limitations
- This tool is informational only. It does not diagnose, treat or replace a qualified health professional. Speak to a doctor before making decisions about your health.
- For work that must comply with a standard or be signed off, check the result against the applicable code and have it reviewed by a qualified engineer.