Heat Index & WBGT Estimator
Enter air temperature and relative humidity to calculate five measures of mugginess, apparent heat, and heat stress. Estimated WBGT is for indoor or shaded conditions without radiant heat.
Estimated WBGT tends to read lower than a true WBGT, so the band shown can be one step below the real one — near a boundary, act on the next band up. These estimates are not a diagnosis or safety guarantee. If anyone feels unwell, move to a cool place regardless of the number and seek medical help when needed.
How to Use It
Enter air temperature from −20 to 50°C and relative humidity from 5 to 99%. Tick "Use °F" and the input field and every temperature result switch to Fahrenheit (−4 to 122°F); a value already entered is converted, and the discomfort index has no unit so it does not change. Every result and the estimated-WBGT risk band update immediately. At the same air temperature, higher humidity reduces evaporative cooling and generally raises Heat Index, wet-bulb temperature, and estimated WBGT.
What Each Result Means
- Discomfort index (DI) is a commonly used temperature-and-humidity indication of perceived mugginess. Comfort varies by person, and DI is not a heat-illness risk measure.
- Apparent temperature (Heat Index) uses the U.S. National Weather Service Rothfusz regression and adjustments. It describes shaded, light-wind hot-weather conditions; direct sunlight can increase heat stress.
- Wet-bulb temperature reflects evaporative cooling. The Stull (2011) approximation assumes standard sea-level pressure and is intended for −20 to 50°C and 5 to 99% RH.
- Estimated WBGT assumes no solar or radiant load and globe temperature equal to air temperature, using
0.7 × wet-bulb + 0.3 × air temperature. It is not an official outdoor WBGT. - Dew point is the approximate temperature at which condensation begins as air cools. It uses the Magnus approximation.
Reading Estimated WBGT
| Estimated WBGT | Band | General daily-life guidance |
|---|---|---|
| Below 25°C (77°F) | Caution | Heat illness is still possible during strenuous activity |
| 25 to below 28°C (77 to 82.4°F) | Warning | Take regular breaks during exercise or heavy work |
| 28 to below 31°C (82.4 to 87.8°F) | Severe warning | Avoid direct sun and watch indoor temperature |
| 31°C or above (87.8°F or above) | Danger | Avoid going out and move to a cool indoor place |
Sources and Limits
WBGT definitions and bands follow Japan's Ministry of the Environment heat-illness guidance; Heat Index follows the U.S. NWS equation; wet-bulb temperature uses Stull, R. (2011), Wet-Bulb Temperature from Relative Humidity and Air Temperature (sources last checked July 23, 2026).
True WBGT uses wet-bulb, globe, and dry-bulb temperatures. This simplified calculation does not include direct sun, reflected heat, wind, nearby heat sources, clothing, workload, age, or individual health. For safety decisions, use current official local guidance or a compliant WBGT meter.
WBGT is also defined with the natural wet-bulb temperature, measured without forced airflow, whereas the Stull (2011) formula used here returns the psychrometric (aspirated) wet-bulb temperature. The natural wet-bulb temperature is always equal to or higher than that, so this estimated WBGT comes out consistently lower than a true WBGT. In other words the error falls on the unsafe side, and the band shown can be one step below the real one (at 35°C and 60% RH this tool shows 30.4°C, "Severe warning", where the real conditions may already be "Danger"). Near a band boundary (25/28/31°C), act on the next band up.