Heat Index Calculator
Why Humidity Makes Heat More Dangerous
The body cools itself mainly through sweat evaporation, and evaporation slows dramatically in humid air because the surrounding air is already close to saturated with moisture. Heat index — the "feels like" temperature the National Weather Service issues advisories around — captures that compounding effect, and at high enough readings it becomes a genuine safety threshold rather than just a discomfort figure.
The Formula
T is air temperature in °F and R is relative humidity in percent. This is the NOAA/NWS Rothfusz regression, the standard formula used for official heat index reporting. It only applies when a simpler Steadman approximation, averaged with air temperature, indicates a result of 80°F or higher — below that the simpler estimate is used directly. Additional adjustment terms apply at low humidity (RH below 13% with T between 80–112°F) and high humidity (RH above 85% with T between 80–87°F).
Worked Examples
| Air temperature | Relative humidity | Heat index |
|---|---|---|
| 85°F | 40% | 84.33°F |
| 90°F | 70% | 105.92°F |
| 95°F | 60% | 113.09°F |
NWS Heat Index Danger Categories
| Heat index range | Category | Risk |
|---|---|---|
| 80–90°F | Caution | Fatigue possible with prolonged exposure or activity |
| 91–103°F | Extreme Caution | Heat cramps and heat exhaustion possible |
| 104–124°F | Danger | Heat cramps and heat exhaustion likely; heatstroke possible |
| 125°F+ | Extreme Danger | Heatstroke highly likely |
Where This Matters
- Outdoor labor and athletics — employers and coaches use heat index thresholds to schedule work breaks, hydration protocols, or cancel outdoor activity entirely.
- Vulnerable populations — young children, older adults, and people with certain health conditions face elevated heat-related illness risk at heat index levels that healthy adults tolerate.
- Event and travel planning — checking heat index rather than raw temperature gives a more accurate picture of what a day will actually feel like outdoors.
How to Use This Calculator
- Enter the Air Temperature in °F.
- Enter the Relative Humidity as a percentage (0–100).
- Select Calculate to see the heat index.
Related Calculations
In cold conditions, use the Wind Chill Calculator instead, or check the underlying humidity reading with the Humidity Calculator.