Heat Index Calculator
Heat index from °F and relative humidity — for hot days. Click any i for detail.
Weather
Air temperature (°F)
i
Temp
Ambient temperature in Fahrenheit. |
|
|---|---|
Relative humidity (%)
i
Humidity
Relative humidity from 0–100%. |
How to use this calculator
- Enter air temperature in °F.
- Enter relative humidity percent.
- Read heat index when temperature is in the valid hot range.
Results explained
Heat index combines temperature and humidity into an apparent temperature for shady conditions. The Rothfusz regression used here is the common NWS-style worksheet for warmer weather (about 80°F and up). Full NWS implementations add small adjustments at the edges; this page keeps the core polynomial for clarity. Heat illness risk also depends on sun, exertion, and acclimatization.
Quick reference: heat index
Apparent temperature from T (°F) and RH (%).
| Item | Detail |
|---|---|
| Typical validity | About 80°F and warmer |
| Caution band | Often discussed near 90°F heat index |
| Danger band | Higher heat-index values — shorten exposure |
| Cousin | Wind chill for cold, windy days |
Not a medical heat-stress monitor — use workplace/WBGT guidance when required.
How the estimate is built
Rothfusz heat-index polynomial in T (°F) and RH (%); below ~80°F we show air temperature.
Example scenario
90°F at 60% RH → heat index about 100°F (rounded).
FAQ
Why is it blank or equal to air temp under 80°F?
The usual heat-index chart is for hot conditions. Below that we show air temperature instead of forcing the polynomial.
Does sun make it worse?
Yes — heat index assumes shade. Direct sun can feel several degrees hotter.
Is this wet-bulb globe temperature?
No. WBGT is a different workplace/military metric. Heat index is the everyday “feels like” number.
Celsius inputs?
Convert to °F first, or use a metric heat-index tool.