Heat Index Calculator
Free heat index calculator finds the 'feels like' temperature from air temperature and humidity using the NWS Rothfusz equation, with caution levels and the formula.
Updated 2026-06-14 · Free · No sign-up · Runs privately in your browser
Show the formula & steps
How the Heat Index Calculator Works
The heat index is the temperature it actually feels like when relative humidity is factored in with the real air temperature. Enter the temperature (in °F or °C) and the relative humidity, and this tool returns the “feels like” temperature plus the National Weather Service caution level.
The Formula
For accuracy, this calculator uses the official NWS Rothfusz regression, which operates in Fahrenheit:
HI = −42.379 + 2.04901523·T + 10.14333127·R − 0.22475541·T·R − 0.00683783·T² − 0.05481717·R² + 0.00122874·T²·R + 0.00085282·T·R² − 0.00000199·T²·R²
where T is air temperature (°F) and R is relative humidity (%). When the simple Steadman average comes out below 80°F, that lighter formula is used instead. Adjustments are applied at very low humidity (R < 13%) and very high humidity (R > 85%) for greater precision.
Worked Example
For 90°F and 70% relative humidity:
- The full Rothfusz equation yields a heat index of about 105°F.
- 105°F falls in the Danger range, where heat cramps and heat exhaustion are likely.
So although the thermometer reads 90°F, it feels like 105°F to the body.
Heat Index Caution Levels
| Heat index (°F) | Category | Risk |
|---|---|---|
| 80-90 | Caution | Fatigue possible with prolonged exposure |
| 91-103 | Extreme Caution | Heat cramps and exhaustion possible |
| 103-124 | Danger | Heat cramps/exhaustion likely; heat stroke possible |
| 125+ | Extreme Danger | Heat stroke highly likely |
Staying Safe in High Heat Index
- Drink water steadily; don’t wait until you’re thirsty.
- Limit strenuous activity to early morning or evening.
- Take frequent breaks in shade or air conditioning.
- Remember that direct sun can add up to 15°F beyond the calculated heat index.
Frequently asked questions
What is the heat index?+
The heat index, or apparent 'feels like' temperature, combines air temperature and relative humidity to estimate how hot it actually feels to the human body. High humidity slows sweat evaporation, so the body cannot cool as efficiently and the air feels hotter than the thermometer reading.
How is the heat index calculated?+
The US National Weather Service uses the Rothfusz multiple-regression equation, which takes temperature in °F and relative humidity in percent and outputs the heat index in °F. Special adjustments are applied when humidity is below 13% or above 85% within certain temperature ranges.
At what heat index is it dangerous?+
80-90°F is Caution, 91-103°F is Extreme Caution, 103-124°F is Danger, and 125°F or higher is Extreme Danger. In the Danger range and above, heat cramps, heat exhaustion and heat stroke become likely with prolonged exposure or activity.
Why does humidity make it feel hotter?+
Your body cools itself by evaporating sweat. When the air is already humid, less sweat can evaporate, so heat stays trapped and your effective temperature rises. That is why 90°F at 70% humidity feels far hotter than 90°F in dry desert air.
Is heat index the same in the shade?+
The standard heat index assumes shade and light wind. Direct sunlight can add up to about 15°F to the apparent temperature, so the real 'feels like' value outdoors in full sun is usually higher than the calculated figure.