Heat Index Calculator
Enter the air temperature and relative humidity to get the “feels like” temperature — the heat index — using the official National Weather Service formula.
How to use this heat index calculator
In the calculator above, choose a mode that takes air temperature, enter your temperature and relative humidity, and read the Heat Index (Feels Like) line in the results. The heat index appears whenever the temperature is 80°F (27°C) or higher — the range where the formula is valid and humidity meaningfully raises heat stress.
What is the heat index?
The heat index — also called apparent temperature — is how hot the air actually feels once humidity is taken into account. Your body cools itself by evaporating sweat. When the air is already loaded with moisture, sweat evaporates slowly, heat builds up, and the same 95°F afternoon feels far hotter in Miami than in Phoenix. The heat index puts a single number on that difference.
How it's calculated
This tool uses the Rothfusz regression, the multi-term polynomial the U.S. National Weather Service publishes. It blends air temperature and relative humidity and then applies two correction terms — one for very dry air, one for very humid air — so the result tracks the official NWS heat index chart.
NWS heat index categories
| Heat index | Category | Risk |
|---|---|---|
| 80–90°F | Caution | Fatigue possible with prolonged exposure and activity |
| 90–103°F | Extreme Caution | Heat cramps and heat exhaustion possible |
| 103–124°F | Danger | Heat cramps or exhaustion likely; heat stroke possible |
| 125°F and up | Extreme Danger | Heat stroke highly likely |
Worked example
At 90°F with 70% relative humidity, the heat index is about 105°F — squarely in the NWS “Danger” band, where heat cramps and exhaustion become likely with prolonged exposure. Drop the humidity to 30% and the same 90°F feels closer to 88°F.
Related reading: How Humidity Affects Your Health, Allergies, and Asthma and the wet-bulb calculator for the survival-limit side of heat stress.
Frequently Asked Questions
- What is the heat index?
- The heat index, or apparent temperature, is how hot it actually feels to the human body once relative humidity is combined with the air temperature. High humidity slows the evaporation of sweat, so the body cannot shed heat as efficiently and the air feels hotter than the thermometer reads.
- How is the heat index calculated?
- This calculator uses the Rothfusz regression, the same multi-term equation the U.S. National Weather Service uses. It combines air temperature and relative humidity, with low-humidity and high-humidity adjustments, to produce the feels-like temperature.
- Why does the heat index need 80°F or higher?
- The Rothfusz regression is only valid at about 80°F (27°C) and above, where heat stress from humidity becomes meaningful. Below that threshold the calculator reports the other humidity metrics but not a heat index, because the formula would not be accurate.
- What is the difference between heat index and temperature?
- Air temperature is what a thermometer measures in the shade. The heat index adds the effect of humidity on top of it. At 96°F and 65% humidity the heat index climbs above 120°F, even though the thermometer still reads 96°F.
- What heat index is dangerous?
- The NWS uses four bands: Caution (80–90°F), Extreme Caution (90–103°F), Danger (103–124°F), and Extreme Danger (125°F and above). Heat cramps and exhaustion become likely in the Danger band, and heat stroke is probable in Extreme Danger.