🥵 Heat Index Calculator
Enter the temperature and humidity to get the heat index — how hot it actually feels — plus the NWS risk level.
Heat index (feels like)
106°F
41°C
Risk level
Danger
The heat index is how hot it feels when humidity slows sweat evaporation, from the NWS Rothfusz regression. It applies in warm, humid conditions (roughly 80°F+ / 27°C+). In direct sun it can feel up to ~15°F hotter still. 🔒 In your browser.
How the heat index calculator works
When it's humid, sweat evaporates slowly, so your body can't cool as well and the air feels hotter than the thermometer reads. The heat index quantifies that using the National Weather Service's Rothfusz regression — a formula fitted to temperature and relative humidity. The tool applies it (with the NWS low- and high-humidity adjustments) and shows the matching heat-risk category.
The heat index is calculated for shade and light wind — in full sunshine it can feel up to about 15°F (8°C) hotter still. It becomes meaningful in warm conditions, roughly 80°F (27°C) and above. Above a heat index of 103°F it's in the "danger" zone: limit exertion, hydrate and seek cool spaces.
Frequently asked questions
What is the heat index?
A "feels like" temperature that combines air temperature and humidity. Because humidity slows sweat evaporation, humid heat feels hotter — a 90°F day at 70% humidity has a heat index of about 105°F.
How is the heat index calculated?
From the National Weather Service Rothfusz regression, a formula in temperature (°F) and relative humidity (%). The NWS adds small corrections at very low and very high humidity. This tool implements the full official method.
What heat index is dangerous?
The NWS bands are: 80–90°F caution, 90–103°F extreme caution, 103–125°F danger, and above 125°F extreme danger. In the danger zone, heat cramps, exhaustion and heatstroke become likely with activity.
Does the heat index account for sun?
No — it assumes shade and a light breeze. Direct sunlight can add up to about 15°F (8°C) to how hot it feels, so treat the value as a minimum on a sunny day.
What's the difference between heat index and temperature?
Air temperature is what the thermometer reads; the heat index adds the effect of humidity on your body's cooling. When it's dry the two are similar; when it's humid the heat index is higher.
Related weather tools
From the blog
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Dew Point vs Relative Humidity: Why 100% Humidity Can Feel Fine and 50% Can Feel Awful
Relative humidity depends on temperature, so it tells you almost nothing about comfort. Dew point measures the actual moisture in the air. Here's the comfort scale, why sweat is the mechanism, and why 100% RH at 30°F feels dry.
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Why 'Feels Like' Isn't the Temperature: Heat Index, Wind Chill and Wet-Bulb
The thermometer only tells half the story. In humid heat, sweat can't evaporate — the heat index. In cold wind, heat is stripped away — the wind chill. And the wet-bulb temperature sets the hard limit on how much the body can cool at all. What each means and when it matters.