Heat Index Calculator
The "feels like" temperature when high humidity stops your sweat from evaporating.
Apparent temperature
Heat Index
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Category
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Air temp difference
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Why humidity matters
Your body cools itself by sweating — but only if the sweat actually evaporates. In high humidity, the air is already nearly saturated with water, so evaporation slows. Same air temperature, but much harder to lose heat. The heat index captures this with the Rothfusz regression developed by the National Weather Service.
Categories
- Below 80°F: Comfortable — heat index equals air temp.
- 80–90°F: Caution — fatigue with prolonged exertion.
- 91–103°F: Extreme caution — heat cramps and exhaustion possible.
- 104–124°F: Danger — heat exhaustion likely; stroke possible.
- 125°F+: Extreme danger — heat stroke imminent.
Limits of the formula
This is calibrated for shaded conditions and assumes a light wind. Direct sunlight can add another 15°F to the perceived temperature. The formula is also less accurate at the extremes; below 80°F it doesn't apply.