Heat Index Calculator
Calculate the heat index (feels-like temperature) using the Rothfusz regression formula. Supports °F and °C with risk category indicators.
Heat Index (Feels Like)
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In Celsius
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Feels Hotter By
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Risk Category
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What is Heat Index Calculator?
The heat index is the perceived air temperature when humidity is factored in. High humidity reduces the body's ability to cool itself through sweat evaporation, making it feel hotter than the actual temperature. This calculator uses the official NWS Rothfusz regression equation.
How to use
- 1 Select your temperature unit (°F or °C).
- 2 Enter the current air temperature (must be at least 80°F / 27°C for the formula to apply).
- 3 Enter the relative humidity as a percentage.
- 4 Click Calculate to get the heat index and risk level.
- 5 Take the risk level seriously — heat illness can escalate quickly.
Formula
Example calculation
At 95°F and 60% humidity: HI ≈ 114°F. This falls in the Extreme Danger category, where heat stroke is highly likely with continued exposure.
Frequently asked questions
What temperature does the heat index apply to?
The Rothfusz formula is valid at temperatures of 80°F (27°C) or higher. Below that, the humidity effect is minimal.
What is Extreme Danger heat index?
Extreme Danger is when the heat index exceeds 125°F (52°C). Heat stroke is imminent. Avoid all strenuous outdoor activity.
How is heat index different from actual temperature?
Heat index accounts for how humidity affects your body's ability to cool itself. At 90°F and 90% humidity, it can feel like 132°F.
What humidity level is dangerous?
At high temperatures, any humidity above 60% significantly raises the heat index. Above 80% humidity at 90°F+ is considered dangerous.
Does shade reduce the heat index?
Shade reduces radiant heat but not the heat index itself, since heat index only measures air temperature and humidity. Shade still helps reduce heat stress.