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.

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Heat Index Calculator - Feels Like Temperature Online

Calculate the heat index (feels-like temperature) using the Rothfusz regression formula. Supports °F and °C with risk category indicators.

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How to use this calculator

👉 Fill in the boxes below and your answer appears instantly — no maths needed, we do it all for you! 🎉

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In plain English — what does this do?

📏 Some countries say ‘30 degrees’ and others say ‘86 degrees’ for the same weather! Some say ‘5 km’, others say ‘3 miles’. This tool swaps between them instantly so you always know what it means.

📌 Direct Answer & Summary

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.

Temperature Unit

Air Temperature

Relative Humidity (%)

What is Heat Index Calculator - Feels Like Temperature Online?

How to use it

1️⃣ Select your temperature unit (°F or °C).
2️⃣ Enter the current air temperature (must be at least 80°F / 27°C for the formula to apply).
3️⃣ Enter the relative humidity as a percentage.
4️⃣ Click Calculate to get the heat index and risk level.
5️⃣ Take the risk level seriously — heat illness can escalate quickly.

Formula

HI = −42.379 + 2.04901523T + 10.14333127RH − 0.22475541TRH − 0.00683783T² − 0.05481717RH² + 0.00122874T²RH + 0.00085282TRH² − 0.00000199T²RH² (T in °F, RH in %). Adjusted when RH < 13% and 80°F < T < 112°F.

💡 See it in action — a real example

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.

❓ Common 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.

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