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Heat Index Calculator

Estimate heat index (feels like temperature) from air temperature and humidity. Includes danger level, dehydration risk, and safe outdoor timing guidance.

Feels Like Heat Humidity Impact Danger Levels Hydration Guidance

All-in-One Heat Index, Humidity And Safety Suite

Switch between heat index calculator, dehydration risk, exposure guidelines, humidity conversion, heat stress index and safety levels.

Uses NOAA heat index formula for temperatures ≥ 80°F (27°C) and humidity ≥ 40%.

Uses heat index to estimate dehydration risk and outdoor time safety.

See how humidity affects feels like temperature at a fixed air temperature.

Heat stress index (0–10) based on heat index severity.

Convert °F ↔ °C and humidity dew pointationships.

General NWS-style guidance for heat index danger levels.

Heat Index (°F) Heat Index (°C) Risk Category Guidance
80–90 27–32 Caution Fatigue possible with prolonged exposure or activity.
90–103 32–39 Extreme Caution Heat cramps and heat exhaustion possible.
103–125 39–52 Danger Heat cramps and heat exhaustion likely; heat stroke possible.
125+ 52+ Extreme Danger Heat stroke highly likely with continued exposure.

Important Reminder

Heat index assumes shaded conditions with a light breeze. Direct sunlight can increase the effective heat index by 10–15°F (6–8°C). Hydration, clothing, medical conditions and activity play major roles in real-world heat stress.

Heat Index Calculator – Feels Like Heat, Humidity And Safety

This Heat Index Calculator helps you estimate how hot it really feels when humidity interacts with air temperature. High humidity slows the body’s ability to cool itself, making the real-feel temperature higher than the thermometer reading. You can use this tool to estimate heat index, dehydration risk, danger levels and safe outdoor timing.

How the Heat Index Formula Works

The heat index is calculated using a NOAA-style regression formula based on temperature and humidity. It is most accurate when the air temperature is 80°F (27°C) or higher and humidity is above 40%. Outside this range, the formula still gives a useful approximation but may not reflect official heat advisories.

Danger Levels and Exposure Guidance

As heat index increases, the risk of heat exhaustion and heat stroke rises sharply. Light activity may feel manageable at 90°F heat index but dangerous at 105°F. Hydration, shade and activity intensity all play major roles. The danger-level tab provides detailed hydration and safety suggestions.

Humidity-Driven Feels Like Table

Humidity dramatically changes perceived heat. Even modest temperature shifts can feel extreme when humidity spikes. The table tab shows how feels like temperature changes at humidity levels from 20% to 100% at the same air temperature.

Heat Stress Index (0–10)

The heat stress index gives a simple 0–10 scale rating of thermal stress. Zero is mild; ten is extreme. This can help with training logs, outdoor work planning and shift safety management.

Unit Conversion

Many weather sources mix Fahrenheit, Celsius and humidity readings. The converter tab helps you align readings across US-style and metric-style forecasts.

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