Heat Index Calculator
Calculate the heat index (feels-like temperature) from air temperature and relative humidity using the Rothfusz regression equation. Includes danger categories, activity guide, and hydration recommendations.
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Heat Index
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Danger Category —
Caution Note —
Extended More scenarios, charts & detailed breakdown ▾
°F
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Heat Index
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Heat Index (°C) —
Danger Category —
Feels Hotter By —
Professional Full parameters & maximum detail ▾
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Temperature Indices
Heat Index (°F) —
Heat Index (°C) —
Humidex (Canadian Scale) —
WBGT Estimate —
Risk & Activity
Danger Category —
Activity Modification Zone —
Additional Factors
UV Index Note —
Acclimatization Note —
How to Use This Calculator
- Enter the air temperature in °F and relative humidity as a percentage.
- The calculator shows the heat index and danger category.
- Use the Activity Guide tab for outdoor work and exercise recommendations.
- Use the Hydration tab for water intake recommendations based on heat index and activity level.
- Use the Professional tab for WBGT estimate, humidex, and hourly water intake guidance.
Formula
Rothfusz Regression (NWS):
HI = −42.379 + 2.04901523T + 10.14333127RH − 0.22475541T·RH − 0.00683783T² − 0.05481717RH² + 0.00122874T²·RH + 0.00085282T·RH² − 0.00000199T²·RH²
HI = −42.379 + 2.04901523T + 10.14333127RH − 0.22475541T·RH − 0.00683783T² − 0.05481717RH² + 0.00122874T²·RH + 0.00085282T·RH² − 0.00000199T²·RH²
Example
Example: T = 95°F, RH = 60%. Heat Index ≈ 114°F — Danger category. Outdoor activity should be severely limited.
Frequently Asked Questions
- The heat index (also called apparent temperature) combines air temperature and relative humidity to estimate how hot it feels to the human body. High humidity reduces the body's ability to cool itself by sweating, making it feel hotter.
- This calculator uses the Rothfusz regression equation developed for the National Weather Service: HI = −42.379 + 2.04901523T + 10.14333127RH − ..., where T is temperature in °F and RH is relative humidity in percent.
- Caution: 80–90°F (fatigue possible). Extreme Caution: 90–103°F (heat cramps/exhaustion possible). Danger: 103–124°F (heat cramps/exhaustion likely, heat stroke possible). Extreme Danger: 125°F+ (heat stroke highly likely).
- The Rothfusz equation is most accurate when the temperature is above 80°F (27°C) and relative humidity is above 40%. At lower temperatures or humidities, it may overestimate the perceived temperature.