☀️ Heat Index Calculator
💡 Tip: Heat index is only valid for temperatures above 80°F (26.7°C) and humidity above 40%.
How to Use This Tool
Enter the current air temperature and relative humidity percentage from your weather app or home sensor. Select the temperature unit (Fahrenheit or Celsius) you used for the temperature input. Click Calculate Heat Index to see the feels-like temperature, risk level, and recommended actions. Use the Reset button to clear all fields and start over. You can copy your results to your clipboard using the Copy Results button.
Formula and Logic
This calculator uses the National Weather Service’s Rothfusz regression formula for heat index, which factors in air temperature and relative humidity to determine how hot the air feels to the human body. The formula is calibrated for temperatures between 80°F and 112°F and relative humidity between 13% and 85%, with adjustments for outliers. Heat index values are converted to your selected temperature unit for easy reference.
The risk levels and recommended actions align with CDC guidelines for heat-related illness prevention.
Practical Notes
- Heat index only applies to outdoor shaded areas; direct sunlight can increase feels-like temperature by up to 15°F.
- Use this tool to plan outdoor workouts, grocery trips, or yard work: if the heat index is above 90°F, schedule activities for early morning or evening.
- For home cooling: if the heat index is above 80°F, set your air conditioner to 78°F or lower to maintain safe indoor temperatures.
- High humidity reduces sweat evaporation, making hot temperatures feel even hotter—this tool accounts for that effect.
- Always check heat index for children and elderly family members, who are more susceptible to heat-related illness.
Why This Tool Is Useful
Many weather apps only show air temperature, which can be misleading on humid days. This tool gives you the real feels-like temperature to help you make informed daily decisions: whether to send kids to outdoor camp, how to adjust your home cooling, or when to reschedule a picnic. It removes guesswork from hot weather planning for everyday life.
Frequently Asked Questions
Is heat index the same as dew point?
No, dew point measures the amount of moisture in the air, while heat index combines temperature and humidity to show how hot it feels. Dew point is used to calculate humidity, which is one input for this tool.
Can I use this tool for indoor spaces?
Heat index is designed for outdoor shaded areas, but you can use it for indoor spaces without air conditioning if the temperature and humidity are similar to outdoor conditions. Indoor spaces with AC will have lower feels-like temperatures than the calculated heat index.
What if my humidity sensor only shows dew point?
You can convert dew point to relative humidity using a dew point calculator, or estimate: if dew point is above 70°F, humidity is likely above 60%; if dew point is below 60°F, humidity is likely below 40%.
Additional Guidance
Always prioritize official weather service alerts over this tool’s results during extreme heat events. If you or a family member experience symptoms of heat exhaustion (dizziness, nausea, rapid pulse), move to a cool area immediately and hydrate, regardless of the calculated heat index. This tool is for planning purposes only, not medical advice.