🌡️ Heat Index Calculator
Calculate the heat index , what the temperature actually feels like to the human body when relative humidity is combined with air temperature. Based on the official NOAA Rothfusz regression formula.
What Is the Heat Index?
The heat index, also known as the "apparent temperature," is what the temperature actually feels like to the human body when relative humidity is combined with the air temperature. When the body gets too hot, it perspires to cool itself. If the perspiration cannot evaporate (due to high humidity), the body cannot regulate its temperature effectively.
The official heat index formula used by the U.S. National Weather Service is the Rothfusz regression equation, developed by Lans P. Rothfusz in 1990. This calculator implements the full Rothfusz formula along with the necessary adjustments for low and high humidity conditions.
How the Calculator Works
This calculator uses the NOAA Rothfusz regression formula:
HI = -42.379 + 2.04901523·T + 10.14333127·RH − 0.22475541·T·RH
− 0.00683783·T² − 0.05481717·RH² + 0.00122874·T²·RH
+ 0.00085282·T·RH² − 0.00000199·T²·RH²
Where T = temperature in °F and RH = relative humidity in %
The calculator automatically handles:
- Temperature unit conversion , enter in °F or °C, the calculator converts as needed.
- Low humidity adjustment , when RH < 13% and temperature is between 80–112°F.
- High humidity adjustment , when RH > 85% and temperature is between 80–87°F.
- Simple Steadman formula , for conditions below about 80°F heat index.
- Direct sunlight adjustment , adds up to 15°F for sun exposure.
Heat Index Risk Categories
The National Weather Service uses these heat index categories to communicate heat risk:
- Caution (80–90°F): Fatigue possible with prolonged exposure or physical activity.
- Extreme Caution (90–103°F): Heat stroke, heat cramps, or heat exhaustion possible.
- Danger (103–124°F): Heat cramps or heat exhaustion likely; heat stroke possible.
- Extreme Danger (125°F+): Heat stroke highly likely.
Why Use This Heat Index Calculator?
- Official NOAA Formula: Uses the exact Rothfusz regression equation used by the NWS.
- Multiple Units: Enter in °F or °C, get results in both.
- Risk Assessment: Get instant heat risk category with detailed descriptions.
- Visual Chart: See how the heat index varies with temperature and humidity.
- Sun Exposure: Optional adjustment for direct sunlight.
- Free & Private: No registration, no data storage.
❓ Heat Index Calculator FAQ
What is the heat index?
The heat index, also known as the apparent temperature, is what the temperature feels like to the human body when relative humidity is combined with the air temperature.
What formula does this calculator use?
This calculator uses the official NOAA Rothfusz regression equation: HI = -42.379 + 2.04901523·T + 10.14333127·RH − 0.22475541·T·RH − 0.00683783·T² − 0.05481717·RH² + 0.00122874·T²·RH + 0.00085282·T·RH² − 0.00000199·T²·RH².
What is a dangerous heat index?
A heat index of 103°F or higher is considered "Danger" , heat cramps or heat exhaustion are likely, and heat stroke is possible. Above 125°F is "Extreme Danger" , heat stroke is highly likely.
Does direct sunlight affect the heat index?
Yes. Heat index values are calculated for shaded locations. If you are exposed to direct sunlight, the heat index can be increased by up to 15°F.
What is the difference between heat index and actual temperature?
Actual temperature is the measured air temperature. The heat index is what the temperature feels like when humidity is factored in. For example, 100°F with 55% humidity feels like 124°F.
What is the valid range for the heat index formula?
The Rothfusz regression is valid for temperatures between 80–110°F (26.6–43.3°C). For conditions below about 80°F heat index, a simpler Steadman formula is used.
Can the heat index be lower than the actual temperature?
Yes. When relative humidity is low, the apparent temperature can be lower than the air temperature. For example, 100°F with 15% humidity has a heat index of 96°F.
How do I convert heat index from °F to °C?
Use the formula: °C = (°F − 32) × 5/9. This calculator shows both units automatically.
What is the Rothfusz regression equation?
It is the standard formula used by the National Weather Service to calculate heat index, developed by Lans P. Rothfusz in 1990 and described in NWS Technical Attachment SR 90-23.
What are the heat index risk categories?
Caution (80–90°F), Extreme Caution (90–103°F), Danger (103–124°F), and Extreme Danger (125°F+).
How does humidity affect the heat index?
Higher humidity slows the evaporation of perspiration, making the body feel warmer. As relative humidity increases, the heat index increases.
Is this calculator accurate for all weather conditions?
This calculator uses the official NOAA formula and adjustments. However, the heat index is a model and may not capture all factors like wind speed, clothing, or individual physiology.
What is the dew point and how does it relate to heat index?
The dew point is the temperature at which air becomes saturated with moisture. A higher dew point means more moisture in the air, which increases the heat index. This calculator includes an optional dew point input.
Can I use this calculator for cold weather?
The heat index is designed for hot weather conditions. For cold weather, use a wind chill calculator instead. The heat index formula is not valid below about 80°F.
Is this calculator free?
Yes, this calculator is completely free to use. No registration or personal data storage is required. All calculations are performed in your browser.