CalcHeat Index Calculator
Last updated: 2026-08-24
How to Use This Calculator
To calculate the heat index, simply enter the current air temperature (in degrees Celsius) and the relative humidity (as a percentage) into the respective fields. Then click the "Calculate" button. The result will be displayed in degrees Celsius, representing the apparent temperature, or how hot it actually feels to the human body. This tool is especially useful during summer heatwaves or for planning outdoor work, sports events, or any activity where heat stress is a concern.
Formula and Methodology
The heat index is computed using the comprehensive Rothfusz regression equation, which accounts for the nonlinear relationship between temperature and humidity. The formula is: HI = -8.78469475556 + 1.61139411*T + 2.33854883889*H – 0.14611605*T*H – 0.012308094*T² – 0.0164248277778*H² + 0.002211732*T²*H + 0.00072546*T*H² – 0.000003582*T²*H², where T is the air temperature in °C and H is the relative humidity in percent. This formula is valid for temperatures above 27°C and humidity above 40%. For lower values, the heat index is approximately equal to the air temperature.
Practical Examples
Example 1: A hot summer day with an air temperature of 35°C and relative humidity of 60%. Using the formula, the heat index becomes approximately 46°C. This means the body feels a scorching 46°C, significantly increasing the risk of heat exhaustion. Example 2: A temperature of 30°C and humidity of 50% results in a heat index of about 33°C — still a noticeable increase. Example 3: For a moderate day of 25°C with 40% humidity, the heat index remains close to 25°C, as the formula is not intended for cooler conditions.
Tips and Best Practices
Always use a reliable thermometer and hygrometer to get accurate input values. Remember that the heat index is designed for shaded areas; direct sunlight can increase the apparent temperature by up to 15°C. If the heat index exceeds 40°C, take precautions: stay hydrated, take breaks in air-conditioned spaces, and avoid strenuous outdoor activities during peak heat hours. The heat index should not be used for temperatures below 27°C or humidity below 40%, as the formula loses accuracy. For heat index values above 54°C, extreme caution is required as heat stroke becomes highly likely. Also, consider that wind and individual health factors can alter the perceived temperature.
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. High humidity reduces the evaporation of sweat, making the body feel hotter.
When should I use this calculator?
Use it on hot, humid days, especially when the temperature is above 27°C (80°F) and relative humidity exceeds 40%. It helps assess heat stress risk and plan outdoor activities.
Is the heat index the same as the humidex?
The humidex is a Canadian variant that uses a different formula but serves a similar purpose. Our calculator uses the widely accepted Rothfusz regression equation developed by the US National Weather Service.