Heat Index Calculator
Calculate the heat index (apparent temperature) based on air temperature and relative humidity. Useful for understanding how hot it feels.
Result
Please check your inputs.
How to Use This Tool
Enter the current air temperature in Fahrenheit or Celsius (the tool supports both).
Enter the relative humidity percentage (e.g., 50 for 50%).
Click the 'Calculate' button to instantly see the heat index (apparent temperature).
Review the result and the associated heat risk category (e.g., caution, extreme caution).
Adjust inputs to explore how different conditions affect perceived heat.
What Is Heat Index Calculator?
The heat index, also known as the apparent temperature, combines air temperature and relative humidity to measure how hot it really feels to the human body. High humidity reduces sweat evaporation, making temperatures feel much hotter than a thermometer reading. Understanding the heat index is crucial for preventing heat-related illnesses like heat exhaustion and heatstroke, especially during outdoor activities or heatwaves. This calculator helps you quickly assess real-time heat stress risk so you can plan your day safely.
Formula
The tool uses the NOAA (National Weather Service) heat index formula: HI = -42.379 + 2.04901523×T + 10.14333127×R - 0.22475541×T×R - 6.83783×10⁻³×T² - 5.481717×10⁻²×R² + 1.22874×10⁻³×T²×R + 8.5282×10⁻⁴×T×R² - 1.99×10⁻⁶×T²×R², where T is air temperature in °F and R is relative humidity in percent. For temperatures below 80°F or when the result would be lower than the actual temperature, the tool returns the simpler Steadman regression. Plainly, the formula adds energy from moisture to the thermometer reading to reflect the extra stress humidity places on your body’s cooling system.
Tips for Best Results
🌡️ Use the heat index to plan outdoor work or exercise — take breaks every 20 minutes when HI exceeds 90°F (32°C).
💧 Stay hydrated even if you don’t feel thirsty; humidity tricks your body into thinking it’s cooler than it is.
🕶️ The heat index is usually 10–15°F higher in direct sunlight — always seek shade when possible.
⏰ Check the heat index before 10 AM and after 4 PM to avoid the worst hours for heat stress.
Frequently Asked Questions
What is the difference between heat index and actual temperature?
Actual temperature is measured by a thermometer in the shade, while the heat index accounts for humidity to estimate how hot it feels to your body. For example, 95°F (35°C) with 70% humidity can feel like 125°F (52°C), drastically increasing health risks.
Why does humidity make the heat index higher?
Your body cools itself by sweating — when sweat evaporates, it removes heat. High humidity slows evaporation because the air is already saturated with moisture, so your body can’t cool down as efficiently, making you feel much hotter.
Can the heat index be lower than the actual temperature?
Yes, in very dry conditions the heat index can be slightly lower than the actual temperature because dry air allows sweat to evaporate faster, cooling the body more effectively. However, the effect is usually small (a few degrees).