🌡 Heat Index Calculator
Humidity slows sweat evaporation, which is how your body cools itself — so 90°F at 60% humidity feels like nearly 100°F. This calculator runs the Rothfusz regression used by the National Weather Service to give the apparent temperature.
It also reports the risk band, from Caution up to Extreme Danger, so you know when to take extra water, shade and rest breaks during heat waves.
How to use this calculator
Enter the air temperature in Fahrenheit and the relative humidity percentage. The calculator runs the Rothfusz regression to give the apparent temperature plus a heat risk band.
- Type the Air temperature in Fahrenheit.
- Type the Relative humidity as a percentage.
- Read the heat index result instantly.
- Check the risk level, from Caution up to Extreme Danger.
Frequently asked questions
What does the Heat Index Calculator compute?
It estimates how hot the air feels to the human body by combining temperature and humidity with the Rothfusz regression used by the National Weather Service. It also reports a risk band so you know how seriously to take the heat.
Why does humidity make it feel hotter?
Sweat cools you by evaporating, and humid air slows evaporation. That is why 90 degrees at 60 percent humidity feels like nearly 100 degrees: your body cannot shed heat efficiently.
Can you give a worked example?
At 90 degrees Fahrenheit and 60 percent humidity the heat index is 99.7 degrees, which falls in the Extreme caution band where heat cramps or exhaustion are possible.
What do the risk bands mean?
Under 80 is no heat risk, 80 to 90 is Caution, 91 to 103 is Extreme caution, 104 to 124 is Danger, and 125 plus is Extreme danger. The bands describe risk for prolonged exposure or physical activity.