Heat Index Calculator: How Hot Does Humid Air Really Feel?
The thermometer alone misses how humidity changes comfort. The heat index combines temperature and humidity into one feels-like number.
What is the Heat Index Calculator?
Humid air feels hotter than dry air at the same temperature. Enter temperature in Fahrenheit and relative humidity and this tool calculates the heat index with the standard Rothfusz formula. Below about 80 Fahrenheit it simply returns the temperature, since the formula is not meant for cooler air.
How to use it
- Enter your values in the fields provided.
- Click Calculate Heat Index.
- Read the result and adjust an input if you want a different outcome.
Worked example
At 90 F and 70 percent humidity, the heat index is about 105.92 F.
What is the heat index at 90 F and 70 percent humidity?
The Rothfusz formula gives about 105.92 F, well above the air temperature, because humid air slows cooling.
Frequently asked questions
What do I enter to get a result?
Fill in every field with your own numbers or choices, then run the tool. Empty number fields count as zero, so check them before you trust the output.
Can I reuse this result elsewhere?
Yes. The result is calculated in your browser from the values you entered, so you can copy the numbers into your own notes or plan.