This heat index calculator shows how hot it feels when humidity is combined with air temperature, and the heat risk level used by the US National Weather Service.
How the heat index is calculated
The calculator follows the National Weather Service method: a simple formula first, then the Rothfusz regression equation with its adjustments when the heat index is 80°F or higher. High humidity slows sweat evaporation, so the body cools less and it feels hotter.
Risk levels
- 80–90°F: caution
- 90–103°F: extreme caution
- 103–124°F: danger
- 125°F and above: extreme danger
Worked example
At 90°F with 60% relative humidity, the heat index is about 100°F (38°C), in the extreme caution range.
Frequently asked questions
Does the heat index include sunshine?
No. It assumes shade and light wind. Direct sun can add up to about 15°F to how hot it feels.
Who is most at risk?
Older adults, young children, outdoor workers, athletes and people with heart conditions or on certain medicines. Drink water, rest in shade and watch for signs of heat exhaustion.