Heat Index Calculator
Calculate the heat index — how hot it feels when humidity slows the body's evaporative cooling.
How to use the Heat Index Calculator
- Enter your inputs into the Heat Index Calculator above.
- Results update instantly as you type — no submit button needed.
- Adjust any value to see how the result changes in real time.
The NWS heat index formula
HI = −42.379 + 2.04901523·T + 10.14333127·RH − 0.22475541·T·RH − 0.00683783·T² − 0.05481717·RH² + ...
T is air temperature in °F, RH is relative humidity (%). The full Rothfusz regression has 9 terms plus corrections at low RH. Valid for T ≥ 80°F and RH ≥ 40%.
Worked example
90°F with 70% relative humidity: HI ≈ 106°F. 85°F with 90% humidity: HI ≈ 101°F. Heat index can exceed 130°F in extreme combinations (113°F at 60% humidity).
Frequently asked questions
Why does humidity make heat worse?
Sweating cools you by evaporation. Humid air is closer to saturation, so sweat evaporates slower. The body's primary cooling mechanism becomes less effective.
When should I worry about heat?
HI 90–105°F: caution, monitor for symptoms. 105–129°F: extreme caution, limit exposure. 130°F+: extreme danger, heatstroke possible quickly.
Is heat index for shade or sun?
Standard heat index is for shaded conditions. Direct sunlight adds about 15°F to the perceived temperature — a 100°F heat index in shade feels like 115°F in direct sun.