Heat Index Calculator
Feel-like temperature from heat and humidity.
Feels like
105°F
40°C
NWS risk level
Danger
Humidity adds
13°F
Humidity blocks your cooling
Sweat cools by evaporating; humid air slows evaporation, so the same 92°F feels — and physiologically is — hotter at 60% humidity (feels like 105°F) than at 20%. The Rothfusz regression behind this is the NWS standard.
Reading the risk levels
Above a 91°F index, the NWS advises limiting exertion; above 103°F, heat cramps and exhaustion become likely with activity. Heat is the deadliest U.S. weather hazard — hydration, shade breaks, and afternoon avoidance are the whole playbook.
Frequently asked questions
- How does the Heat Index Calculator work?
- Feel-like temperature from heat and humidity.
- Is the Heat Index Calculator free to use?
- Yes. The Heat Index Calculator is completely free, runs entirely in your browser, and requires no account or sign-up. Your inputs never leave your device.
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