Heat Index Calculator
90°F at 60% humidity feels like 100°F. Enter the temperature and relative humidity for the official feels-like reading and the matching heat-safety level.
Feels like
100°F
38°C
Extreme caution: Heat cramps and exhaustion possible with prolonged exposure
Uses the NWS Rothfusz regression with the official low- and high-humidity adjustments. Humidity makes heat dangerous because it blocks evaporative cooling — sweat stops working. The index assumes shade and light wind; full sun can add up to 15°F.
Why Humidity Makes Heat Dangerous
Your main cooling system is evaporation — sweat absorbing body heat as it dries. High humidity slows that evaporation, so the same air temperature stresses the body harder. The NWS heat index quantifies it with the Rothfusz regression, fit to Steadman's physiological model of a walking adult in the shade. In direct summer sun, add as much as 15°F to the result.
Feels-like examples
40% RH 60% RH 80% RH
85°F → 86°F 90°F 97°F
90°F → 91°F 100°F 113°F
95°F → 98°F 114°F 134°F
NWS risk levels: 80–90 caution · 90–103 extreme caution
103–124 danger · 125+ extreme dangerFrequently Asked Questions
At what heat index do outdoor events get cancelled?
Many athletic associations move to modified play around a heat index of 95–99°F and suspend outdoor activity in the danger zone at 103°F+. OSHA's guidance for outdoor work adds mandatory rest/water cycles as the index climbs. Local policies vary, but 103°F is the most common bright line.
Why does 95°F feel worse in Florida than in Arizona?
Humidity. At 20% humidity, 95°F 'feels like' about 91°F because sweat evaporates instantly; at 70% it feels like 124°F because evaporation nearly stops. Same thermometer, completely different load on your body — that's exactly what the heat index measures.
What's the difference between heat index and wet-bulb globe temperature?
Heat index combines temperature and humidity for a person in the shade. WBGT — used by the military and many sports leagues — also folds in direct sun and wind, so it's the stricter standard for outdoor exertion. A shaded heat index can look acceptable while the WBGT on a sunlit field is not.
Does the heat index apply indoors?
Yes — the formula doesn't care where the hot, humid air is. Un-air-conditioned interiors during heat waves often exceed outdoor readings because humidity accumulates and walls radiate stored heat, which is why most heat fatalities happen indoors.