Heat Index Calculator
Feels-like temperature from heat & humidity (NWS formula)
Heat index is defined for shady, light-wind conditions — full sun can add up to 15°F. Wind chill applies at or below 50°F with wind of at least 3 mph (NWS 2001 formula).
NWS heat index chart (°F)
| Temp＼RH | 40% | 50% | 60% | 70% | 80% | 90% | 100% |
|---|---|---|---|---|---|---|---|
| 80°F | 80 | 81 | 82 | 83 | 84 | 86 | 89 |
| 85°F | 84 | 86 | 89 | 93 | 97 | 102 | 108 |
| 90°F | 91 | 95 | 100 | 106 | 113 | 122 | 132 |
| 95°F | 99 | 105 | 113 | 123 | 134 | 147 | 161 |
| 100°F | 109 | 118 | 129 | 143 | 158 | 176 | 195 |
| 105°F | 121 | 134 | 149 | 166 | 187 | 209 | 234 |
| 110°F | 136 | 152 | 171 | 194 | 219 | 247 | 278 |
Yellow: extreme caution (90°F+) · Orange: danger (103°F+) · Red: extreme danger (125°F+)
FAQ
Why does humidity make it feel hotter?
Your body cools itself by evaporating sweat. High humidity slows evaporation, so heat builds up — 90°F at 70% humidity feels like about 106°F.
What heat index is dangerous?
NWS categories: 80–90°F caution, 90–103°F extreme caution, 103–124°F danger, 125°F and above extreme danger. Heat cramps and heat exhaustion become likely in the danger range, and heat stroke is possible with continued exposure.
How is wind chill calculated?
The NWS 2001 wind chill formula uses air temperature and wind speed: 35.74 + 0.6215T − 35.75V^0.16 + 0.4275TV^0.16 (°F, mph). It applies at 50°F or below with winds of 3 mph or more.
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