Heat Index Calculator
95°F never feels the same twice — humidity decides whether it's a warm day or a dangerous one. Set the temperature and relative humidity to get the official NWS heat index, its danger category with safety guidance, and a full feels-like chart.
92°F at 60% humidity feels like
105°F
Heat index (NWS Rothfusz regression) = 40.4°C — measured for shade; full sun can add up to 15°F
🌡️ NWS category: Danger
Heat cramps and heat exhaustion are likely; heat stroke is possible with prolonged exposure or activity. Cancel strenuous outdoor plans and drink water constantly.
Heat index chart (°F)
| RH ↓ / Temp → | 80°F | 85°F | 90°F | 95°F | 100°F | 105°F | 110°F |
|---|---|---|---|---|---|---|---|
| 40% | 80° | 84° | 91° | 99° | 109° | 121° | 136° |
| 50% | 81° | 86° | 95° | 105° | 118° | 134° | 152° |
| 60% | 82° | 89° | 100° | 113° | 129° | 149° | 171° |
| 70% | 83° | 93° | 106° | 123° | 143° | 166° | 194° |
| 80% | 84° | 97° | 113° | 134° | 158° | 187° | 219° |
| 90% | 86° | 102° | 122° | 147° | 176° | 209° | 247° |
| 100% | 89° | 108° | 132° | 161° | 195° | 234° | 278° |
Shading follows the NWS categories: yellow = Caution (80–90°F), orange = Extreme Caution (90–103°F), light red = Danger (103–124°F), red = Extreme Danger (125°F+).
Why humidity makes heat dangerous
Your body's main cooling system is evaporation: sweat absorbs heat from your skin as it turns to vapor. That only works if the air has room for more moisture. As relative humidity climbs, evaporation slows to a crawl — sweat drips instead of evaporating, the cooling stops, and your core temperature starts to rise. The heat index captures this by answering one question: what dry-air temperature would stress your body the same amount as these actual conditions?
One important caveat: the heat index is defined for shade. Standing in full sunshine can add up to 15°F on top of the calculated value, which is why a "95°F feels-like" afternoon can be brutal on a treeless ballfield.
The formula and a worked example
The NWS heat index is built on the Rothfusz regression, a nine-term fit of Robert Steadman's 1979 "apparent temperature" model. For milder conditions the simple formula is used first:
HI = 0.5 · [T + 61 + (T − 68) · 1.2 + RH · 0.094]
If that result is 80°F or above, the full regression takes over (with extra corrections when humidity is below 13% or above 85%). Example: at 96°F and 65% humidity, the simple formula gives well over 80°F, so the regression applies — and it returns a heat index of about 121°F. That's deep in the NWS Danger zone: heat exhaustion is likely with continued outdoor activity, and heat stroke is possible. The same 96°F at 30% humidity would feel like roughly 98°F — a completely different day.
More weather and heat tools
The dew point calculator measures the mugginess itself — many meteorologists prefer dew point over relative humidity for judging comfort. In winter, the wind chill calculator is the cold-weather counterpart to this page. On hot days you also need more fluids than usual — the water intake calculator estimates how much, and the temperature converter handles °F ↔ °C conversions.