Heat index calculator with danger levels
As of 2 November 2026, a heat index of 105°F (41°C) is in the US National Weather Service "Danger" band (103°F to 124°F), where heat cramps or heat exhaustion are likely; 95°F (35°C) air at 50% humidity already gives a heat index of about 105°F (41°C), and full sun can add up to 15°F.
Temperature and humidity
105°F heat index
(41°C) Danger Heat cramps or heat exhaustion likely, and heat stroke possible with prolonged exposure and/or physical activity.
Rule: NOAA/NWS Weather Prediction Center, The Heat Index Equation, checked 2026-11-02
Rule: NWS Amarillo, Heat Index categories, checked 2026-11-02
NWS heat index categories
- Caution 80–90°F (27–32°C): Fatigue possible with prolonged exposure and/or physical activity.
- Extreme Caution 90–103°F (32–39°C): Heat stroke, heat cramps, or heat exhaustion possible with prolonged exposure and/or physical activity.
- Danger 103–124°F (39–51°C): Heat cramps or heat exhaustion likely, and heat stroke possible with prolonged exposure and/or physical activity.
- Extreme Danger 125°F+ (52°C+): Heat stroke highly likely.
Values are for shade. In direct sunlight, the NWS says the heat index can be up to 15°F (about 8°C) higher.
Canada uses humidex
Environment and Climate Change Canada's humidex is a different formula in °C: 20–29 little discomfort, 30–39 some discomfort, 40–45 great discomfort (avoid exertion), 46 and over dangerous, possible heat stroke. How to use the Humidex, checked 2026-11-02.
Worked examples · °FHeat index by temperature and humidity
| Air \ Humidity | 20% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|
| 80°F | 79 | 80 | 81 | 82 | 83 | 84 | 86 |
| 84°F | 81 | 83 | 85 | 88 | 90 | 94 | 98 |
| 88°F | 85 | 88 | 91 | 95 | 100 | 106 | 113 |
| 92°F | 88 | 94 | 99 | 105 | 112 | 121 | 131 |
| 96°F | 93 | 101 | 108 | 116 | 126 | 138 | beyond chart |
| 100°F | 97 | 109 | 118 | 129 | beyond chart | beyond chart | beyond chart |
| 104°F | 103 | 119 | 131 | beyond chart | beyond chart | beyond chart | beyond chart |
| 108°F | 109 | 130 | beyond chart | beyond chart | beyond chart | beyond chart | beyond chart |
Worked examples · °CHeat index by temperature and humidity
| Air \ Humidity | 20% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|
| 27°C | 26 | 27 | 27 | 28 | 29 | 30 | 31 |
| 30°C | 28 | 30 | 31 | 33 | 35 | 38 | 41 |
| 32°C | 30 | 32 | 34 | 37 | 40 | 44 | 49 |
| 35°C | 33 | 37 | 41 | 45 | 50 | 57 | beyond chart |
| 38°C | 37 | 43 | 49 | 55 | beyond chart | beyond chart | beyond chart |
| 40°C | 39 | 48 | 55 | beyond chart | beyond chart | beyond chart | beyond chart |
| 43°C | 44 | 57 | beyond chart | beyond chart | beyond chart | beyond chart | beyond chart |
Marked cells fall in an NWS category. "Beyond chart" means above 140°F: the NWS says the regression is not valid for extreme combinations outside the data it was built on.
Related on Global Incident Tracker
Heatstroke and heat exhaustion: warning signs Weather alerts by state (US) Weather warnings in Europe Wind chill calculator Emergency water calculator Heat waves recorded
What this page shows, and what it misses
The heat index is how hot it feels in the shade when humidity slows sweating. It applies the National Weather Service procedure to the numbers you enter; it does not know about sun, wind, work, clothing, age or health, all of which change the real risk. It is not the wet-bulb globe temperature used for sport and work rules. For symptoms, follow your health service's advice.
Equation and categories: US National Weather Service (public domain). Humidex bands: Environment and Climate Change Canada.