Heat Index Calculator
Humidity is why 95°F can feel like 115°F. Enter the temperature and relative humidity to get the apparent temperature using the National Weather Service formula.
What the heat index categories mean
| Heat index | Category | What can happen |
|---|---|---|
| 80–90°F | Caution | Fatigue possible with prolonged exposure or activity. |
| 90–103°F | Extreme Caution | Heat cramps and heat exhaustion possible; heat stroke with prolonged exertion. |
| 103–125°F | Danger | Heat cramps and exhaustion likely; heat stroke probable with continued activity. |
| 125°F and above | Extreme Danger | Heat stroke highly likely. |
Why humidity makes heat dangerous
Your body sheds heat mainly by evaporating sweat. When the air is already loaded with moisture, sweat evaporates slowly, so the cooling system stalls and core temperature climbs. That is why a humid 90°F day is far more punishing than a dry 100°F day.
The heat index assumes shade and a light breeze. In full sunshine it can read up to 15°F too low. If you are working or exercising in direct sun, treat the number as optimistic.
The wet-bulb limit
There is a hard ceiling on human tolerance. Once the wet-bulb temperature approaches about 35°C (95°F), sweat cannot evaporate at all, and no amount of shade or water prevents overheating. Even healthy people cannot survive many hours at that level. Real-world danger begins well below it.
Heat safety basics
Drink water before you are thirsty, take breaks in shade or air conditioning, and shift hard work to early morning. Watch for heavy sweating that suddenly stops, confusion, or hot dry skin — those signal heat stroke, which is a medical emergency. Cool the person fast and call for help immediately.
Never leave a child or pet in a parked car. Interiors can climb 40°F above the outside air within an hour, and cracking a window barely slows it.