Heat Index Calculator
What the air actually feels like, and when it is dangerous.
Work out Heat Index. What the air actually feels like, and when it is dangerous. Shows the working, not just the answer.
The heat index applies at or above 80 °F.
Feels like
105.9 °F
41.1 °C · 15.9 degrees hotter than the air
Humidity matters because cooling depends on sweat evaporating, and humid air cannot absorb much more water. At 95% humidity sweat simply runs off, so the body loses its main cooling mechanism entirely. This is the Rothfusz regression used by the National Weather Service, computed for shade. In full sun the felt temperature can be up to 15 °F higher than the figure above. Wet-bulb temperature is the measure that matters at the extremes: above about 35 °C wet-bulb, no amount of sweating cools a person, and survival becomes a matter of hours regardless of fitness.
How the Heat Index Calculator works
Enter temperature and humidity for the heat index and its risk band. This is the Rothfusz regression used by the National Weather Service, valid at or above 80 °F and 40% humidity — below that the concept has little content and the page says so.
Also known as: feels like temperature with humidity · heat index chart · apparent temperature calculator · humidex calculator
Frequently asked questions
Why does humidity make heat worse?
Because cooling depends on sweat evaporating, and humid air can absorb very little more water. At high humidity sweat runs off instead of evaporating, so the body loses its main cooling mechanism.
Is the heat index measured in shade or sun?
Shade. In full sunlight the felt temperature can be up to 15 °F higher than the index, which is why the official figure often seems low on a bright day.
What is wet-bulb temperature?
The lowest temperature reachable by evaporation, and the measure that matters at the extremes. Above roughly 35 °C wet-bulb no amount of sweating cools a person, and survival becomes a matter of hours regardless of fitness.
At what heat index is it dangerous?
Above 103 °F heat exhaustion becomes likely and heat stroke possible; above 125 °F heat stroke is highly likely. Those thresholds assume healthy adults — the old, the very young and anyone unwell are affected sooner.
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