Heat Index Calculator
Calculate the heat index — how hot it feels when relative humidity is factored in with air temperature.
Danger
The National Weather Service classifies 113.1°F as "Danger" — heatstroke risk rises sharply as this number climbs.
Most accurate for temperatures at or above 80°F and humidity at or above 40%.
About the Heat Index Calculator
This calculator works out the heat index — how much hotter humid air actually feels compared to the thermometer reading — using the National Weather Service's Rothfusz regression formula, from air temperature and relative humidity.
It's used to judge real heat risk before outdoor work, sports practice, or events in hot weather, since humidity is what makes heat genuinely dangerous rather than just uncomfortable — the same air temperature can range from tolerable to hazardous depending on how much moisture is in the air.
High humidity blocks sweat from evaporating efficiently, which is the body's main cooling mechanism — this calculator quantifies exactly how much that effect raises the perceived (and physiologically felt) temperature.
How it’s calculated
The Rothfusz regression is a statistical formula developed by the National Weather Service that combines air temperature and relative humidity into a single "feels like" figure, fitted from actual data on how humans perceive combined heat and humidity. It's most accurate for temperatures at or above 80°F and humidity at or above 40%, which is the range it was calibrated against.
The risk categories (Caution, Extreme Caution, Danger, Extreme Danger) reflect NWS guidance on the likelihood of heat-related illness — sunstroke, heat cramps, heat exhaustion — at sustained exposure to a given heat index value, with risk increasing sharply as the number climbs.
Frequently asked questions
Why does high humidity make heat feel so much worse?
Sweat evaporating off skin is the body's primary cooling mechanism, and high humidity slows that evaporation dramatically — so on a humid day, sweat lingers on the skin without cooling you nearly as effectively, making the same air temperature feel significantly hotter.
What's a dangerous heat index level?
The NWS "Danger" category starts around 103°F, where heat cramps and heat exhaustion become likely with continued exposure or physical activity, and "Extreme Danger" above roughly 125°F carries a high risk of heat stroke — both warrant limiting outdoor exposure and activity.
Is heat index the same as the temperature shown on a weather app?
No — the plain temperature is just the air reading, while heat index (often labeled "feels like" or "real feel") factors in humidity's effect on perceived heat. On a humid day the two numbers can differ substantially.
Does heat index account for direct sunlight?
No — the standard heat index formula assumes shade. Direct sun exposure can add significantly to how hot conditions actually feel and to real heat stress on the body, beyond what the heat index number alone suggests.
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