Heat Index Calculator

Calculate the heat index (feels-like temperature) based on air temperature and humidity. Assess heat danger level.

Heat index — the 'feels like' temperature you hear in summer weather forecasts — measures how hot the human body actually perceives the air to be once humidity is factored in, because humidity interferes with the evaporative cooling your skin relies on to shed heat. This calculator uses the National Weather Service's Rothfusz regression, a statistical fit to a heat-stress model developed by Steadman, once the simple estimate and actual temperature together average to 80°F or higher (below that, a simpler linear approximation is used, since the multi-term regression isn't validated at cooler temperatures — using the average of both formulas near the 80°F handover, per NWS's own documented procedure, avoids a jump right at the switch point). At high humidity, sweat evaporates more slowly, trapping heat near the skin and pushing the perceived temperature above the actual air temperature; the formula includes extra correction terms for unusually low humidity above 80°F (which pulls heat index slightly below the base regression, since dry air still evaporates sweat efficiently even when hot) and for unusually high humidity in the 80-87°F range (which pushes it slightly higher). The resulting danger-level categories — Caution (80-90°F) through Extreme Danger (125°F+) — mirror the current official NWS heat index chart used in public heat advisories. What this tool does not account for: direct sunlight exposure (the NWS heat index assumes shade; full sun can add 15°F or more to the perceived heat), wind, individual acclimatization, or physical exertion level, all of which shift how hot a given heat index actually feels to a specific person. The regression is also a polynomial fit, not a physical law — pushed far outside the hot, humid conditions it was calibrated against, it can return values well beyond anything the NWS actually publishes on its chart; this calculator caps the displayed result and flags it as an off-chart extreme rather than reporting an unbounded number as a precise reading.