Heat Index Calculator

Temperature and humidity

How it works

The heat index is the Rothfusz regression used by the National Weather Service — a nine-term polynomial in temperature T (°F) and relative humidity R (%), fitted to how human skin actually sheds heat. Following the NWS procedure, this page first tries a simple estimate; if that estimate averages to 80 °F or more, it switches to the full regression and then applies the two standard adjustments: a small subtraction in very dry air (humidity under 13%, temperatures 80–112 °F) and a small addition in very humid air (humidity over 85%, temperatures 80–87 °F).

Categories (the NWS wording): 80–90 °F Caution — fatigue possible with prolonged exposure or activity; 90–103 °F Extreme Caution — heat cramps and heat exhaustion possible; 103–124 °F Danger — heat cramps and heat exhaustion likely, heat stroke possible; 125 °F and up Extreme Danger — heat stroke highly likely.

Read it as a general estimate, not medical advice. The index assumes shade and light wind; full sun can add up to 15 °F, and your own health, clothing, exertion, and acclimatization move the real risk a lot. The standard public guidance stands regardless of the exact number: on hot humid days, take breaks, drink water, and move heavy work to cooler hours.

Frequently asked questions

What is the heat index at 90 °F and 70% humidity? About 106 °F — the worked example here, matching the published NWS heat index chart.

Why does the number jump around 80 °F? Below about 80 °F the heat index is not defined — humidity barely changes how the air feels — so the simple-estimate branch of the NWS procedure reports values close to the air temperature itself.

Is the heat index the same as "feels like"? In summer, yes — weather services' "feels like" is the heat index in hot weather and the wind chill in cold weather.