Heat Index Calculator

The heat index — often called the 'feels-like' temperature — combines air temperature and relative humidity into a single number that reflects how hot it actually feels to the human body. High humidity slows the evaporation of sweat, which is the body's main cooling mechanism, so on humid days it can feel much hotter than the thermometer reading alone suggests. This calculator uses the official National Weather Service (NWS) formula, so a reading of 100F air temperature and 55% humidity comes out to a heat index of about 124F.

It uses the NWS's two-stage algorithm: first a simplified Steadman approximation is computed and averaged with the air temperature; if that averaged value is 80F or higher, the full Rothfusz regression equation (a nine-term polynomial in temperature and humidity) is applied, along with two conditional corrections for very low humidity and very high humidity. Below about 80F, the simplified form alone is returned because the feels-like temperature is close to the actual air temperature.

The core Rothfusz regression was fit to Steadman's apparent-temperature tables and is slightly less accurate at the humidity extremes. The NWS adds a subtractive correction when relative humidity is below 13% (and temperature is between 80F and 112F) and an additive correction when humidity is above 85% (and temperature is between 80F and 87F), to keep the output aligned with the underlying physical model in those edge conditions.

The Rothfusz regression is stated to be accurate to within about plus or minus 1.3 degrees Fahrenheit (0.7 degrees Celsius) of Steadman's original apparent-temperature model, within its valid range of roughly 80F and above with humidity of 40% or higher. Outside that range, the raw polynomial becomes less reliable, which is why the NWS gates it behind the 80F check rather than applying it universally.

The National Weather Service defines four categories based on the heat index: Caution (80-90F) with possible fatigue on prolonged exposure; Extreme Caution (90-103F) with possible heat cramps or heat exhaustion; Danger (103-124F) with likely heat cramps, heat exhaustion and possible heat stroke; and Extreme Danger (125F or higher) with a high risk of heat stroke. This calculator shows which category your result falls into.

Yes. The heat index formula assumes shady, relatively calm-wind conditions. According to the NWS, full exposure to direct sunlight can increase the effective heat index value by up to about 15 degrees Fahrenheit. If you are working or exercising outdoors in full sun, treat the calculated value as a conservative (lower) estimate of how hot it actually feels.

Yes. Toggle the unit to Celsius and enter your reading directly — the calculator converts to Fahrenheit internally (since the NWS formula's coefficients are defined for Fahrenheit and whole-number humidity percentages), runs the full staged calculation, and converts the result back to Celsius for display, without losing precision in the round trip.

The Rothfusz regression is a curve fit valid for roughly 80F and above with 40% or higher humidity; it has no natural upper bound built in, so at extreme combinations (such as very high temperature with 100% humidity) the raw polynomial can produce a physically implausible number. This calculator caps and flags results in that corner case rather than displaying an unclamped, meaningless figure — real-world conditions rarely reach that extreme combination.

Heat index and wind chill are opposite ends of the 'feels-like' spectrum: heat index describes how hot warm, humid air feels to the body (humidity slows sweat evaporation), while wind chill describes how cold windy, cold air feels (wind accelerates heat loss from skin). They use entirely different formulas and are never combined into a single 'feels-like' number — you use heat index in hot weather and wind chill in cold weather.

In hot weather, most weather apps' 'feels like' or 'real feel' figure is based on the same NWS heat index methodology (or a very similar humidity-based model), so it should be close to what this calculator produces for the same temperature and humidity. Small differences can occur if the app also factors in wind speed or a proprietary adjustment on top of the base heat index.

Older adults, young children, people with chronic heart or respiratory conditions, outdoor workers and athletes, and anyone without reliable access to air conditioning face elevated risk at a given heat index. The NWS categories describe general population risk, not a guarantee for any individual — if you fall into one of these higher-risk groups, take extra precaution even at the lower end of the Caution range.

Limit strenuous outdoor activity, stay in air-conditioned spaces where possible, drink water regularly, wear light-colored breathable clothing, and check on children, older adults and pets who cannot regulate their own exposure. This calculator is a general educational guide, not medical advice — follow official heat-advisory guidance from your local weather service during extreme heat events.