Heat Index Calculator — Feels Like Temperature
Find out how hot it actually feels by combining air temperature and relative humidity. The calculator uses the NOAA Rothfusz regression formula and maps the result to the standard danger-category scale.
Temperature unit
°C
%
Danger — 116.3 °F
How does this calculator work?
Heat index = apparent temperature from air temperature + relative humidity using the NOAA Rothfusz equation (accurate for T ≥ 27°C / 80°F and RH ≥ 40%). Above 40°C heat index = Danger zone (heat exhaustion likely). High humidity prevents sweat evaporation, making hot days feel much hotter than the thermometer shows.
Formula
How this is calculated
Heat stress is not determined by air temperature alone — humidity is equally important because it affects the body's ability to cool itself through evaporation of sweat. At high relative humidity, sweat evaporates slowly, reducing the cooling effect and making the air feel hotter than a thermometer reads.
The heat index quantifies this "apparent temperature." The standard method is the Rothfusz (1990) regression equation, published by NOAA / the National Weather Service. It was developed by fitting Steadman's 1979 human physiology model to a grid of temperature and humidity combinations. The formula operates in degrees Fahrenheit; this calculator converts Celsius inputs before applying it, then converts the result back. Two small correction factors are applied when humidity is below 13% (air feels slightly cooler than the equation predicts) or above 85% at moderate temperatures.
For temperatures below approximately 27°C (80°F), the regression becomes less accurate and a simpler linear Steadman approximation is used instead. The NOAA danger categories — Caution, Extreme Caution, Danger, Extreme Danger — correspond to specific heat-illness risks and guide public health heat-advisory decisions.
Frequently asked questions
They are the same concept — both describe how hot conditions feel to the human body accounting for humidity, not just the air thermometer reading. "Feels like" is the consumer-friendly label; heat index is the meteorological and scientific term. Wind-chill is the analogous concept for cold weather.
The body cools itself primarily by sweating. Evaporation of sweat into the surrounding air removes heat. When relative humidity is high, the air is already nearly saturated with water vapour, so sweat cannot evaporate efficiently. This traps heat in the body, raising core temperature and making the air feel much hotter than the thermometer shows.
NOAA classifies heat index ≥ 40°C (103°F) as "Danger" — heat cramps and heat exhaustion become likely and heatstroke is possible. Above 51°C (125°F) is "Extreme Danger" where heatstroke is imminent. Even the lower "Extreme Caution" band (32–39°C / 90–103°F) warrants care during outdoor activity, especially for elderly people, children, and athletes.
TG we-Calculate Editorial Team. (2026). Heat Index Calculator — Feels Like Temperature [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/heat-index-calculator
TG we-Calculate Editorial Team. "Heat Index Calculator — Feels Like Temperature." TG we-Calculate. 2026. https://we-calculate.com/calculator/heat-index-calculator.
TG we-Calculate Editorial Team, "Heat Index Calculator — Feels Like Temperature," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/heat-index-calculator
@misc{wecalculate_heat_index_calculator, title = {Heat Index Calculator — Feels Like Temperature}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/heat-index-calculator}}, year = {2026}, note = {TG we-Calculate} }
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