Heat Index Calculator
Combine air temperature and humidity into the 'feels-like' temperature — the number the National Weather Service uses for heat advisories.
Two inputs, the real feel
Enter the air temperature in °F and the relative humidity in % and the calculator returns the heat index from the full Rothfusz regression, in both °F and °C.
Built for hot, humid air
The regression is fitted for T ≥ 80 °F and RH ≥ 40 %. Below that the heat index is close to the air temperature itself, so the formula matters most on a sticky summer afternoon.
What is a heat index calculator?
Temperature and humidity in, feels-like out
A heat index calculator turns two everyday weather readings — the shade air temperature and the relative humidity — into the "feels-like" temperature your body actually experiences. When the air is humid, sweat evaporates more slowly, so your body cools less efficiently and a given temperature feels hotter than the thermometer says. The National Weather Service captures that effect with the Rothfusz regression, a polynomial fitted to a detailed human heat-balance model. The result is the apparent temperature used in heat advisories, sports cancellations, and workplace safety rules.
Enter the air temperature and relative humidity to get the heat index in both °F and °C instantly.
One regression with nine terms, all built from the temperature T (°F) and the relative humidity RH (%).
HI = -42.379 + 2.04901523·T + 10.14333127·RH − 0.22475541·T·RH − 0.00683783·T² − 0.05481717·RH² + 0.00122874·T²·RH + 0.00085282·T·RH² − 0.00000199·T²·RH²The nine coefficients combine the temperature, the humidity, and their cross-terms, so humidity can add several degrees to the apparent temperature on a muggy day. To read the result in Celsius, the calculator applies the standard conversion (HI − 32) × 5/9 to the Fahrenheit value.
Suppose the air temperature is 90 °F and the relative humidity is 70 %.
Plug in T and RH
Set T = 90 and RH = 70 in the Rothfusz regression — temperature, humidity, their squares, and their cross-terms.
Heat index in °F
The nine terms sum to about 105.9 °F — almost 16 degrees above the thermometer reading.
Convert to °C
(105.9 − 32) × 5/9 ≈ 41.1 °C — the same feels-like temperature in metric.
The heat index is a danger gauge, not just a curiosity. At 90 °F with 70 % humidity it feels like about 106 °F, which sits in the National Weather Service "danger" band where heat cramps and heat exhaustion become likely with prolonged exposure. As a rough guide, an apparent temperature of 80–90 °F means caution, 90–103 °F means extreme caution, 103–124 °F is danger, and above 125 °F is extreme danger. The single most useful insight is how much humidity matters: at a fixed temperature, raising the humidity steadily pushes the feels-like value up, which is why a humid 90 °F day is far more dangerous than a dry one. Use the result to decide whether to move a workout indoors, schedule heavy outdoor work for the early morning, or simply drink more water — the higher the heat index, the shorter the safe exposure.
The regression is the official NWS formula, but a couple of practical points are worth keeping in mind.
Shade values, and a valid range
The heat index assumes shade air temperature and a light wind; full sunshine can add up to about 15 °F to how it actually feels. The Rothfusz regression is fitted for T ≥ 80 °F and RH ≥ 40 % — outside that range (cooler or drier air) the heat index is essentially the air temperature itself, and the National Weather Service uses simpler adjustments. The index is also a population average: age, fitness, clothing, and hydration all change how a given feels-like temperature affects you, so treat it as guidance, not a personal medical limit.