Heat Index Calculator

Calculate heat index, the feels-like temperature accounting for humidity, using the NWS Rothfusz regression.

⏱ Updated: 18 Sep 2026

Calculator

Free heat index calculator: find the "feels like" temperature from air temperature and relative humidity using the NWS Rothfusz regression.

This falls outside the Rothfusz formula's core validated range (roughly 80°F+ and 40%+ relative humidity). The result below is a rough approximation, not the NWS's official adjusted value.
Heat Index
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Enter a temperature and relative humidity to calculate the heat index.

How to Use the Heat Index Calculator

Pick °F or °C above the temperature field — whichever you choose, the calculator converts internally to Fahrenheit, since that's the unit the underlying formula was built around — then enter relative humidity. A warning banner appears automatically if the combination falls outside the formula's tested range.

The body's main defense against overheating is sweat evaporation, which pulls heat off the skin as it turns to vapor. Humid air is already closer to saturated, so it can't absorb that extra moisture as readily — sweat evaporates more slowly, less heat leaves the body, and the air ends up feeling hotter than the thermometer alone would suggest. Heat index puts a number on that gap: roughly, the temperature it would need to be at more moderate humidity to produce the same physiological heat load.

The Rothfusz Regression

This calculator implements the Rothfusz regression, published by Lans P. Rothfusz in NWS Technical Attachment SR 90-23 (1990). It's a multiple-regression curve fit to Steadman's 1979 apparent-temperature model, and it's still the formula behind the heat index values in every US National Weather Service forecast:

HI = −42.379 + 2.04901523T + 10.14333127R − 0.22475541TR − 0.00683783T²
     − 0.05481717R² + 0.00122874T²R + 0.00085282TR² − 0.00000199T²R²

T is temperature in °F, R is relative humidity as a percentage.

That's a lot of coefficients for something modeling a fairly simple physical intuition, and that's the tell: this isn't a physics equation derived from first principles, it's an empirical curve fit tuned to match Steadman's model over a specific range of conditions. Fits behave well inside their tested range and get shaky at the edges, which is why the NWS applies separate correction terms below roughly 80°F or 40% humidity — corrections this calculator doesn't implement. Outside that range, treat the number here as a ballpark, not the official published adjustment.

What the Number Means in Practice

The NWS groups heat index values into rough risk bands for outdoor exposure: 80–90°F is caution territory, where fatigue becomes possible with prolonged activity; 90–103°F is extreme caution, where heat cramps and exhaustion get more likely; 103–124°F is danger, where heat exhaustion is likely and heatstroke possible; and above 125°F is extreme danger, where heatstroke is highly likely. Those bands assume the value sits inside the formula's valid range in the first place — a heat index estimate for a cool, dry day carries a lot less confidence than one for a hot, humid one.