Heat Index Calculator

The heat index, also called apparent temperature, is how hot the air feels to a human body once relative humidity is combined with the actual air temperature. Humid air slows the evaporation of sweat, the body's main cooling method, so the same temperature feels hotter as humidity climbs.

It uses the National Weather Service Rothfusz regression. First it computes a simpler estimate and averages it with the air temperature. If that average is below 80 F the simple value is returned, otherwise the full nine-term regression runs, with an adjustment subtracted for very dry air and added for very humid air.

The Rothfusz regression was fit to warm, humid conditions and is only meaningful once the averaged estimate reaches about 80 F. Below that threshold the calculator returns the simple averaged value, which humidity affects only slightly, so numbers under 80 F are approximate and not an official NWS heat index.

Yes. In hot, very dry air the low-humidity adjustment can pull the heat index below the thermometer reading. At 100 F and 10 percent humidity this tool returns 94.1 F, because sweat evaporates so freely that the body cools more efficiently than in still, average air.

The tool follows the NWS scale: below 80 F is No risk, 80 to 89 F is Caution, 90 to 102 F is Extreme caution, 103 to 124 F is Danger, and 125 F or above is Extreme danger. Higher bands mean heat cramps, exhaustion, and eventually heat stroke become progressively more likely with prolonged exposure.

No. The formula assumes shade and a light breeze. Standing in full sunshine can add as much as 15 F to how hot it actually feels, while a stronger wind or active sweating can make it feel cooler than the calculated value.

The temperature field is Fahrenheit only, so convert a Celsius reading before entering it. Relative humidity must be greater than 0 and at most 100. A value at or below zero, or above 100, is rejected because it is not a physically valid relative humidity.

No. The heat index is a comfort index calibrated to how heat feels, while the wet bulb temperature is a direct physical measurement of the lowest temperature reachable by evaporation. Wet bulb is the stricter survivability limit, so use a dedicated wet bulb or dew point tool for that question.

Yes. The calculation runs entirely in your browser using the same engine described above, and no temperature or humidity value is uploaded to a server, so nothing you type leaves your device.