Heat index calculator: convert air temperature and humidity into the feels-like heat index and outdoor risk level. Free, private, runs in your browser.
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Heat index calculator: convert air temperature and humidity into the feels-like heat index and outdoor risk level. Free, private, runs in your browser.
A 95 F afternoon at 65% humidity feels like 118 F.
Transparent methodology
How this calculator works
Reviewed August 2026 · BoringToolsKit Editorial Team
Formula
Rothfusz regression: HI = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 6.83783e-3T^2 - 5.481717e-2R^2 + 1.22874e-3T^2R + 8.5282e-4TR^2 - 1.99e-6T^2R^2, T in Fahrenheit, R in %.
Worked example
90 Fahrenheit at 60% relative humidity: a heat index of about 100 Fahrenheit.
BoringToolsKit. “Heat Index Calculator.” boringtoolskit.com/heat-index-calculator/ (reviewed August 2026). Free to reference in articles, syllabi, and answer posts with a link.
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Worked example: 95 degrees and 65 percent humidity
The Rothfusz regression behind this page sums nine terms. At an air temperature of 95 F and 65 percent relative humidity, the terms evaluate as follows: -42.379, plus 2.04901523 x 95 = 194.66, plus 10.14333127 x 65 = 659.32, minus 0.22475541 x 95 x 65 = 1,387.86, minus 0.00683783 x 9,025 = 61.71, minus 0.05481717 x 4,225 = 231.60, plus 0.00122874 x 9,025 x 65 = 720.81, plus 0.00085282 x 95 x 4,225 = 342.30, and minus 0.00000199 x 9,025 x 4,225 = 75.88.
Running total: 152.28, 811.59, -576.27, -637.98, -869.58, -148.78, 193.53, then 117.65 after the final term. The calculator rounds to 118 F, so a 95 F afternoon at 65 percent humidity feels like 118 F. Notice the huge canceling terms: the T x RH cross term subtracts nearly 1,388 while the cubic terms add back over 1,063. Small input changes swing the result; at 85 F and 65 percent the same regression gives about 95 F instead. When conditions fall outside the regression's valid zone (below 80 F or below 40 percent RH), the tool simply returns the air temperature itself, matching how weather services handle the low end.
Where the heat index breaks down
The regression was fitted on data between roughly 80 and 110 F and 40 to 100 percent RH, so extrapolation outside that box is fiction. Below 80 F the formula can produce values below the actual air temperature; the NWS clamps to the temperature instead, which is why this calculator skips the regression entirely under 80 F or 40 percent. Even in range, the model assumes shade: full sun can push the felt temperature 10 to 15 F higher, per NWS guidance.
The index also assumes a 5 ft 7 in adult walking at about 3 mph in light wind, so it is not personalized. It cannot see wind chill effects (the heat index is undefined for wind speeds above about 10 mph affecting convective cooling), direct solar radiation, clothing, or fitness. Two adjustments the NWS applies but this simplified tool omits: at RH above 85 percent and temps 80 to 87 F, add about ((RH-85)/10) x ((87-T)/5); at RH under 13 percent and temps 80 to 112 F, subtract a correction that can reach 6 F or more. At 88 F and 80 percent RH the raw regression gives 106.25 F, while the NWS humid adjustment adds 0.1 F for 106.4 F, a small difference here but larger near the edges of the valid range.
