Heat Index Calculator Widget

How it works

The widget follows the NWS Weather Prediction Center procedure step by step, in Fahrenheit. It first evaluates Steadman's simple formula, 0.5 x (T + 61 + (T - 68) x 1.2 + RH x 0.094), and averages it with the temperature. If that average is below 80 °F the simple value is the answer. Otherwise it applies the nine-term Rothfusz regression, then the two NWS corrections: a deduction when humidity is under 13% and the temperature is 80-112 °F, and an addition when humidity exceeds 85% at 80-87 °F. Celsius inputs are converted before the calculation and the result converted back. The answer is matched to the NWS chart bands: Caution from 80 °F, Extreme Caution from 90 °F, Danger from 103 °F and Extreme Danger from 125 °F. At 90 °F and 60% the heat index is 100 °F.

Calculation method

• Simple: HI = 0.5 x (T + 61 + (T - 68) x 1.2 + RH x 0.094); used when (HI + T) / 2 < 80 °F
• Rothfusz: HI = -42.379 + 2.04901523T + 10.14333127RH - 0.22475541T.RH - 0.00683783T² - 0.05481717RH² + 0.00122874T².RH + 0.00085282T.RH² - 0.00000199T².RH²
• If RH < 13% and 80 <= T <= 112 °F: HI -= ((13 - RH) / 4) x sqrt((17 - |T - 95|) / 17)
• If RH > 85% and 80 <= T <= 87 °F: HI += ((RH - 85) / 10) x ((87 - T) / 5)
• Bands: 80-90 °F Caution; 90-103 °F Extreme Caution; 103-124 °F Danger; 125 °F and above Extreme Danger

Worked examples

General heat-safety guidance from public weather-service categories, not medical advice. Follow local heat warnings and workplace rules.

Limitations

• Applies to shade and light wind; direct sun, heavy clothing or exertion raise heat stress beyond the index.
• The regression was fitted to typical conditions; extreme combinations outside the NWS chart are less reliable.
• Not a wet-bulb globe temperature (WBGT), which many athletic and occupational rules use instead.

Where publishers use it

• Local TV and newspaper weather pages during a heat wave
• Construction, roofing and agricultural employers' heat-safety pages
• Youth sports leagues and marathon organisers deciding on extra water breaks
• School and summer-camp outdoor activity policies
• Gardening and outdoor-event blogs in humid climates

Questions

Sources

1. The Heat Index Equation - NOAA National Weather Service, Weather Prediction Center . Simple formula, the 80 F averaging rule, the Rothfusz regression coefficients and both adjustments, used exactly. Checked 2026-10-01.
2. What is the heat index? - National Weather Service, Amarillo TX office . Category bands and effect text: Caution 80-90 F, Extreme Caution 90-103 F, Danger 103-124 F, Extreme Danger 125 F or higher; worked values 100 F / 55% = 124 F.
3. Heat Forecast Tools - National Weather Service . 96 F and 65% relative humidity gives a heat index of 121 F; full sunshine can increase heat index values by up to 15 F.

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A2Z Tools Heat Index Calculator
https://a2z.tools/embed/heat-index-calculator

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Heat Index Calculator by A2Z Tools - https://a2z.tools/embed/heat-index-calculator