Heat Index Calculator

Written by Great Medico

Updated September 25, 2026 •2 min read

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Air-property calculations are tightly coupled: temperature, moisture, pressure, and density can all move together. Heat Index Calculator focuses that system on the page’s heat index, so the number can be traced back to the exact atmospheric inputs rather than treated as a generic weather value.

What this calculator does

The Heat Index Calculator brings together Temperature, Relative Humidity, Dew point around the page’s heat index. The formula section below identifies which values actually drive that result and which fields are supporting or derived quantities, so you can check the page without assuming every visible box is an independent input.

How to use it

The main fields on this page are Temperature, Relative Humidity, Dew point. Enter the quantities you actually know, keep their units consistent, and leave derived/output-style fields blank unless the formula explicitly allows solving in the opposite direction. For a clean check of heat index, change one driving quantity at a time and confirm that the result moves in the direction predicted by the equation.

How the calculation works

The page uses the NOAA Rothfusz heat-index regression in the Fahrenheit domain, combining air temperature with relative humidity. It also applies the familiar low-humidity/high-humidity adjustments around the hot-weather range; the displayed value is then reported as an apparent temperature, not a measured air temperature.

Example

For example, start with the page’s populated scenario: Temperature = 30; Relative Humidity = 70. Apply the equation above using the units shown on the page, then compare the calculated heat index with the displayed result. As a second check, change one physical input while holding the others fixed and confirm that the direction of change makes sense for this formula.

How to interpret the result

Interpret heat index as an estimate for the entered air state and the specific approximation described above. Compare it with the related supporting values, and remember that a numerically precise result can still be wrong if pressure type, temperature scale, or humidity basis is inconsistent.

Limitations and notes

Heat index is designed for hot, humid conditions and shaded/light-wind assumptions; it is not a universal thermal-stress index. Direct sun, wind, clothing, exertion, health status, and acclimatization can change human heat strain substantially.