Heat index calculator
The full National Weather Service procedure, including the simpler formula it uses at the cool end and the two adjustments most implementations drop — and what it assumes about where you are standing. Nothing you type here leaves your browser — it is worked out on your own device, so there is nothing to send and nothing to log.
The heat index assumes you are in the shade
That is the part nobody mentions. The Rothfusz regression is fitted to a model of human heat balance that assumed shade, a light wind, and a person of a particular size walking at a particular speed.
In full sun the real apparent temperature can be up to 8 °C higher than the figure above. So a heat index is a floor rather than an estimate, if you are standing outside in it.
Below 80 °F there is a different formula
The NWS procedure is not one equation. Below a heat index of 80 °F it uses a simpler linear form, and only switches to the full regression above that.
Most implementations use the regression everywhere, because it is the formula that gets quoted. At the cool end that is wrong, and it is wrong in a way that looks entirely reasonable. This page follows the published procedure, and says which one it used.
And two corrections almost nobody applies
The regression is least accurate at very low and very high humidity, so the NWS publishes two adjustments — one below 13% humidity, one above 85%. Both are conditional on the temperature being in a particular band as well.
They are small, a degree or two, and they are part of the method rather than an optional refinement. When one applies here, the page says so and shows what the raw regression gave.