Heat Index Calculator
The heat index is what a temperature feels like once humidity is taken into account. This uses the US National Weather Service regression, which is defined at 80 °F (26.7 °C) and above with humidity of 40% or more.
Work out the heat index
Frequently Asked Questions
An estimate of how hot it feels to a person, combining air temperature with relative humidity. Humid air slows the evaporation of sweat, so the same temperature feels hotter.
The National Weather Service regression is only defined at 80 °F and above with humidity of 40% or more. Below that there is no heat index to report, so this calculator says so rather than printing a number the formula does not support.
Shade. Direct sunshine can raise the apparent temperature considerably above the figure shown here.
It is a regression fitted to a heat-balance model, so it is an estimate. Treat it as guidance on risk rather than a measurement.