The heat index is how hot the air feels to the human body once relative humidity is combined with the air temperature. Enter the air temperature and relative humidity to see the "feels like" heat index, the risk level, and a full heat index chart. Uses the US National Weather Service Rothfusz regression, with the simpler Steadman estimate below the 80 °F threshold. Free, in °F or °C.
National Weather Service heat index values assume shade and a light wind — in direct sun it feels hotter still. Below about 80 °F the simpler Steadman estimate is used instead of the full regression. Risk categories are general guidance, not medical advice.
The feels-like temperature (°F) for each combination of air temperature and humidity, coloured by risk level.
Temperature plus humidity equals how hot it really feels.
Type the air temperature (switch between °F and °C) and the relative humidity as a percentage. The feels-like value updates as you type.
The calculator applies the National Weather Service formula and shows the feels-like temperature plus its risk category — from Caution up to Extreme Danger.
Use the chart to see how the risk climbs with humidity, and check the UV Index tool and our extreme heat warning guide before heading out.