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Heat index calculator

Humid air stops sweat from cooling you. The heat index combines temperature and humidity into how hot it feels.

National Weather Service heat index levels

Heat indexLevel
80 °F (27 °C) to 90 °FCautionFatigue possible with prolonged exposure and/or physical activity.
90 °F (32 °C) to 103 °FExtreme cautionHeat stroke, heat cramps or heat exhaustion possible with prolonged exposure and/or physical activity.
103 °F (39 °C) to 125 °FDangerHeat cramps or heat exhaustion likely, and heat stroke possible with prolonged exposure and/or physical activity.
125 °F (52 °C) or moreExtreme dangerHeat stroke highly likely.

How it is worked out

We use the NWS formula: a simple estimate first, then, when that comes out at 80 °F or more, the full Rothfusz regression with its corrections for very dry and very humid air. The heat index is for shade and light wind; in full sun it can be up to 15 °F (8 °C) higher.

In dangerous heat: drink water before you are thirsty, rest in the shade or air conditioning, avoid hard work in the hottest hours, and check on older neighbours. Heat stroke — confusion, hot skin, fainting — is a medical emergency: call your emergency number.

For planning and general information. In an emergency, follow the instructions of your local authorities and call your emergency number (911, 112, 999 or 000).

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Source: NOAA Weather Prediction Center, the heat index equation and the National Weather Service heat index categories.