Heat Index Calculator

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 formula. Free, in °F or °C.

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Heat index (feels like) —

Heat Index Chart

The feels-like temperature (°F) for each combination of air temperature and humidity, coloured by risk level.

How the Heat Index Works

Temperature plus humidity equals how hot it really feels.

1

Enter temperature and humidity

Type the air temperature (switch between °F and °C) and the relative humidity as a percentage. The feels-like value updates as you type.

2

Read the heat index and risk

The calculator applies the National Weather Service formula and shows the feels-like temperature plus its risk category — from Caution up to Extreme Danger.

3

Plan around the heat

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.

Frequently Asked Questions

What is the heat index?
The heat index, or apparent temperature, is how hot it actually feels when relative humidity is combined with the air temperature. High humidity slows the evaporation of sweat, so your body cannot cool itself as effectively and the air feels hotter than the thermometer reads. This calculator uses the US National Weather Service formula.
How is the heat index calculated?
It uses the National Weather Service Rothfusz regression, which combines air temperature and relative humidity. Below about 80°F a simpler formula is used, and small adjustments are applied at very low or very high humidity. You just enter the temperature and the humidity and the feels-like value and risk level update instantly.
What heat index is dangerous?
The National Weather Service flags 80 to 90°F as Caution, 90 to 103°F as Extreme Caution, 103 to 125°F as Danger, and 125°F or above as Extreme Danger. From the Danger level up, heat cramps, heat exhaustion, and heatstroke become likely with prolonged exposure or activity, so limit time outdoors, hydrate, and find shade or air conditioning.
Is the heat index the same as the temperature?
No. The temperature is what a thermometer reads; the heat index adds the effect of humidity to estimate how hot it feels to your body. When humidity is high the heat index is higher than the air temperature, sometimes by 15°F or more, which is why a humid 90°F day can feel far more dangerous than a dry one.
Why does humidity make it feel hotter?
Your body cools itself by sweating, and sweat cools you as it evaporates. When the air is already full of moisture, sweat evaporates slowly, so less heat leaves your body and you feel hotter and more uncomfortable. That is why the same temperature feels worse on a humid day than a dry one.