Heat Index Calculator

How to use the heat index calculator

1. Enter the air temperature and choose °F or °C.
2. Enter the relative humidity as a percentage.
3. Read the heat index, the feels-like temperature.
4. Copy the link to save or share it.

What the heat index is

The heat index is how hot the air actually feels once humidity is included, sometimes called the apparent temperature. Your body cools itself by sweating, and sweat works by evaporating. When the air is humid, evaporation slows, so you cool less and the same temperature feels hotter. This heat index calculator combines temperature and humidity to give that feels-like figure, a humidity heat index that tells you how hot does it feel rather than just the reading.

The formula and its limit

The tool uses the US National Weather Service heat index, the Rothfusz regression, the standard model behind official heat advisories. It is defined at 80°F (27°C) and above, because below that humidity has little effect on comfort. If you enter a lower temperature, the tool says the heat index does not apply rather than returning a misleading value.

Source: US National Weather Service heat index (Rothfusz regression).

Why it matters for safety

A high heat index is a health warning. When the feels-like temperature climbs, the body struggles to shed heat, raising the risk of heat exhaustion and heatstroke, especially during exertion or for vulnerable people. Weather services issue heat advisories based on the heat index, not the raw temperature, which is why a humid 90°F day can be more dangerous than a dry 100°F one.

A worked example

At 90°F with 70% relative humidity, the heat index is about 105°F. The thermometer says 90, but the humidity makes it feel like 105, firmly in the range where caution is needed.

FAQ

What is the heat index?

The heat index is the feels-like temperature that combines air temperature and humidity. Humid air slows sweat evaporation, so it feels hotter than the thermometer reads. The tool works it out for you.

When does the heat index apply?

At 80°F (27°C) and above. Below that, humidity barely affects how hot it feels, so the formula is not defined and the tool says so.

Why does humidity make it feel hotter?

Your body cools by evaporating sweat. When the air is already moist, sweat evaporates slowly, so you cool less and the heat feels more intense.

Is the heat index the real temperature?

No. It is how hot the air feels to the body, not the actual air temperature, which the thermometer still shows.

Is my information private?

Yes. Everything is calculated in your browser and nothing you enter is uploaded or saved.