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Heat Index Calculator

Apparent temperature from heat and humidity, with risk levels.

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Risk categories

Heat indexCategoryWhat it means

Heat index chart

Feels like

—

Details

Heat index (°C)—
Heat index (°F)—
Added by humidity—
In full sun—

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Heat illness risk category

The heat index assumes shade and a light breeze — direct sunlight can add as much as 8 °C to the effective figure. Humidity matters because sweat evaporation is how the body sheds heat, and evaporation slows as the air approaches saturation.

Quick Tool Overview

The Heat Index Calculator applies the Rothfusz regression the US National Weather Service publishes, including the low- and high-humidity adjustments most tools omit, and maps the result onto the official heat illness risk categories.

What it does
Calculates apparent temperature from heat and humidity
You give it
Air temperature and relative humidity
You get back
Heat index in both scales with a risk category
Best for
Summer work planning, sport and heat safety
Calculators Free, unlimited use No sign-up Runs in your browser

About the Heat Index Calculator

Calculate the heat index — what the temperature feels like once humidity is taken into account — using the Rothfusz regression the US National Weather Service publishes, including the low-humidity and high-humidity adjustments most calculators omit.

Humidity matters because evaporating sweat is how the body sheds heat, and evaporation slows as the surrounding air approaches saturation. At 32 °C and 70 percent humidity the heat index is around 41 °C, well into the danger range, because cooling has largely stopped working. The index assumes shade and a light breeze — direct sun can add as much as 8 °C to what the figure suggests.

Key features of the Heat Index Calculator

The full NWS regression

Including both adjustment terms for very low and very high humidity, which many calculators skip.

Risk categories

Caution, extreme caution, danger and extreme danger, with what each means physiologically.

Explains the mechanism

Sweat evaporation is how the body cools, and humidity is what stops it working.

Sun adjustment noted

The index assumes shade; direct sunlight can add as much as 8 °C to the effective figure.

Full chart

A temperature-by-humidity grid, colour-banded by risk level.

How to use the Heat Index Calculator

  1. 1

    Enter the temperature

    In Celsius or Fahrenheit. The formula applies above about 27 °C.

  2. 2

    Enter the relative humidity

    As a percentage. Dew point can be used instead if that is what you have.

  3. 3

    Read the heat index

    The apparent temperature in both scales, with the risk category.

  4. 4

    Adjust for sun

    The index assumes shade — the page shows what direct sunlight adds.

Frequently asked questions

The apparent temperature: how hot it feels when relative humidity is combined with the actual air temperature. It reflects how much harder the body has to work to cool itself.

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