Heat Index Calculator — what the temperature feels like

How it works

The heat index combines air temperature and relative humidity into a single "feels like" temperature. Your body cools itself by evaporating sweat; when the air is already humid, that evaporation slows, so you shed heat less efficiently and the air feels hotter than the thermometer reads.

This calculator uses the National Weather Service heat-index formula — the Rothfusz regression, a nine-term polynomial fitted to human comfort research. It takes the air temperature in degrees Fahrenheit and the relative humidity in percent and returns the apparent temperature in °F. The regression is defined only at or above 80 °F (26.7 °C); below that the heat index is not meaningful, so the result is left blank.

Worked example

At 90 °F with 70% relative humidity, the heat index is about 105.9 °F — it feels roughly 16 degrees hotter than the air temperature. Drier air is easier to bear: 100 °F at 40% humidity gives 109.3 °F, while more moisture at the same temperature pushes it higher. A muggy 85 °F at 80% already feels like 96.8 °F, and a dangerous 95 °F at 55% feels like 108.9 °F.

Frequently asked questions

What is the heat index?

The heat index, or apparent temperature, is how hot it feels to the human body when relative humidity is combined with the air temperature. Because humidity limits how fast sweat evaporates, humid heat feels hotter than the same temperature in dry air.

How is the heat index calculated?

The National Weather Service uses the Rothfusz regression — a polynomial in temperature (°F) and relative humidity (%) fitted to biometeorological models. This calculator applies that base regression; the result is the apparent temperature in °F.

Why does humidity make it feel hotter?

The body cools itself mainly by evaporating sweat, which carries heat away. When the air is humid it already holds a lot of water vapour, so sweat evaporates more slowly and less cooling happens. The hotter-feeling result is what the heat index captures.

Why does the calculator need a temperature of at least 80 °F?

The Rothfusz regression was fitted for hot conditions and is only valid at or above 80 °F (26.7 °C). Below that, humidity has little effect on how hot it feels, so the NWS does not define a heat index and this calculator leaves the result blank.

What heat index is dangerous?

The NWS flags caution at about 80–90 °F, extreme caution at 90–103 °F, danger at 103–124 °F, and extreme danger at 125 °F and above, where heat stroke is likely. These are guidelines — direct sun, exertion and wind all shift the real risk.

Is the heat index the same as the temperature "feels like"?

In hot weather, yes — "feels like" on weather apps is usually the heat index. In cold weather the equivalent is the [wind chill](wind-chill-calculator), which accounts for wind rather than humidity. Both express an apparent temperature, just for opposite ends of the thermometer.

How accurate is this heat index?

It uses the base Rothfusz regression, which matches the official NWS heat index across the ordinary range. The full NWS algorithm adds two small corrections — a downward adjustment in very dry heat (relative humidity below 13% at 80–112 °F) and a small upward one in very humid heat (above 85% at 80–87 °F). This calculator omits those edge adjustments, so in those corners the value can differ from the NWS figure by up to about a degree; everywhere else it agrees.