# Heat Index Calculator

URL: https://speedycalculator.com/calculators/physics/heat-index/
Last updated: 2026-10-25

> The heat index calculator estimates how hot it feels when humidity is combined with air temperature, using the U.S. National Weather Service Rothfusz regression. It includes the official risk categories.

## Inputs

- Air temperature (°C, °F, K)
- Relative humidity (%)

## Results

- Heat index (feels like)

## Formula

```
HI = −42.379 + 2.04901523T + 10.14333127RH − 0.22475541T·RH − 6.83783×10⁻³T² − 5.481717×10⁻²RH² + 1.22874×10⁻³T²RH + 8.5282×10⁻⁴T·RH² − 1.99×10⁻⁶T²RH²  (T in °F)
```

## Explanation

High humidity slows sweat evaporation, so the body cools less effectively and hot weather feels hotter.

### How to use the Heat Index Calculator

- **Air temperature**
- **Relative humidity**

Results update as you type, and every field accepts the units shown in its menu.

### How it works

The NWS regression is applied for temperatures above about 80 °F; below that a simpler average is used.

### Inputs explained

FieldWhat to enterAir temperatureUnits: °C, °F, KRelative humidityIn %

### Example values

How the main result changes when **Air temperature** changes and everything else stays at the example values above:

Air temperature (°C)Heat index (feels like) (°C)1615.224243237.14062.64810264221.9

## Worked example

Inputs — Air temperature: 32 °C; Relative humidity: 60 %.

Results — **Heat index (feels like) = 37.1 °C**.

## FAQ

**Does sunshine affect heat index?**
Yes, full sun can add up to about 8 °C (15 °F) to the value.

**Who is most at risk?**
Older adults, young children, outdoor workers and people with chronic conditions.


## Sources

- [U.S. National Weather Service – Heat index equation](https://www.wpc.ncep.noaa.gov/html/heatindex_equation.shtml)