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
| Field | What to enter |
|---|---|
| Air temperature | Units: °C, °F, K |
| Relative humidity | In % |
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) |
|---|---|
| 16 | 15.2 |
| 24 | 24 |
| 32 | 37.1 |
| 40 | 62.6 |
| 48 | 102 |
| 64 | 221.9 |
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)Worked example
Inputs — Air temperature: 32 °C; Relative humidity: 60 %.
Results — Heat index (feels like) = 37.1 °C.
Frequently asked questions
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.