High humidity slows sweat evaporation, so the body cools less effectively and hot weather feels hotter. The heat index is the “feels like” temperature in the shade with a light wind.
How to use the Heat Index Calculator
- Air temperature
- Humidity given as
- Relative humidity (%)
- Dew point
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Enter the air temperature and the relative humidity or dew point. The NWS regression applies above about 27 °C (80 °F); below that a simpler formula is used.
Inputs explained
| Field | What to enter |
|---|---|
| Air temperature | Units: °C, °F, K |
| Humidity given as | Choose one: Relative humidity, Dew point |
| Relative humidity (%) | A number |
| Dew point | Units: °C, °F, K |
What each result means
- Heat index (feels like) = ([hif] – 32)×5÷9
- Relative humidity used = [rh]
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 |
| 24 | 23.8 |
| 32 | 34.4 |
| 40 | 54.8 |
| 48 | 86.6 |
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; Humidity given as: Relative humidity; Relative humidity (%): 50.
Results: Heat index (feels like) = 34.4 °C; Relative humidity used = 50 %.
Key terms
- Heat index
- Apparent temperature combining heat and humidity.
Frequently asked questions
Does sunshine affect heat index?
Yes, full sun can add up to about 8 °C (15 °F).
At what heat index is it dangerous?
Above about 40 °C (103 °F) heat exhaustion and heatstroke become likely.
Why does humidity matter?
Sweat evaporates more slowly in humid air.
Is it the same as humidex?
Similar idea; Canada's humidex uses dew point and a different formula.
Sources
Guides on this topic
- Celsius to Fahrenheit: Formula, Mental Shortcuts and Reference Table: The exact °C to °F formula and why it has two steps, a mental trick that is never wrong, a table of everyday temperatures and how kelvin fits in.