Heat Index Calculator guide
Heat Index Calculator: How Hot Does It Feel?
Use our heat index calculator to work out the feels-like temperature from the air temperature and humidity, and the heat risk level.
By the AllCalculator editorial teamUpdated 16 October 20261 min read
Humid heat feels worse than dry heat, because sweat can't evaporate to cool you down. Our Heat Index Calculator works out how hot it really feels.
How to use the calculator (Heat Index Calculator)
- Choose °C or °F.
- Enter the air temperature.
- Enter the relative humidity.
How it works
The calculator uses the US National Weather Service's Rothfusz regression, a formula fitted to how the human body loses heat at different temperatures and humidity levels. It's most accurate above about 27°C (80°F).
A worked example
With the calculator's default settings, 32°C with 60% humidity feels like 37.1°C, in the "extreme caution" range.
Heat risk levels
| Heat index | Risk |
|---|---|
| 27–32°C (80–90°F) | Caution: fatigue is possible |
| 32–39°C (90–103°F) | Extreme caution: heat cramps and exhaustion are possible |
| 39–51°C (103–124°F) | Danger: heat exhaustion is likely |
| 52°C and above | Extreme danger: heatstroke is highly likely |
Staying safe in a heatwave
- Stay out of the sun between 11am and 3pm.
- Drink plenty of water and avoid alcohol.
- Keep rooms cool by closing curtains on windows that face the sun.
- Check on older people, babies and anyone with a long-term health condition.
Frequently asked questions
What's the difference between heat index and humidex?
They measure the same thing in different ways. Humidex is used in Canada and gives slightly different figures.
When is heat dangerous?
Call 999 if someone has signs of heatstroke, such as confusion, a seizure or loss of consciousness.
To check condensation and comfort, try our Dew Point Calculator.
Sources and further reading
Links open official and independent websites in a new tab.
This guide is general information to help you understand your options. It isn't financial, tax, legal or building advice and doesn't take your personal circumstances into account. Figures were checked when the guide was last updated and can change, so confirm anything important with GOV.UK, HMRC, a regulated adviser or a qualified tradesperson.