Heat index calculator
This heat index calculator shows how hot it feels: enter the air temperature and relative humidity and see the heat index at once, the National Weather Service level and a chart for other values.
Example: Air temperature 32 °C / °F · Unit: Degrees Celsius (°C) · Relative humidity 60 % → Heat index: Heat index 37.1 °C – Extreme caution (from 90 °F). Source: US National Weather Service (WPC) – The Heat Index Equation. Updated: .
How it is calculated
At 90 °F (32 °C) and 60 % humidity the heat index is about 100 °F (37 °C); at 95 °F and 70 % it is already around 123 °F. The heat index turns temperature and humidity into a feels-like value.
How it is calculated
The calculator uses the Rothfusz regression equation that the US National Weather Service (NWS) applies. It gives the heat index from temperature in °F and relative humidity. Adjustments apply for very dry air under 13 % and for very humid air above 85 % at 80 to 87 °F. Below about 80 °F (27 °C) a simpler formula replaces the regression because values there barely differ from the air temperature.
Why humidity matters
The body cools by sweating. In humid air sweat evaporates less well, so the same temperature feels hotter. The heat index sums that up in one number.
The levels
The National Weather Service distinguishes caution (from 80 °F, 27 °C), extreme caution (from 90 °F, 32 °C), danger (from 103 °F, 39 °C) and extreme danger (from 125 °F, 52 °C).
Related: wind chill, the dew point and the temperature converter.
Frequently asked questions
What is the heat index?
It describes how hot an air temperature feels at a given relative humidity, the feels-like temperature in the heat.
How is the heat index calculated?
With the National Weather Service formula from temperature and humidity. The calculator does it and converts between °C and °F if you like.
At what heat index does it become dangerous?
The National Weather Service speaks of danger from 103 °F (39 °C) and extreme danger from 125 °F (52 °C). That is a weather service classification, not health advice.
Is the heat index the same as the feels-like temperature?
In the heat, similar. Weather reports often compute feels-like temperature with their own models, so numbers can differ.
Why does the calculator show little difference below 80 °F?
At low temperatures humidity hardly affects how hot it feels. The index is not defined there and only an approximation.
Sources and legal basis
- US National Weather Service (WPC) – The Heat Index Equation
- US National Weather Service – Heat Index (levels and table)
As of: · Editorial policy
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