Heat Index Calculator
Calculate the NWS heat index (feels-like temperature) from air temperature and relative humidity. Flags caution, danger, and extreme-danger thresholds.
Heat index conditions
Feels like
113.1 °F
Heat risk: Danger- Air temperature
- 95 °F
- Relative humidity
- 60%
- Heat index (F)
- 113.1 °F
- Heat index (C)
- 45.1 °C
Frequently Asked Questions about the Heat Index Calculator
What is heat index?
Heat index is the apparent (feels-like) temperature when relative humidity is factored into the actual air temperature. High humidity reduces evaporative cooling from sweat, so the air feels much hotter than the thermometer shows.
What formula does this use?
The US National Weather Service Rothfusz regression with the standard low-humidity (under 13% RH, T 80 to 112 °F) and high-humidity (over 85% RH, T 80 to 87 °F) corrections.
What do the risk levels mean?
Caution starts at 80 °F (heat fatigue possible). Extreme caution at 90 °F (heat cramps and exhaustion possible). Danger at 103 °F (heat exhaustion likely, heatstroke possible). Extreme danger at 125 °F (heatstroke imminent).
Why does the calculator approximate at low temperatures?
Below 80 °F the Rothfusz formula is not meaningful, since humidity has little effect on perceived heat. The NWS reference algorithm uses a simple weighted average for cooler conditions, which the calculator displays with a warning.
Does heat index account for sun exposure?
No. Heat index is measured in the shade. Direct sun can add 10 to 15 °F to the perceived temperature on top of the heat-index value. Use the result as a baseline, not a precise outdoor measurement.
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