Heat Index Calculator
Find the 'feels like' temperature when humidity stops sweat from cooling you, using the NWS Rothfusz regression.
Feels like
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In Celsius —
NWS category —
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How it works
NWS Rothfusz regression on temperature (°F) and relative humidity (%)Frequently Asked Questions
Why does humidity make heat feel worse?
Your body cools by evaporating sweat; humid air absorbs less of it. 90°F at 70% humidity feels like 105°F because evaporation has nearly stalled.
At what heat index is it dangerous?
NWS categories: 80–90°F caution, 90–103°F extreme caution, 103–124°F danger (heat cramps/exhaustion likely), 125°F+ extreme danger (heat stroke imminent with exposure).
Is the heat index measured in shade or sun?
Shade — direct sunlight can add up to 15°F on top of the calculated value.