Heat Index Calculator
When you need to check heat Index, the Heat Index gives you a clear starting point. Enter one realistic example first, then compare another option if useful.
Feels like
—
In Celsius —
NWS category —
✉ Email me this result
</> Embed this calculator
Paste this into any website or blog post. It is free — the small "powered by" link must stay visible.
How it works
NWS Rothfusz regression on temperature (°F) and relative humidity (%)How to use the Heat Index
Enter the requested values and start with one realistic example. Read the result together with its unit or label, then change one input at a time if you want to compare options.
Review the assumptions on the page and confirm any result that affects money, safety, technical work or another important decision.
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.