Heat Index Calculator
How hot it feels when humidity is added.
Your numbers
°F
%
Your results
Heat index (°F)
Heat index (°F)
—
Heat index (°C)
—
Risk level
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Heat Index Calculator by TheCalcTools
How the Heat Index Calculator works
Enter air temperature and relative humidity.
The Rothfusz regression combines the two into an apparent temperature.
The risk label follows the NWS heat-index bands.
The formula
HI = −42.379 + 2.049 T + 10.143 RH − 0.2248 T·RH − 0.00684 T² − 0.0548 RH² + 0.00123 T²·RH + 0.000853 T·RH² − 0.00000199 T²·RH²
Worked example
Using the default values shown in the calculator:
| Air temperature (°F) | 92 °F |
|---|---|
| Relative humidity | 60 % |
- At 92°F and 60% humidity the heat index is 104.7°F (40.4°C).
- Risk: Danger: heat exhaustion likely.
Result:
| Heat index (°F) | 104.7 |
|---|---|
| Heat index (°C) | 40.4 |
| Risk level | Danger: heat exhaustion likely |
Frequently asked questions
Why does humidity matter?
Sweat evaporates more slowly in humid air, so the body cools less efficiently.
Is the heat index valid in direct sun?
Values assume shade. Full sunshine can raise the effective heat index by up to 15°F.
Sources and further reading
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This calculator gives estimates for information only. Check important results independently before you rely on them. See our disclaimer.