Calculator tool
How this calculator works
Use the explanation to understand the formula, assumptions, and practical limits behind the calculator result.
What Does the Heat Index Measure?
The heat index estimates how hot conditions feel to the human body when air temperature and relative humidity are considered together. High humidity slows sweat evaporation, so the body cools less effectively.
When This Calculator Applies
This tool uses the National Weather Service-style Rothfusz regression, which is intended for hot and humid conditions. The calculator is constrained to inputs of at least 80°F and 40% relative humidity so it stays inside the practical range where the heat-index method is normally used.
Risk Bands Used in the Result
| Heat index | Calculator band |
|---|---|
| below 91°F | Caution |
| 91 to below 103°F | Extreme caution |
| 103 to below 125°F | Danger |
| 125°F or higher | Extreme danger |
The National Weather Service also notes that full sunshine can make the heat index feel up to about 15°F hotter than the shaded value. The calculator result should therefore be read as a baseline, not a guarantee of safety outdoors.
Worked Example
At 95°F with 60% relative humidity, the calculator returns a heat index of about 113.1°F. That falls in the Danger band.
The actual thermometer reading has not changed, but the body's ability to cool itself has. That is why the heat index can be much higher than the dry-bulb temperature.
How to Use the Result
Use the band first, then the exact number. A shift from 102°F to 104°F is not just a small arithmetic change; it crosses into the calculator's danger band.
For outdoor activity, also consider direct sun, workload, clothing, hydration, acclimatization, and local official warnings. Heat index is a useful screening tool, but it does not replace situational judgment.
Frequently asked questions
Why does humidity make hot weather feel worse?
Because sweat cools the body mainly by evaporating. When the air is already humid, evaporation slows down, so the body retains more heat.
Why does this calculator start at 80°F and 40% humidity?
Those limits keep the tool inside the range where the Rothfusz heat-index method is normally applied. Below that range, a simpler feels-like estimate may be more appropriate than this regression.
Does shade matter?
Yes. The standard heat-index value is based on shaded conditions with light wind. Direct sun can make conditions feel substantially hotter, so outdoor exposure can be riskier than the number alone suggests.
What should I do when the result enters the danger band?
Treat it as a warning to reduce heat load: limit strenuous outdoor activity, use shade or cooling, drink fluids, and follow local heat advisories. The calculator can flag risk, but it cannot evaluate a person's medical condition.
