Transparent method
The formula
Worked example
Example inputs
How to interpret the result
The heat index formula is calculated for shaded conditions specifically, so direct, unobstructed sun exposure can push the real felt heat noticeably higher than the calculated figure shows. The number is a genuine baseline, not the ceiling, for anyone actually standing in direct sunlight rather than shade.
Before you rely on it
What to check
Treat a calculated heat index as a floor, not a ceiling, when direct sun exposure is part of the actual real-world conditions being planned for.
The common error
Where people go wrong with heat index calculator
Treating a shaded-condition heat index calculation as the full picture for someone working or exercising in direct sun. Direct sun exposure can add meaningfully to the real felt heat beyond what the shaded-condition formula reflects.
Sensitivity evidence
How air temperature (°f) changes the feels-like temperature (°f)
| Air temperature (°F) | Feels-like temperature (°F) | Difference from actual temperature |
|---|---|---|
| 76 | 78 | 2 |
| 86 | 91.1 | 5.1 |
| 95worked example | 113.1 | 18.1 |
| 105 | 148.9 | 43.9 |
| 114 | 191.3 | 77.3 |
Every input, tested
Which input moves the feels-like temperature (°f) most
| Input | Tested from | To | Feels-like temperature (°F) at each end | Swing |
|---|---|---|---|---|
| Air temperature (°F) | 86 | 105 | 91.1 to 148.9 | 57.8 (51%) |
| Relative humidity | 58.0% | 62.0% | 111.4 to 114.9 | 3.5 (3.1%) |
Two variables at once
Feels-like temperature (°F) by air temperature (°f) and relative humidity
| Air temperature (°F) \ Relative humidity | 56.0% | 60.0% | 64.0% |
|---|---|---|---|
| 76 | 78.1 | 78 | 77.9 |
| 86 | 89.7 | 91.1 | 92.6 |
| 95 | 109.7 | 113.1 | 116.7 |
| 105 | 142.6 | 148.9 | 155.6 |
| 114 | 181.7 | 191.3 | 201.5 |
Step by step
The worked example, input by input
| Input | Value used |
|---|---|
| Air temperature (°F) | 95 |
| Relative humidity | 60.0% |
| Feels-like temperature (°F) | 113.1 |
| Difference from actual temperature | 18.1 |
Where the inputs come from
Enter the air temperature in degrees Fahrenheit, in the shade, and the relative humidity as a percent. The National Weather Service Rothfusz regression estimates how hot it feels as humidity slows the evaporation of sweat. It applies at 80 degrees and above with at least 40% humidity. Use readings taken in the shade, and remember that direct sun can add up to about 15 degrees to the heat index.
When the result stops applying
The formula is calibrated for shade, light wind and a person of average size, and it can be off at very high or very low humidity. Sun, wind, clothing, fitness and acclimatization change actual heat stress. Wet-bulb globe temperature is used for occupational and sports guidance. Children, older adults and outdoor workers are at higher risk.
What to compare it with
See the cold version on the wind chill calculator, humidity on the dew point page, and cooling demand on the degree days calculator. The temperature converter changes units, and the calories burned page shows exercise energy. Drink water and rest in the shade in hot weather.
People also ask
Frequently asked questions
What is the heat index?
A measure of how hot it feels when relative humidity is combined with the air temperature, because humid air slows the cooling effect of sweat.
At what heat index is it dangerous?
The National Weather Service treats 103 to 124 as dangerous, with heat exhaustion likely, and 125 or higher as extremely dangerous.
Does the sun change the heat index?
Yes. The published values are for shade, and full sun can add up to about 15 degrees.