Heat index calculator with the full NWS regression
Enter temperature and relative humidity. HitEquals follows the NWS decision sequence, returns apparent temperature in Fahrenheit and Celsius, shows the difference from air temperature and identifies the method used.
Inputs used by this calculator
Heat Index Calculator uses the fields below. The starting values reproduce the page example; replace them with values from the same units and time period before using the result.
| Input | Starting example | Accepted value | How it is used |
|---|---|---|---|
| Temperature unit | Degrees Fahrenheit | Degrees Fahrenheit · Degrees Celsius | Selects the unit or conversion path used by Heat Index Calculator. |
| Air temperature | 96 | -100 to 200 | Supplies the air temperature quantity used by the published equation or result breakdown. |
| Relative humidity (%) | 65 | 0 to 100 | Supplies the relative humidity (%) quantity used by the published equation or result breakdown. |
What the heat index represents
Heat index estimates how hot conditions feel when relative humidity is combined with actual air temperature.
NWS calculation sequence
The method begins with a simpler Steadman-consistent expression. When its averaged result reaches 80°F, it applies the nine-term Rothfusz regression.
Rothfusz regression
The regression contains temperature, humidity, squared terms and interaction terms.
HI=−42.379+2.04901523T+10.14333127RH−0.22475541TRH−…Humidity adjustments
NWS subtracts an adjustment for very low humidity from 80–112°F and adds one for humidity above 85% from 80–87°F.
Worked 96°F example
The NWS safety page gives 96°F and 65% RH as approximately 121°F heat index. The engine reproduces the full unrounded calculation before display rounding.
Shade, sun and safety limits
The index assumes shade and light wind. Full sun can add up to 15°F, while work, clothing, age and health can change individual heat stress. Follow current local warnings.
Illustrative NWS heat-index combinations
| Air | RH | Heat index |
|---|---|---|
| 80°F | 40% | ≈80°F |
| 90°F | 60% | ≈100°F |
| 96°F | 65% | ≈121°F |
| 100°F | 40% | ≈109°F |
Frequently asked questions
What is heat index?
An apparent temperature combining heat and relative humidity.
Is it calculated in the sun?
No. Standard values are for shade and light wind.
How much can direct sun add?
NWS says up to about 15°F.
Why can dry air lower the result?
Evaporation can cool skin more effectively when air is dry.
Does heat index equal body temperature?
No.
Can I use Celsius?
Yes; the equation runs in Fahrenheit and returns both units.
What to keep in mind
Heat index describes shade with light wind. Direct sun can raise apparent heat by up to 15°F. The equation is not a substitute for local warnings, hydration, rest or medical guidance.
Results are rounded for display in US format, with money in dollars and cents; CSV exports keep full calculation precision. This is a calculation summary, not an official certificate.