Heat index risk categories
Based on the NOAA heat index chart (approximate air temperature, in shade).
- Caution
- Extreme Caution
- Danger
- Extreme Danger
How is the heat index calculated?
The heat index combines air temperature and relative humidity into a single 'feels like' number using the NOAA Rothfusz regression, a polynomial fit to the original Steadman heat index model. Example: at 90°F with 70% relative humidity, the heat index is about 105.9°F — noticeably hotter than the actual air temperature because high humidity slows the evaporation of sweat, the body's main cooling mechanism.
How do I calculate the heat index?
The heat index combines air temperature and relative humidity into a single 'feels like' number using the NOAA Rothfusz regression. Example: at 90°F with 70% relative humidity, the heat index is about 105.9°F, which falls in the 'Danger' category.
Steps to calculate the heat index
- Enter the air temperature, choosing °F or °C.
- Enter the relative humidity as a percentage from 0 to 100.
- If the simplified formula gives a result below 80°F, that value is used directly — the full Rothfusz regression is only meaningful at 80°F and above.
- At 80°F and above, the calculator applies the full NOAA Rothfusz regression, with a small downward adjustment for very low humidity (below 13%) and a small upward adjustment for very high humidity (above 85%) within their respective valid temperature bands.
- The result is shown in your chosen temperature unit alongside a risk category from the NOAA heat index chart.
Formula
HI = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 0.00683783T^2 - 0.05481717R^2 + 0.00122874T^2R + 0.00085282TR^2 - 0.00000199T^2R^2 (T in F, R in %)
- T = air temperature in degrees Fahrenheit
- R = relative humidity as a percentage (0-100)
- HI = heat index in degrees Fahrenheit, the 'feels like' value; below 80F, a simpler formula is used instead
Example heat index calculations
| Temperature | Humidity | Heat index | Category |
|---|
| 90°F | 70% | 105.9°F | Danger |
| 95°F | 50% | 105.2°F | Danger |
| 85°F | 90% | 101.8°F | Extreme Caution |
| 100°F | 40% | 109.3°F | Danger |
| 80°F | 13% | 78.3°F | None |
Frequently asked questions
Why does the calculator use a simple formula below 80°F?
The full Rothfusz regression was fitted to data in the range where heat stress becomes a real concern, roughly 80°F and above. Below that, NOAA recommends a simpler average of temperature and a humidity term, and the result is not usually reported as an official heat index.
What are the low-humidity and high-humidity adjustments for?
The base Rothfusz regression can slightly overestimate heat index at very low humidity and underestimate it at very high humidity within certain temperature ranges. NOAA publishes small correction terms for relative humidity below 13% (subtracted, for 80-112°F) and above 85% (added, for 80-87°F) to correct for this.
How is the heat index different from wind chill?
The heat index estimates how much hotter high humidity makes you feel in warm weather, because it slows sweat evaporation. Wind chill estimates how much colder moving air makes you feel in cold weather. They use different formulas and apply in opposite temperature ranges.
Does the heat index account for direct sunlight?
No. The NOAA heat index is calculated for shaded conditions. Full sun exposure can add as much as 15°F to the effective heat index, so outdoor risk in direct sunlight can be higher than the calculated value suggests.
This calculator uses the official NOAA Rothfusz heat index regression, most accurate for temperatures at or above 80°F. It assumes shaded conditions and does not account for wind, direct sunlight, individual heat tolerance, or clothing, so treat the result as a general reference rather than a precise medical threshold.
Sources: NOAA/NWS Heat Index Equation