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
Quick heat index answers
What is the heat index at 32°C with 70% humidity?
About 40.4°C (104.7°F) — in the 'Danger' category on the NOAA heat index chart.
What is the heat index at 90°F with 70% humidity?
About 105.9°F (41.1°C) — this calculator's own example, in the 'Danger' category.
At what heat index value does 'Danger' start?
103°F (about 39°C) — this calculator's categories are None below 80°F, Caution 80-89°F, Extreme Caution 90-102°F, Danger 103-124°F, and Extreme Danger 125°F and above.
What is the heat index at 95°F with 50% humidity?
About 105.2°F — reused from this calculator's example table, also in the 'Danger' category.
Why does high humidity make heat feel worse?
High humidity slows the evaporation of sweat, the body's main cooling mechanism, so the same air temperature feels hotter when the air is already moist.
Does the heat index apply to direct sunlight?
No — NOAA's heat index is calculated for shaded conditions; full sun exposure can add up to about 15°F (8°C) to how hot it actually feels.
What formula does the heat index use below 80°F?
A simpler average of temperature and a humidity term, rather than the full Rothfusz regression — NOAA doesn't report an official heat index value in that range since heat stress isn't a real concern yet.
How is heat index different from wind chill?
Heat index estimates how much hotter high humidity makes you feel in warm weather; wind chill estimates how much colder moving air makes you feel in cold weather — different formulas, opposite temperature ranges.
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