This tool shows how to calculate heat index using temperature and humidity, in °F or °C, with the NWS formula. You get the feels-like temperature and the heat-risk level.
Enter the air temperature and the relative humidity (from a weather app or thermometer). The heat index is most meaningful in warm, humid conditions, roughly above 80 F (27 C).
Based on the US National Weather Service (Rothfusz) heat index. High humidity slows sweat evaporation, so the body cools less effectively and the air feels hotter than the thermometer reading.
How to Calculate Heat Index Using Temperature and Humidity
Enter the air temperature and relative humidity, and the formula combines them into one feels-like value. At 90°F and 50% humidity the heat index is about 95°F, so it feels 5 degrees hotter than the thermometer reads. Holding 90°F steady, more humidity pushes it up fast:
| Air temperature | Relative humidity | Heat index |
|---|---|---|
| 90°F | 40% | about 91°F |
| 90°F | 50% | about 95°F |
| 90°F | 60% | about 100°F |
| 90°F | 70% | about 106°F (danger zone) |
| 90°F | 80% | about 113°F (danger zone) |
Values assume shade and light wind; direct sun can add up to 15°F, so plan outdoor work and drink water accordingly.
The Heat Index Formula in Fahrenheit
The tool uses the official U.S. National Weather Service (Rothfusz) formula, which works in °F. It reflects how humidity slows sweat evaporation and makes hot weather feel hotter.
Formula: for temperatures of 80°F and above, Heat Index = −42.379 + 2.04901523×T + 10.14333127×R − 0.22475541×T×R − 0.00683783×T² − 0.05481717×R² + 0.00122874×T²×R + 0.00085282×T×R² − 0.00000199×T²×R², where T is air temperature in °F and R is relative humidity in %. Below 80°F, the calculator instead uses a simpler estimate: Heat Index = 0.5 × (T + 61 + (T − 68)×1.2 + R×0.094). Celsius entries are converted to °F before either formula runs, and the result is converted back if you’ve selected °C.
- Read the shaded air temperature and relative humidity: 32.2°C and 50%.
- Convert to Fahrenheit: 32.2 × 9/5 + 32 = 90°F.
- Run the formula with T = 90 and R = 50: about 94.6°F.
- Convert back if needed: about 34.8°C.
That is also how to calculate heat index in Celsius by hand; the °C toggle handles both conversions for you.
Example Scenarios
| Temperature (°F) | Relative Humidity (%) | Heat Index (°F) |
|---|---|---|
| 80 | 40 | 80 |
| 85 | 70 | 93 |
| 90 | 50 | 95 |
| 95 | 60 | 113 |
| 100 | 55 | 124 |
What Heat Index Is Considered Dangerous?
The National Weather Service treats a heat index above 103°F as a danger zone and above 125°F as extreme danger. The number estimates how hot it feels in the shade with a light wind, and real conditions can feel worse.
- The US National Weather Service notes that full sunshine can raise the heat index by up to 15 °F.
- Take temperature and humidity from a shaded local reading; a car dashboard or sunny wall reads high.
- Hard exertion, heavy clothing and protective gear add heat strain beyond what the number can show.
Plan outdoor work and exercise for early morning, drink water regularly, and schedule rest in the shade. Recheck through the day, since humidity and temperature change together.
Older adults, young children, pregnant people and anyone with heart disease or on certain medicines are more vulnerable. Heavy sweating, dizziness and nausea can signal heat exhaustion; confusion or a very high body temperature can mean heat stroke, a medical emergency, so call emergency services. This is general guidance only, and the National Weather Service heat index page explains the risk levels the calculator reports.
Common questions about the heat index
How do you calculate heat index using temperature and humidity?
Put the air temperature in °F and relative humidity in % into the NWS Rothfusz formula, which applies from 80°F up. At 90°F and 50% humidity the result is about 95°F.What heat index is considered dangerous?
The National Weather Service generally treats heat index values above 103°F as a danger zone where heat-related illness becomes more likely with continued exposure, and values above 125°F as extreme danger. Always check your local forecast for official heat advisories.Is heat index the same as the feels like temperature?
Not always. Many weather apps blend heat index with wind chill and sometimes wind speed into one year-round feels-like figure, while this tool computes the heat index only, from temperature and humidity, as used in official heat advisories.How do you calculate heat index in Celsius?
Convert the temperature to °F, run the formula, then convert back. 32.2°C (90°F) at 50% humidity gives about 94.6°F, or roughly 34.8°C. The calculator does the conversion for you.Why does the calculator use a different formula below 80°F?
The full Rothfusz regression was fitted to conditions of 80°F and above, where humidity has a real impact on comfort and safety. Below that, a simpler additive estimate is more reliable.Is there an adjustment for very dry or very humid air?
Yes. The NWS formula includes small corrections for humidity below 13% (at 80–112°F) and above 85% (at 80–87°F), which slightly lower or raise the raw result in those cases.Related Calculators
For cold-weather conditions, the wind chill calculator uses the same feels-like concept for temperature and wind instead of humidity. The pressure converter and speed of sound calculator are also handy if you’re working with other weather or atmospheric physics figures alongside heat index.