Find how hot it really feels. This free heat index calculator combines air temperature and humidity (°F or °C) using the NWS formula, showing the feels-like temperature and 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 the Heat Index Calculator Works
The heat index turns air temperature and relative humidity into a single “feels like” value, reflecting how humidity slows sweat evaporation and makes hot weather feel hotter. The calculator uses the official U.S. National Weather Service (Rothfusz) formula.
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.
- T (Air temperature) — the value entered in the Air temperature field, in °F or °C depending on the toggle
- R (Relative humidity) — the percentage entered in the Relative humidity field, from 0 to 100
- Temperature unit toggle — switches the calculator between °F and °C, converting the temperature before the formula runs
Example Scenarios
| Temperature (°F) | Relative Humidity (%) | Heat Index (°F) |
|---|---|---|
| 80 | 40 | 80 |
| 85 | 70 | 93 |
| 90 | 50 | 95 |
| 95 | 60 | 113 |
| 100 | 55 | 124 |
Heat Index Calculator FAQ
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 threshold the calculator switches to a simpler additive estimate, since the full regression becomes unreliable outside its fitted range.What relative humidity should I enter if I only have a dew point?
This calculator needs relative humidity as a percentage, not a dew point. If your weather source only reports dew point, look for a relative humidity reading on the same app or forecast — most weather services list both.Why does heat index rise so sharply once it passes 90°F?
The Rothfusz regression includes several multiplied and squared terms involving temperature and humidity together, so their combined effect grows faster than either variable alone as both climb — which is why small increases in humidity at high temperatures push the feels-like value up quickly.Is there an adjustment for very dry or very humid air?
Yes. The National Weather Service formula this calculator is based on includes small correction terms for relative humidity below 13% (in the 80-112°F range) and above 85% (in the 80-87°F range), which slightly lower or raise the regression’s raw output in those edge cases.What heat index values are 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.How is heat index different from the “feels like” temperature on my weather app?
Many weather apps blend heat index with wind chill and sometimes wind speed into one year-round “feels like” figure. This calculator computes the heat index specifically, using only temperature and humidity, matching the NWS formula used in official heat advisories.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.