Heat Index Calculator: What the Temperature Actually Feels Like
The heat index is what a temperature feels like once humidity is taken into account. This runs the National Weather Service's own published equation, and returns the classification band the result falls in. It reproduces both worked examples the NWS prints on its own page — 100°F at 55% humidity gives 124°F, and 100°F at 15% gives 96°F.
Conditions
Humidity is the number a forecast gives as “RH”. If you only have a dew point, the heat index needs the relative humidity, not the dew point, so look that up rather than guessing.
Heat index
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These are shade values. The National Weather Service states that full sunshine can raise the heat index by up to 15°F.
What the bands mean
This is the National Weather Service's own classification, printed here as it appears on its site. The table renders without JavaScript; only the calculator above needs it.
| Band | Heat index | Effect on the body |
|---|---|---|
| Caution | 80°F – 90°F | Fatigue possible with prolonged exposure and/or physical activity |
| Extreme Caution | 90°F – 103°F | Heat stroke, heat cramps, or heat exhaustion possible with prolonged exposure and/or physical activity |
| Danger | 103°F – 124°F | Heat cramps or heat exhaustion likely, and heat stroke possible with prolonged exposure and/or physical activity |
| Extreme Danger | 125°F or higher | Heat stroke highly likely |
Method, and what it cannot tell you
- The formula is the Rothfusz regression published by NOAA's Weather Prediction Center, including both of its adjustments: one subtracted when humidity is at or below 13% and the temperature is between 80 and 112°F, one added when humidity is above 85% and the temperature is between 80 and 87°F.
- Below about 80°F the regression is not appropriate, so the simpler formula the WPC specifies is used instead. That switch is automatic here, exactly as the WPC describes it.
- The NWS states the regression carries an error of ±1.3°F. Results are rounded to the nearest degree, which is finer than the method's own accuracy.
- Very hot air cannot hold very high humidity. Combinations beyond the range the regression was fitted to produce numbers that look precise and are not, so this tool says so rather than printing them silently.
- Nothing here is a forecast, and it is not medical advice. Heat illness depends on exertion, acclimatisation, age, medication and how warm the previous night was — none of which a temperature and a humidity can capture.