Heat Index Calculator
Find out how hot it actually feels by combining air temperature with humidity, using the same formula the National Weather Service uses for heat advisories.
Enter the air temperature and relative humidity to see how hot it actually feels.
Heat cramps and heat exhaustion are possible with prolonged exposure or activity.
Heat index reference chart
| Air temp | 40% | 50% | 60% | 70% | 80% | 90% | 100% |
|---|---|---|---|---|---|---|---|
| 26.7°C | 26.5 | 27.1 | 27.7 | 28.3 | 29 | 30.2 | 31.8 |
| 28.9°C | 28.5 | 29.5 | 30.8 | 32.5 | 34.4 | 36.9 | 39.8 |
| 31.1°C | 31.1 | 32.8 | 35.1 | 37.9 | 41.3 | 45.1 | 49.6 |
| 33.3°C | 34.3 | 37 | 40.4 | 44.5 | 49.5 | 55.1 | 61.6 |
| 35.6°C | 38.3 | 42 | 46.7 | 52.4 | 59.1 | 66.7 | 75.3 |
| 37.8°C | 42.9 | 48 | 54.2 | 61.5 | 70.1 | 79.8 | 90.7 |
| 40°C | 48.3 | 54.8 | 62.6 | 71.9 | 82.5 | 94.5 | 107.9 |
| 42.2°C | 54.3 | 62.5 | 72.2 | 83.5 | 96.3 | 110.7 | 126.7 |
| 43.3°C | 57.6 | 66.6 | 77.4 | 89.7 | 103.8 | 119.4 | 136.8 |
Sources
- NWS Heat Index
- NWS Wind Chill Chart
- Dew point calculated with the Magnus-Tetens formula (Alduchov & Eskridge, 1996)
These are general weather calculations based on standard meteorological formulas, not official warnings. For severe-weather alerts, always check your local National Weather Service or meteorological agency.
About the Heat Index Calculator
The heat index, also called the 'feels like' or apparent temperature, is how hot it actually feels to the human body when relative humidity is factored in with the actual air temperature. High humidity reduces the body's ability to cool itself through evaporating sweat, so the same air temperature can feel far more dangerous on a humid day than a dry one. This calculator uses the official US National Weather Service heat index algorithm: a simple Steadman estimate below 80°F, and the more precise Rothfusz regression — with NWS correction terms for very dry or very humid air — at or above 80°F. The result is classified into the NWS's heat-risk categories, from caution to extreme danger, the same bands used in real heat advisories and warnings. Source: NWS Weather Prediction Center heat index reference (wpc.ncep.noaa.gov/html/heatindex.shtml).
How to use this calculator
- 1 Enter the current air temperature using the slider or the number field.
- 2 Enter the relative humidity as a percentage.
- 3 Read the heat index result, its danger band, and the safety advice for that band.
How the heat index formula works
Below 80°F, the heat index is a simple average of the air temperature and a Steadman estimate: simple = 0.5 · (T + 61 + (T - 68) · 1.2 + RH · 0.094) At or above 80°F, the National Weather Service uses the Rothfusz regression, a more accurate multiple-regression fit to Steadman's full heat-index table: HI = -42.379 + 2.04901523·T + 10.14333127·RH - 0.22475541·T·RH - 0.00683783·T² - 0.05481717·RH² + 0.00122874·T²·RH + 0.00085282·T·RH² - 0.00000199·T²·RH² Two correction terms are then applied: the heat index is lowered slightly when humidity is under 13% and the temperature is between 80-112°F, since dry air evaporates sweat faster than the base formula assumes, and raised slightly when humidity is over 85% and the temperature is 80-87°F. For example, 90°F at 70% humidity gives a heat index of about 106°F — squarely in the 'danger' category.
Frequently asked questions
What is a dangerous heat index?
The National Weather Service considers a heat index of 103–124°F (39–51°C) 'danger,' where heat cramps and heat exhaustion are likely and heat stroke is possible with continued exposure or activity. A heat index of 125°F (52°C) or higher is 'extreme danger,' where heat stroke is highly likely. Even the 90–102°F (32–39°C) 'extreme caution' range can cause heat cramps and exhaustion with prolonged exposure, so it's worth taking seriously well before the highest bands.
Why does humidity make heat feel worse?
Your body cools itself mainly by sweating and letting that sweat evaporate. Evaporation removes heat, but it only works well when the surrounding air is relatively dry. When humidity is high, the air is already close to saturated with water vapor, so sweat evaporates more slowly, or not at all, and the body can't shed heat as effectively — making the same air temperature feel, and actually be, more dangerous.
What's the difference between heat index and air temperature?
Air temperature is simply what a thermometer reads. Heat index (or 'feels like' temperature) combines that reading with relative humidity to estimate how hot it actually feels to a person, since humidity affects how well the body can cool itself through sweating. On a humid day, the heat index can be 15°F (8°C) or more above the actual air temperature.
Is the heat index the same as 'RealFeel' or 'feels like' on weather apps?
It's closely related but not always identical. This calculator uses the official NWS heat index formula, which factors in air temperature and humidity only, in the shade with light wind. Proprietary 'feels like' or 'RealFeel' figures from weather apps often add other factors like wind speed, sun exposure, and cloud cover, so they can differ slightly from the NWS heat index for the same conditions.
Related tools
Heat is one half of the feels-like picture — see the wind chill calculator for how cold air and wind combine in winter, and the dew point calculator for a humidity measure that doesn't depend on temperature. For raw unit conversions, the temperature converter handles °C, °F and kelvin directly.