Calculate the heat index instantly
Enter the air temperature and relative humidity to get the heat index using the National Weather Service Rothfusz regression.
Heat index reference chart
Heat index for common temperature and humidity combinations.
| Air temperature | Relative humidity | Heat index |
|---|---|---|
| 85°F | 40% | 84.2°F |
| 90°F | 50% | 94.4°F |
| 95°F | 60% | 112.8°F |
| 100°F | 50% | 118.1°F |
| 90°F | 70% | 105.6°F |
| 105°F | 40% | 121.3°F |
| 85°F | 70% | 92.4°F |
| 100°F | 70% | 142.4°F |
Different temperature or humidity? Use the heat index calculator above.
About the heat index calculator
Heat index — sometimes called the "feels like" temperature in hot weather — combines air temperature and relative humidity into a single number that reflects how hot it actually feels to the human body. High humidity slows the evaporation of sweat, which is the body's main cooling mechanism, so the same air temperature feels noticeably hotter on a humid day than a dry one.
This calculator uses the Rothfusz regression, the equation the US National Weather Service uses operationally to compute heat index. It's a polynomial fit derived from Steadman's original heat index model, expressed as: HI = −42.379 + 2.04901523T + 10.14333127R − 0.22475541TR − 0.00683783T² − 0.05481717R² + 0.00122874T²R + 0.00085282TR² − 0.00000199788T²R², where T is air temperature in Fahrenheit and R is relative humidity as a percentage.
That regression is only statistically valid at higher temperatures. Per NWS guidance, it isn't applicable below 80°F, since the polynomial's error grows large and heat stress isn't really a meaningful concern at cooler temperatures anyway. This calculator implements that guard directly: below 80°F, the heat index simply equals the air temperature, with no adjustment for humidity.
At the extremes of temperature and humidity — very hot and very dry, or borderline warm and extremely humid — the NWS additionally applies small correction terms to the base Rothfusz regression to keep the result physically reasonable. This calculator uses the core Rothfusz formula as specified without those edge-case corrections, which keeps results accurate across the vast majority of everyday temperature and humidity combinations people actually encounter.
Heat index is a widely used measure for issuing heat advisories and warnings, since it captures real heat-stress risk better than air temperature alone. As a general guide, a heat index above 90°F calls for caution during extended outdoor activity, and values above 105°F are considered dangerous, though individual tolerance varies with hydration, exertion level, and health conditions.
Common heat index calculations
Popular temperature and humidity combinations, already calculated.
Heat index calculator questions
How the National Weather Service heat index formula works.
What is the heat index formula?
The US National Weather Service Rothfusz regression is: HI = −42.379 + 2.04901523T + 10.14333127R − 0.22475541TR − 0.00683783T² − 0.05481717R² + 0.00122874T²R + 0.00085282TR² − 0.00000199788T²R², where T is temperature in °F and R is relative humidity as a percentage.
How do you calculate heat index?
Enter the air temperature in Fahrenheit and the relative humidity as a percentage, and the calculator applies the National Weather Service's Rothfusz regression to get the heat index, or "feels like" temperature.
Does the heat index formula work below 80 degrees?
No. Per National Weather Service guidance, the Rothfusz regression isn't applicable below 80°F. Below that threshold, this calculator returns the heat index as equal to the air temperature, without a humidity adjustment.
What is a dangerous heat index?
A heat index above 90°F calls for caution during extended outdoor activity, and above 105°F is generally considered dangerous, with a higher risk of heat exhaustion or heat stroke. Individual risk varies with hydration, exertion, and health.
What's the difference between heat index and temperature?
Air temperature is what a thermometer reads. Heat index factors in relative humidity to estimate how hot it actually feels to your body, since high humidity slows sweat evaporation and reduces your body's ability to cool itself.
What is the heat index at 95°F with 60% humidity?
At 95°F with 60% relative humidity, the heat index is roughly 113°F, well into the dangerous range for extended outdoor exposure.
Sources & references
The formulas and guidelines used on this page are based on publicly available data from: