About the Heat Index Calculator
The heat index is the "feels like" temperature in hot weather: it combines air temperature and relative humidity to estimate how hot it feels to the human body, because humid air slows the evaporation of sweat that normally cools you. This heat index calculator uses the exact algorithm published by the US National Weather Service, including the Rothfusz regression and the NWS low- and high-humidity adjustments.
Enter the air temperature in °F or °C and the relative humidity, and the calculator returns the heat index in both units along with the NWS hazard category — Caution, Extreme Caution, Danger or Extreme Danger — and what that level means for heat illness. It is useful for planning outdoor work, sports practices, runs and events, and for understanding heat advisories.
Heat index values assume shady, light-wind conditions. Full sunshine can raise the heat index by up to 15 °F, and strenuous activity, heavy clothing, age and health conditions all increase risk. The heat index is an estimate of perceived temperature, not a medical measurement; follow local heat advisories and seek medical help for signs of heat stroke.
How to use the heat index calculator
- 1Choose °F or °C.
- 2Enter the air temperature measured in the shade.
- 3Enter the relative humidity from a weather report or hygrometer.
- 4Read the heat index, its NWS category and the possible health effects.
- 5Add up to 15 °F if you will be in direct sunshine.
Formula and method
Following the National Weather Service method, the calculator first applies Steadman's simple formula HI = 0.5 × [T + 61.0 + (T − 68.0) × 1.2 + RH × 0.094]. If the average of that result and the temperature is below 80 °F, the simple value is used. Otherwise the full Rothfusz multiple regression above is used, with T in °F and RH in percent.
Two NWS adjustments are then applied. When RH is below 13% and T is between 80 and 112 °F, [(13 − RH)/4] × √{[17 − |T − 95|]/17} is subtracted. When RH is above 85% and T is between 80 and 87 °F, [(RH − 85)/10] × [(87 − T)/5] is added. Celsius inputs are converted to °F first and the result is converted back.
- HI
- Heat index (°F)
- T
- Air temperature (°F)
- RH
- Relative humidity (%)
Worked examples
Humid summer afternoon: 92 °F and 60% RH
The simple formula averaged with 92 °F is well above 80, so the Rothfusz regression applies and gives a heat index of about 104.7 °F (40.4 °C). That is in the NWS Danger band of 103–124 °F.
Hot, dry desert air: 100 °F and 10% RH
With humidity under 13% the NWS low-humidity adjustment is subtracted from the regression result, giving a heat index of about 94.1 °F — cooler than the air temperature, because sweat evaporates easily.
Muggy morning: 85 °F and 90% RH
Humidity above 85% with a temperature between 80 and 87 °F triggers the high-humidity adjustment of (90 − 85)/10 × (87 − 85)/5 = 0.2 °F, giving a heat index of about 101.8 °F.
Celsius: 35 °C and 50% RH
35 °C is 95 °F. The regression gives a heat index of about 105.2 °F, which converts back to 40.7 °C — the NWS Danger category.
Frequently asked questions
What is the heat index?+
The heat index, or apparent temperature, is what the temperature feels like to the human body when relative humidity is combined with air temperature. High humidity slows sweat evaporation, so 92 °F at 60% humidity feels like about 105 °F.
What heat index is dangerous?+
The National Weather Service classes 80–90 °F as Caution, 90–103 °F as Extreme Caution, 103–124 °F as Danger and 125 °F or higher as Extreme Danger, where heat stroke is highly likely with continued exposure.
Does the heat index account for sunshine?+
No. Heat index values are calculated for shady conditions with a light wind. The National Weather Service notes that exposure to full sunshine can raise heat index values by up to 15 °F.
Why can the heat index be lower than the actual temperature?+
In very dry air sweat evaporates quickly and cools the skin efficiently, so the body feels slightly cooler than the air. The NWS formula includes a low-humidity adjustment for relative humidity below 13%.
What is the difference between heat index and wet-bulb globe temperature?+
Heat index uses only temperature and humidity in the shade. Wet-bulb globe temperature (WBGT) also accounts for sun, wind and cloud cover, which is why it is preferred for athletics and military heat-stress guidance.
This tool gives general estimates and is not medical advice. Talk to a doctor or qualified professional about your health, diet or training.