Heat Index Calculator
Use Relative Humidity
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Use Dew Point Temperature
If you know the dew point instead of the humidity, enter it here; the relative humidity is worked out first.
Press Calculate to see the results.
Enter the temperature and the relative humidity, or use the second tool with the dew point, and the heat index calculator shows how hot it feels and the National Weather Service risk level.
What is Heat Index?
The heat index, also called the apparent temperature, combines air temperature and relative humidity into a single number that describes how hot it feels to the human body. The body cools itself mainly by sweating; as sweat evaporates it carries heat away from the skin. In humid air sweat evaporates slowly, so the body loses heat less efficiently and feels hotter than the thermometer reading.
At 85 °F with 70% humidity, it feels like about 93 °F. The heat index assumes shade and a light wind. Full sunshine can raise the felt temperature by up to 15 °F, and strong hot winds can add to the stress too, so conditions in the open can be considerably more dangerous than the number suggests.
How to Calculate Heat Index?
The US National Weather Service uses a regression equation published by Lans Rothfusz in 1990, fitted to tables of apparent temperature developed by R. G. Steadman from a detailed model of human heat balance. It is applied when a simpler formula gives 80 °F or more; below that the simple formula is used. Results are close to Steadman's values, within about 1.3 °F across most of the range.
HI = −42.379 + 2.04901523T + 10.14333127RH − 0.22475541T·RH − 0.00683783T² − 0.05481717RH² + 0.00122874T²RH + 0.00085282T·RH² − 0.00000199T²RH²
- HI
- heat index in °F
- T
- air temperature in °F
- RH
- relative humidity in percent
Used when a simpler estimate is 80 °F or more; the NWS adds small corrections at very low or very high humidity.
Potential effects of heat index
Heat is the leading weather-related cause of death in the United States in most years. As the heat index rises, so does the risk of heat illness, especially during physical activity, for older adults, infants and young children, outdoor workers, and people with heart disease or who take certain medications. The NWS uses the categories in the table to issue heat advisories and warnings.
Heat exhaustion causes heavy sweating, cold clammy skin, a fast weak pulse, nausea, dizziness and fainting; move to a cool place, loosen clothing and sip water. Heat stroke, a medical emergency, brings a body temperature of 103 °F or more, hot red skin, a strong rapid pulse and confusion; call 911 and cool the person immediately. Never leave children or pets in a parked car, where temperatures climb to dangerous levels within minutes.
| Heat index | Classification | Possible effects with prolonged exposure or activity |
|---|---|---|
| 80 – 90 °F (27 – 32 °C) | Caution | Fatigue possible |
| 90 – 103 °F (32 – 39 °C) | Extreme caution | Heat cramps and heat exhaustion possible |
| 103 – 124 °F (39 – 51 °C) | Danger | Heat cramps or heat exhaustion likely; heat stroke possible |
| 125 °F (52 °C) or higher | Extreme danger | Heat stroke highly likely |
Frequently asked questions
What is the heat index at 90 °F and 50% humidity?
About 95 °F, in the extreme caution category.
Is the heat index measured in the shade?
Yes. It assumes shade and light wind. Direct sun can make it feel up to 15 °F hotter.
What heat index is dangerous?
The NWS classes 103 °F and above as dangerous, with heat stroke possible, and 125 °F and above as extreme danger.
How is heat index different from temperature?
Temperature measures the air. Heat index adds the effect of humidity on how well sweat can cool the body.
What is the simple heat index formula?
The National Weather Service first computes HI = 0.5 × [T + 61 + (T − 68) × 1.2 + RH × 0.094], with T in °F. If that result, averaged with the air temperature, is 80 °F or more, it switches to the full Rothfusz regression. At 80 °F and 40% humidity the simple formula gives about 79.6 °F.
How do I calculate the heat index in Celsius?
The standard formulas work in Fahrenheit, so convert first with °F = °C × 9/5 + 32, compute the heat index, then convert back with °C = (°F − 32) × 5/9. At 32 °C (89.6 °F) and 50% humidity the heat index is about 94 °F, or 34 °C. The calculator accepts Celsius and does the conversion for you.
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