A heat index calculator shows how hot it actually feels by combining the air temperature with relative humidity, using the National Weather Service equation. At 92 °F and 60% humidity, it feels like about 105 °F.
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How to use the heat index calculator
- Enter the air temperature in °F.
- Enter the relative humidity in percent.
- Read the heat index and check its NWS category.
What the heat index is
The heat index, sometimes called the apparent temperature, combines air temperature and relative humidity to show how hot it feels to the human body. When humidity is high, sweat evaporates more slowly, so the body cools less effectively and the air feels hotter than the thermometer reading. The heat index calculator uses the regression equation from the National Weather Service (NWS).
What is the heat index formula?
The calculator follows the NWS Weather Prediction Center procedure. For hot conditions it uses the Rothfusz regression, with temperature T in °F and relative humidity RH in percent:
- HI = −42.379 + 2.04901523T + 10.14333127RH − 0.22475541T·RH − 0.00683783T² − 0.05481717RH² + 0.00122874T²·RH + 0.00085282T·RH² − 0.00000199T²·RH²
The regression applies when the heat index is about 80 °F or higher. Below that, the heat index calculator uses the simpler NWS formula, HI = 0.5 × [T + 61 + (T − 68) × 1.2 + 0.094 × RH]. It also applies the NWS adjustments for very dry air (below 13% humidity at 80 °F to 112 °F) and very humid air (above 85% at 80 °F to 87 °F), and shows the NWS risk category with the result.
Heat index calculator worked example
At 92 °F and 60% relative humidity, the heat index is about 105 °F, which is in the danger category. At the same temperature with lower humidity it would feel noticeably cooler.
| Temperature | Humidity | Heat index | NWS category |
|---|---|---|---|
| 84 °F | 70% | 90 °F | Extreme caution |
| 86 °F | 60% | 91 °F | Extreme caution |
| 90 °F | 50% | 95 °F | Extreme caution |
| 92 °F | 60% | 105 °F | Danger |
| 95 °F | 40% | 99 °F | Extreme caution |
| 96 °F | 65% | 121 °F | Danger |
| 100 °F | 30% | 102 °F | Extreme caution |
| 104 °F | 55% | 137 °F | Extreme danger |
Heat index categories
The National Weather Service groups heat index values by their effect on the body:
- Caution, 80 °F to 90 °F: fatigue possible with prolonged exposure and activity.
- Extreme caution, 90 °F to 103 °F: heat cramps and heat exhaustion possible.
- Danger, 103 °F to 124 °F: heat cramps or heat exhaustion likely; heat stroke possible with prolonged exposure.
- Extreme danger, 125 °F or higher: heat stroke highly likely.
NWS notes that heat index values are for shady locations; direct sunlight can raise the heat index by up to 15 °F.
Staying safe in the heat
- Drink water regularly, before you feel thirsty.
- Schedule strenuous work and exercise for early morning or evening.
- Take breaks in shade or air conditioning.
- Wear light, loose clothing and a hat.
- Check on older adults, young children and people with chronic illnesses, who are most at risk.
- Never leave children or pets in a parked car.
Signs of heat stroke include confusion, a very high body temperature and hot skin; it is a medical emergency, so call 911.
Heat index and dew point
Dew point is another way to judge how humid it feels, alongside the heat index calculator. A dew point above about 65 °F feels oppressive, and on those days the heat index climbs well above the air temperature. Check both on hot afternoons.
Heat at work and play
Outdoor workers, athletes and people exercising in the heat face higher risk because activity produces extra body heat. Employers can plan water, rest and shade breaks based on the heat index, and give new or returning workers time to get used to the heat over one to two weeks. Coaches often adjust practice length and equipment when the heat index is high. Check the heat index calculator before heading out, and remember that the risk adds up over hours of exposure.
Indoors without air conditioning, heat can build up over several days, especially in upper floors. Fans help when the air is cooler than the body, but in very hot conditions they can do little, so cooling centers and air-conditioned public places are safer. During heat waves, the heat index calculator can help you decide when to check on vulnerable neighbors.
Related converters and calculators
Related calculators:
- Dew point calculator: dew point
- Wind chill calculator: cold
- Temperature converter: °F/°C
- BTU calculator: cooling
Sources
National Weather Service: Heat index; NWS Weather Prediction Center: The heat index equation.
Questions about this calculator
What is the heat index at 92 °F and 60% humidity?
About 105 °F.
When is the heat index dangerous?
103 °F or higher is in the NWS danger category.
Does sunlight change the heat index?
Yes, it can add up to 15 °F.
Which formula is used?
The NWS procedure: a simple formula below about 80 °F, otherwise the Rothfusz regression with humidity adjustments.
What should I do in extreme heat?
Drink water, rest in shade or AC, and avoid strenuous activity.
Check the assumptions and units before relying on a result. Financial, tax and health estimates do not replace advice for your individual circumstances.
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