Heat Index Calculator: Feels-Like Temperature and Heat Risk
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The heat index is how hot it feels when humidity is combined with air temperature. At 92 °F and 65% humidity, it feels like about 108 °F.
At this heat index, limit time outdoors, drink water often and take breaks in the shade or air conditioning.
At 92°F and 65% relative humidity, the heat index is 108°F. NWS level: danger. Heat cramps and heat exhaustion are likely; heat stroke is possible.
Same temperature at other humidity levels
| Humidity | Feels like |
|---|---|
| 30% | 90°F |
| 40% | 94°F |
| 50% | 99°F |
| 60% | 105°F |
| 70% | 112°F |
| 80% | 121°F |
| 90% | 131°F |
Show the steps
- Temperature 92.0 °F with 65% humidity.
- Simple estimate first: 0.5 × (T + 61 + (T − 68) × 1.2 + RH × 0.094). If the average of that and T is 80 °F or higher, use the full regression.
- Heat index = 108.3 °F.
NWS heat index (Rothfusz regression)
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
T in °F, RH in percent. Adjustments apply at very low or very high humidity
- The index assumes shade and light wind. Full sun can add up to 15 °F.
- Older adults, young children and people with health conditions are at higher risk.
Estimates for planning and learning, not financial, tax, legal or medical advice. Check figures with your lender, tax preparer or doctor before you act.
About this calculator
When the air is humid, sweat evaporates slowly and your body cools less well, so the heat feels worse than the thermometer says. The National Weather Service heat index puts a number on that.
The calculator uses the NWS formula, which applies from about 80 °F (27 °C). It shows the risk level and how the heat index changes with humidity.
How to use it
- Choose °F or °C.
- Enter the air temperature and relative humidity.
- Read the heat index, the NWS level and the table for other humidity levels.
Heat risk levels (NWS)
- 80 to 90 °F: caution. Fatigue is possible with prolonged exposure.
- 91 to 103 °F: extreme caution. Heat cramps and exhaustion are possible.
- 103 to 124 °F: danger. Heat exhaustion is likely.
- 125 °F or more: extreme danger. Heat stroke is highly likely.
Staying safe
Drink water before you feel thirsty, rest in the shade, wear light clothing and never leave children or pets in a parked car.
Sources
Quick check: test yourself
The heat index combines temperature with…
Correct answer: Relative humidity. High humidity slows sweat evaporation, so it feels hotter.
At the same temperature, higher humidity makes the heat index…
Correct answer: Higher. Sweat evaporates more slowly in humid air.
Which is a sign of heat stroke?
Correct answer: High body temperature and confusion. Heat stroke is an emergency. Call for help right away.
Pick an answer for each question.
Common questions
How is heat index calculated?
With a regression formula that uses air temperature in °F and relative humidity in percent.
What is a dangerous heat index?
A heat index of 103 °F or more is dangerous for prolonged exposure. 125 °F or more is extreme.
Does the heat index apply in the sun?
It assumes shade. Direct sun can raise the feels-like temperature by up to 15 °F.
What is the lowest temperature for the heat index?
The formula is meant for 80 °F and above.
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