Heat Index Calculator
Heat Index (°F)
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Heat Index (°C)
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Risk Level
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How the Heat Index Works
The heat index is a measure that combines air temperature and relative humidity to describe how hot the weather actually feels to the human body. According to the U.S. National Weather Service (NWS), the heat index is the most widely used indicator of heat stress risk in public weather forecasts and advisories. Your body cools itself primarily through sweat evaporation, but when humidity is high, sweat evaporates more slowly because the air is already saturated with moisture. The heat index quantifies this effect -- at 95°F with 60% humidity, the heat index reaches approximately 114°F, meaning your body experiences thermal stress equivalent to 114°F in dry conditions.
The concept was developed by Robert G. Steadman in 1979 and adopted by the NWS shortly after. According to the CDC, heat-related illness causes an average of over 700 deaths per year in the United States and thousands of emergency department visits. Steadman's model assumes a person walking outdoors in shade at 3.1 mph, wearing long pants and a short-sleeve shirt, with a light breeze. Direct sunlight can add up to 15°F to the calculated value. For cold weather conditions, use our wind chill calculator, and for moisture measurements, try our dew point calculator.
The Rothfusz Regression Equation
This calculator uses the Rothfusz regression equation, the same polynomial formula employed by the NWS: HI = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 0.00683783T^2 - 0.05481717R^2 + 0.00122874T^2R + 0.00085282TR^2 - 0.00000199T^2R^2, where T is temperature (°F) and R is relative humidity (%).
Worked example: At 95°F and 65% humidity: HI = -42.379 + 2.049(95) + 10.143(65) - 0.225(95)(65) - 0.007(95^2) - 0.055(65^2) + 0.001(95^2)(65) + 0.001(95)(65^2) - 0.000002(95^2)(65^2) = -42.379 + 194.66 + 659.30 - 1387.73 - 63.14 - 232.29 + 722.12 + 362.39 - 76.92 = approximately 136°F. This is in the "Extreme Danger" category. Use our temperature converter to switch between Fahrenheit and Celsius.
Key Terms You Should Know
- Heat Index (Feels Like Temperature): The apparent temperature experienced by the human body when air temperature and humidity are combined. Only meaningful at temperatures of 80°F or above; below 80°F, the heat index equals the air temperature.
- Relative Humidity: The percentage of moisture in the air relative to the maximum the air can hold at that temperature. At 100% relative humidity, the air is fully saturated and sweat cannot evaporate at all.
- Wet Bulb Temperature: The temperature read by a thermometer covered in a water-soaked cloth, reflecting evaporative cooling potential. A sustained wet bulb of 95°F (35°C) is considered the theoretical limit for human survival.
- Heat Exhaustion: A heat illness characterized by heavy sweating, weakness, nausea, dizziness, and cool/clammy skin. Body temperature may be elevated but typically below 104°F. Treatable by moving to a cool location and rehydrating.
- Heatstroke: A life-threatening emergency where body temperature exceeds 104°F with confusion, loss of consciousness, and often the absence of sweating. Mortality rate is 10-50% if untreated. Requires immediate emergency medical care.
NWS Heat Index Danger Levels
The NWS categorizes heat index values into four danger levels based on the risk of heat-related illness. These classifications guide public advisories and workplace safety regulations, as referenced by OSHA's heat exposure guidelines.
| Heat Index | Danger Level | Health Risks | Recommended Action |
|---|---|---|---|
| 80-90°F | Caution | Fatigue possible | Stay hydrated, take breaks |
| 90-103°F | Extreme Caution | Heat cramps, exhaustion possible | Limit outdoor activity, drink every 15-20 min |
| 103-126°F | Danger | Heat cramps, exhaustion likely; heatstroke possible | Avoid outdoor exertion, seek AC |
| 126°F+ | Extreme Danger | Heatstroke highly likely | Stay indoors, avoid all outdoor activity |
Heat Index Reference Chart
The following table shows heat index values for common temperature and humidity combinations. Note how a seemingly moderate 90°F becomes dangerous at high humidity. Values in red indicate Danger or Extreme Danger levels.
| Temp / RH | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|
| 85°F | 83 | 85 | 87 | 89 | 91 | 93 |
| 90°F | 88 | 91 | 95 | 100 | 106 | 113 |
| 95°F | 93 | 98 | 104 | 113 | 123 | 135 |
| 100°F | 99 | 107 | 116 | 128 | 142 | -- |
| 105°F | 105 | 116 | 130 | 148 | -- | -- |
Practical Examples
Example 1: Summer afternoon run. Air temperature is 88°F with 70% humidity. Heat index = approximately 100°F (Extreme Caution). A runner should reduce intensity, drink 16-32 oz of water per hour, wear light clothing, and take walk breaks every 15 minutes. Heat acclimatization takes 7-14 days, so the first week of summer heat poses the highest risk.
Example 2: Outdoor construction work. Air temperature is 95°F with 55% humidity. Heat index = approximately 104°F (Danger level). Per OSHA guidelines, workers should take a 15-minute rest break in shade every 30 minutes, have continuous access to drinking water, and supervisors should monitor for signs of heat illness. Work should be scheduled for early morning or evening hours when possible.
Example 3: Dry heat comparison. Phoenix, Arizona at 110°F with 10% humidity has a heat index of approximately 104°F. Houston, Texas at 95°F with 70% humidity has a heat index of approximately 113°F. Despite Houston being 15°F cooler by thermometer, it feels 9°F hotter than Phoenix due to humidity. This illustrates why humid heat is more dangerous than dry heat -- the body can cool effectively in dry conditions through sweat evaporation.
Tips and Strategies for Heat Safety
- Drink before you are thirsty. By the time you feel thirsty, you are already mildly dehydrated. Drink 16-32 oz of water per hour during moderate outdoor activity in high heat index conditions.
- Follow the OSHA work-rest ratio. At a heat index of 91-103°F, take 15-minute rest breaks every hour. At 103-115°F, increase to 30-minute breaks. Above 115°F, suspend heavy outdoor work entirely.
- Acclimatize gradually over 7-14 days. Gradually increase heat exposure duration and intensity over 1-2 weeks. Inadequately acclimatized individuals are at significantly higher risk of heat illness, especially during the first heatwave of summer.
- Recognize the warning signs. Heat exhaustion symptoms include heavy sweating, weakness, nausea, and dizziness. Heatstroke symptoms include confusion, loss of consciousness, body temperature above 104°F, and often the absence of sweating. Heatstroke is a medical emergency -- call 911 immediately.
- Never leave children or pets in parked vehicles. Vehicle interiors can reach 140°F+ within minutes even with windows cracked. According to the CDC, an average of 38 children die from heat-related causes in vehicles each year in the US.
- Apply sunscreen. Sunburn reduces the body's cooling ability and increases heat index risk. Use SPF 30+ and reapply every 2 hours during outdoor exposure.
Heat Index vs. Wet Bulb Globe Temperature
While the heat index is the standard for public forecasts, occupational safety professionals and the military use the Wet Bulb Globe Temperature (WBGT), which additionally accounts for wind speed, sun angle, and cloud cover. Climate scientists focus on wet bulb temperature as a survivability threshold: a sustained wet bulb of 95°F (35°C) is the theoretical human survival limit, as sweat cannot evaporate at all. While this threshold has only been reached briefly at a few global locations, climate projections suggest it could be exceeded regularly in parts of South Asia and the Persian Gulf by century's end under high-emission scenarios. For humidity-specific calculations, use our humidity calculator.
Frequently Asked Questions
What is the heat index and why is it different from air temperature?
The heat index combines air temperature and relative humidity to represent how hot it actually feels to the human body. High humidity impairs sweat evaporation, reducing your primary cooling mechanism. At 95°F with 60% humidity, the heat index reaches about 114°F because your body cannot cool as efficiently. The heat index is always higher than or equal to the actual air temperature when humidity is present.
What formula does the heat index calculator use?
This calculator uses the Rothfusz regression equation, the same nine-term polynomial used by the U.S. National Weather Service. It models the relationship between temperature, humidity, and perceived heat with accuracy within 1.3°F of the full Steadman model. The formula is valid for temperatures at or above 80°F; below that threshold, the heat index simply equals the air temperature.
What are the NWS heat index danger levels?
The NWS defines four danger levels: Caution (80-90°F heat index) where fatigue is possible with prolonged exposure; Extreme Caution (90-103°F) where heat cramps and exhaustion are possible; Danger (103-126°F) where heat cramps, exhaustion, and heatstroke are likely; and Extreme Danger (126°F+) where heatstroke is highly likely and outdoor activity should be avoided entirely.
How can I protect myself when the heat index is high?
Stay hydrated by drinking water before you feel thirsty (16-32 oz per hour during activity), wear lightweight and light-colored clothing, take frequent breaks in shade or air conditioning, avoid strenuous outdoor activity during peak heat (10 AM to 4 PM), and never leave children or pets in parked vehicles. Use our dew point calculator to assess moisture conditions alongside the heat index.
What is the difference between heat index and wet bulb temperature?
The heat index combines temperature and humidity into a single perceived temperature for shaded conditions with a light breeze. Wet bulb temperature is measured by wrapping a wet cloth around a thermometer bulb, directly reflecting evaporative cooling potential. A sustained wet bulb temperature of 95°F (35°C) is the theoretical upper limit for human survival, as sweat cannot evaporate and the body cannot cool itself at all.
Does the heat index account for sun exposure and wind?
The standard heat index assumes shaded conditions with a light breeze. Direct sunlight can add up to 15°F to the heat index value, and calm wind conditions also increase the effective temperature. For outdoor conditions with sun exposure, the Wet Bulb Globe Temperature (WBGT) is more accurate, incorporating sun angle, wind speed, and humidity. WBGT is used by the U.S. military, NCAA, and OSHA for outdoor activity guidelines.