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Calculate heat index temperature and feels-like temperature based on temperature and humidity. Get accurate heat index, heat exhaustion risk assessment, and hot weather safety tips instantly.
Heat index measures how hot it feels when humidity is factored with actual air temperature. It applies to temperatures above 80°F (27°C) and humidity above 40%.
Enter the actual air temperature in either Fahrenheit or Celsius. Toggle between units using the temperature unit buttons. The calculator accepts any temperature value, but heat index only applies to temperatures above 80°F (27°C).
Input the relative humidity percentage (0-100%). Humidity measures the amount of moisture in the air. Heat index effects become significant when humidity is above 40%. Higher humidity makes it feel much hotter because sweat cannot evaporate efficiently to cool your body.
The calculator instantly displays the heat index temperature (feels-like temperature), risk level assessment, potential symptoms, and safety recommendations. Results update automatically as you change inputs. Color-coded risk levels help you quickly assess danger from green (safe) to purple (extreme danger).
Heat index, also known as apparent temperature, is a measure of how hot it really feels when relative humidity is factored with the actual air temperature. The human body cools itself through perspiration (sweating). When sweat evaporates from your skin, it removes heat and cools you down.
However, when humidity is high, the air is already saturated with moisture, making it harder for sweat to evaporate. This reduces your body's ability to cool itself, making you feel much hotter than the actual temperature. The heat index quantifies this effect, showing what the temperature "feels like" to the human body.
The heat index is calculated using the Rothfusz regression equation, developed by the National Weather Service (NWS) based on extensive physiological research. This scientifically validated formula takes into account both temperature and humidity:
Rothfusz Regression (NWS Official Formula):
HI = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 0.00683783T² - 0.05481717R² + 0.00122874T²R + 0.00085282TR² - 0.00000199T²R²
Where T is air temperature (°F) and R is relative humidity (%). The formula includes adjustments for specific conditions to improve accuracy. It's valid for temperatures at or above 80°F (27°C) and humidity above 40%.
Source: National Weather Service (NOAA) - Official heat index calculation methodology used by meteorologists and weather services worldwide.
Your body maintains a constant internal temperature of about 98.6°F (37°C). When the environment is hot, your body produces sweat to cool down through evaporative cooling. Here's how humidity interferes:
For example, at 90°F with 90% humidity, the heat index can reach 122°F - that's how hot it feels to your body. This extreme heat index poses serious health risks.
This chart shows heat index values (°F) based on air temperature and relative humidity combinations:
| Temp (°F) | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|
| 80 | 80 | 81 | 83 | 85 | 86 | 88 |
| 85 | 85 | 87 | 90 | 93 | 97 | 102 |
| 90 | 91 | 95 | 99 | 106 | 113 | 122 |
| 95 | 97 | 103 | 110 | 120 | 132 | 144 |
| 100 | 104 | 112 | 123 | 137 | 151 | 166 |
| 105 | 111 | 121 | 135 | 151 | 168 | 186 |
Normal (Below 80°F)
No heat-related concerns
Caution (80-90°F)
Fatigue possible with prolonged exposure and physical activity
Extreme Caution (90-103°F)
Heat exhaustion and heat cramps possible
Danger (103-125°F)
Heat exhaustion likely, heat stroke possible
Extreme Danger (Above 125°F)
Heat stroke highly likely with continued exposure
Our calculator uses color-coded zones to help you quickly assess heat danger levels. Green indicates comfortable conditions, yellow suggests caution with outdoor activities, orange warns of heat exhaustion risk requiring frequent breaks, red signals dangerous conditions with high heat illness risk, and purple indicates extreme danger where outdoor exposure should be minimized or avoided entirely.
Medical Disclaimer: This information is for educational purposes based on CDC and OSHA guidelines. Always consult healthcare professionals for medical advice. In emergencies, call 911 immediately.
Heat exhaustion is a serious condition that occurs when your body overheats. According to the CDC, it typically happens after prolonged exposure to high temperatures, especially with high humidity. Symptoms include:
Treatment: Move to a cool place, loosen clothes, apply cool wet cloths, sip water. If symptoms worsen or last longer than 1 hour, seek medical attention immediately.
Source: Centers for Disease Control and Prevention (CDC) - Heat Stress Guidelines
Heat stroke is a life-threatening emergency that requires immediate medical attention. According to OSHA, it occurs when body temperature rises above 103°F (39.4°C). Warning signs include:
EMERGENCY: Call 911 immediately! Move person to cooler place, help lower temperature with cool cloths or bath. Do NOT give fluids if unconscious.
Source: Occupational Safety and Health Administration (OSHA) - Heat Illness Prevention
Heat cramps are painful muscle spasms that usually occur in the legs, arms, or abdomen during or after intense exercise in hot weather. They're caused by loss of salt and fluids through sweating.
Treatment: Stop activity, move to cool place, drink water or sports drink, gently stretch and massage cramped muscles. Seek medical attention if cramps last more than 1 hour.
Dehydration occurs when you lose more fluids than you take in. In hot weather, you can lose significant fluids through sweating. Signs of dehydration include:
Prevention: Drink water regularly throughout the day, even before feeling thirsty. In high heat index conditions, drink 1 cup (8 oz) of water every 15-20 minutes during activity.
Proper hydration is critical in hot weather. Your body loses fluids rapidly through sweating:
What you wear significantly affects how your body handles heat:
Timing your outdoor activities can significantly reduce heat exposure:
Effective ways to stay cool in extreme heat:
| Aspect | Heat Index | Wind Chill |
|---|---|---|
| Purpose | Measures hot weather danger | Measures cold weather danger |
| Factors | Temperature + Humidity | Temperature + Wind Speed |
| When Used | Above 80°F (27°C) | Below 50°F (10°C) |
| Health Risk | Heat Exhaustion, Heat Stroke | Frostbite, Hypothermia |
| Effect | Makes it feel hotter | Makes it feel colder |
Use Heat Index Calculator when:
Use Wind Chill Calculator when:
Heat index is calculated using the Rothfusz regression formula: HI = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 0.00683783T² - 0.05481717R² + 0.00122874T²R + 0.00085282TR² - 0.00000199T²R², where T is temperature (°F) and R is relative humidity (%). Our calculator automatically computes heat index based on your inputs, showing feels-like temperature and heat risk level.
Heat index becomes dangerous at different levels: 80-90°F: Caution (fatigue possible with prolonged exposure), 90-103°F: Extreme caution (heat exhaustion and cramps possible), 103-125°F: Danger (heat exhaustion likely, heat stroke possible), Above 125°F: Extreme danger (heat stroke highly likely). At 103°F heat index, limit outdoor exposure. Above 125°F, avoid outdoor activities unless absolutely necessary.
Humidity affects perceived temperature by preventing sweat evaporation. Your body cools itself through perspiration - when sweat evaporates from skin, it removes heat. High humidity saturates the air with moisture, making it harder for sweat to evaporate. This reduces cooling efficiency, making you feel much hotter. At 90°F with 90% humidity, heat index can reach 122°F.
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