Heat Index Calculator
Calculate the heat index (feels like temperature) based on air temperature and humidity. Understand heat-related health risks and precautions.
Heat Index
Feels Like
91°F
Caution
Temperature Unit
Weather Conditions
Heat Index Result
Feels Like Temperature
91°F
Caution
Actual Temperature
85°F
Difference
+6°
Fatigue possible with prolonged exposure and physical activity.
Caution - Health Risks
Risks
- •Fatigue possible with prolonged exposure
- •Discomfort during physical activity
Precautions
- ✓Stay hydrated
- ✓Take breaks during outdoor activities
- ✓Wear light, loose clothing
Heat Index Reference Chart
| Temp\RH | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|
| 80°F | 80° | 81° | 82° | 83° | 84° | 86° |
| 85°F | 84° | 86° | 89° | 93° | 97° | 102° |
| 90°F | 91° | 95° | 100° | 106° | 113° | 122° |
| 95°F | 99° | 105° | 113° | 123° | 134° | 147° |
| 100°F | 109° | 118° | 129° | 143° | 158° | 176° |
| 105°F | 121° | 134° | 149° | 166° | 187° | 209° |
| 110°F | 136° | 152° | 171° | 194° | 219° | 247° |
Heat Index Categories
Recognizing Heat Illness
Heat Exhaustion
- • Heavy sweating
- • Cold, pale, clammy skin
- • Fast, weak pulse
- • Nausea or vomiting
- • Tiredness, weakness
- • Headache, dizziness
Action: Move to cool place, loosen clothing, cool wet cloths, sip water.
Heat Stroke (EMERGENCY)
- • High body temperature (103°F+)
- • Hot, red, dry or damp skin
- • Fast, strong pulse
- • Confusion, altered speech
- • Loss of consciousness
Action: CALL 911 immediately. Move to cool place, cool with wet cloths, do NOT give fluids.
Quick Answer
Heat index (feels like temperature) combines air temperature and humidity to show how hot it actually feels. High humidity prevents sweat evaporation, making it feel hotter. At 90F with 60% humidity, heat index is 100F. Danger levels: 80-90F caution, 90-103F extreme caution, 103-124F danger, 125F+ extreme danger. Heat index only valid for temperatures above 80F and shade.
Key Facts
- Heat index combines temperature and relative humidity
- Only accurate for temps above 80F (27C) in shade
- 80-90F heat index: caution - fatigue possible
- 90-103F: extreme caution - heat cramps/exhaustion possible
- 103-124F: danger - heat exhaustion likely, heatstroke possible
- Above 125F: extreme danger - heatstroke highly likely
- Direct sun adds up to 15F to heat index
- At 100% humidity, 80F feels like 80F; at 90F feels like 132F
Frequently Asked Questions
Heat index, also called "apparent temperature" or "feels like" temperature, combines air temperature and relative humidity to determine what the temperature actually feels like to the human body. High humidity reduces the body's ability to cool through sweat evaporation.
Humidity affects how efficiently your body can cool itself through sweat evaporation. At 100% humidity, sweat cannot evaporate, making it feel much hotter. At 90°F with 30% humidity, it feels like 90°F. At 90°F with 80% humidity, it feels like 113°F.
Heat index above 103°F is considered "Danger" level where heat exhaustion is likely. Above 125°F is "Extreme Danger" where heat stroke is highly likely. Even "Extreme Caution" levels (91-103°F) can be dangerous for at-risk groups.
High-risk groups include: elderly (65+), infants and children, people with chronic illnesses, those on certain medications, outdoor workers, athletes, and people without air conditioning. These groups should take extra precautions when heat index rises.
Heat exhaustion: heavy sweating, cold/pale/clammy skin, fast/weak pulse, nausea, tiredness. Heat stroke (emergency): high body temp (103°F+), hot/red/dry skin, fast/strong pulse, confusion, unconsciousness. Call 911 for heat stroke.
Heat index uses the Rothfusz regression equation developed by the National Weather Service. It takes air temperature and relative humidity as inputs. The formula includes multiple polynomial terms and adjustment factors for extreme humidity conditions.
Heat Index
Feels Like
91°F
Caution