Heat Index Calculator
Calculate the heat index (feels-like temperature) based on air temperature and relative humidity using the Rothfusz regression equation. See also Wind Chill Calculator and Dew Point Calculator.
What Is Heat Index?
The heat index, also known as the apparent temperature or "feels like" temperature, is a measure of how hot it actually feels when relative humidity is factored in with the actual air temperature. When humidity is high, sweat evaporates more slowly from your skin, reducing your body's ability to cool itself. This makes the effective temperature feel significantly higher than the actual air temperature. The heat index is used by the National Weather Service to issue heat advisories and warnings.
Heat Index Formula (Rothfusz Regression)
The NWS uses the Rothfusz regression equation to calculate heat index. This formula is most accurate for temperatures at or above 80 degrees F and relative humidity at or above 40%:
HI = -42.379 + 2.04901523T + 10.14333127R
- 0.22475541TR - 6.83783 x 10^-3 x T^2
- 5.481717 x 10^-2 x R^2 + 1.22874 x 10^-3 x T^2 x R
+ 8.5282 x 10^-4 x T x R^2 - 1.99 x 10^-6 x T^2 x R^2
Where:
T = Air temperature in degrees Fahrenheit
R = Relative humidity (percentage)
Example Calculation
T = 95 degrees F, R = 55%
HI = -42.379 + 2.04901523(95) + 10.14333127(55)
- 0.22475541(95)(55) - ... (remaining terms)
HI = approximately 110 degrees F
Heat-Related Illness Prevention
High heat index values increase the risk of heat-related illnesses. Here are prevention strategies:
Effects of Heat Index on the Human Body
- Stay hydrated — drink water before you feel thirsty
- Wear lightweight, light-colored, loose-fitting clothing
- Limit outdoor activity during peak heat hours (10 AM to 4 PM)
- Take frequent breaks in shade or air conditioning
- Never leave children or pets in parked vehicles
- Check on elderly neighbors and those without air conditioning
- Know the signs of heat exhaustion: heavy sweating, weakness, nausea, dizziness
Heat Index Reference Chart (degrees F)
| Temp / RH | 40% | 50% | 60% | 70% | 80% | 90% | 100% |
|---|---|---|---|---|---|---|---|
| 80 F | 80 | 81 | 82 | 83 | 84 | 86 | 87 |
| 85 F | 85 | 87 | 90 | 93 | 97 | 102 | 108 |
| 90 F | 91 | 95 | 100 | 106 | 113 | 122 | 132 |
| 95 F | 99 | 104 | 110 | 120 | 130 | 142 | -- |
| 100 F | 109 | 114 | 124 | 136 | -- | -- | -- |
| 105 F | 118 | 126 | 137 | -- | -- | -- | -- |
| 110 F | 130 | 139 | -- | -- | -- | -- | -- |
Values of 103+ indicate Danger level. Values of 124+ indicate Extreme Danger.
Frequently Asked Questions
What is a dangerous heat index?
A heat index of 103 degrees F or higher is considered dangerous. At this level, heat cramps and heat exhaustion are likely, and heat stroke is possible with prolonged exposure. The NWS issues Excessive Heat Warnings when the heat index is expected to reach 105-110 degrees F or higher.
Why does humidity make heat feel worse?
Your body cools itself primarily through sweat evaporation. When humidity is high, the air is already saturated with moisture, so sweat evaporates more slowly. This reduces your body's cooling efficiency, making the temperature feel much hotter than it actually is.
Is heat index measured in the sun or shade?
The heat index is calculated for shady conditions with light wind. Direct sunlight can increase the felt temperature by up to 15 degrees F. Always add extra caution when in direct sunlight.
What is the difference between heat index and wet bulb temperature?
Both measure the effect of humidity on perceived temperature, but they use different methods. Heat index is a calculated "feels like" value. Wet bulb temperature is measured by wrapping a wet cloth around a thermometer and is always lower than or equal to the air temperature. A wet bulb temperature above 95 degrees F is considered unsurvivable.
Can the heat index be lower than the actual temperature?
At very low humidity levels (below about 40%), the heat index can actually be lower than the air temperature because dry air allows very efficient sweat evaporation, providing a cooling effect.
How does heat index affect exercise?
When the heat index exceeds 90 degrees F, outdoor exercise should be modified. Above 103 degrees F, strenuous outdoor activity should be avoided. Athletes should acclimate gradually, hydrate aggressively, and watch for signs of heat illness.
Who is most at risk from high heat index?
Those most vulnerable include elderly adults (65+), young children, people with chronic medical conditions, outdoor workers, athletes, and those without access to air conditioning. Certain medications can also impair the body's heat response.