Heat Index Calculator
The heat index, sometimes called the "feels-like" temperature, combines air temperature and relative humidity to indicate how hot the weather actually feels to the human body. High humidity impairs the evaporation of sweat, which is the body's primary way to shed heat. The result is that 95 F with 80% humidity can feel like 133 F. The National Weather Service uses the Rothfusz regression equation as its official heat index formula. Enter air temperature and relative humidity to see the heat index and danger category.
Heat index formula (Rothfusz regression)
HI = -42.379 + 2.04901523T + 10.14333127RH
- 0.22475541TRH - 0.00683783T^2 - 0.05481717RH^2
+ 0.00122874T^2*RH + 0.00085282T*RH^2
- 0.00000199T^2*RH^2
T is air temperature in degrees Fahrenheit, RH is relative humidity as a percentage. Valid for T at or above 80 F. Two adjustment terms are applied by NWS when RH is below 13% (low humidity adjustment) or above 85% with T between 80-87 F (high humidity adjustment).
Heat index danger categories
- 80-90 F: Caution. Fatigue possible with prolonged exposure and physical activity.
- 91-103 F: Extreme Caution. Heat cramps and heat exhaustion possible.
- 103-124 F: Danger. Heat cramps and heat exhaustion likely; heat stroke possible.
- Above 125 F: Extreme Danger. Heat stroke highly likely with continued exposure.
Heat index calculator: frequently asked questions
What is the NWS heat index formula?
The NWS uses the Rothfusz regression equation: HI = -42.379 + 2.04901523T + 10.14333127RH - 0.22475541TR - 0.00683783T^2 - 0.05481717RH^2 + 0.00122874T^2*RH + 0.00085282T*RH^2 - 0.00000199T^2*RH^2. T is temperature in F and RH is relative humidity as a percentage.
When does the heat index apply?
The heat index applies when air temperature is 80 F or above and relative humidity is at least 40%. In cooler or drier conditions, the heat index approximates the actual air temperature. The NWS issues heat advisories and excessive heat warnings based on heat index values.
Why is humidity so important to the heat index?
High humidity slows evaporation of sweat from the skin, which is the body's primary cooling mechanism. When sweat cannot evaporate, the body cannot cool itself as effectively, making the air feel much hotter than the thermometer reading.
What heat index values are dangerous?
The NWS categorizes heat index danger: Caution (80-90 F), Extreme Caution (91-103 F), Danger (103-124 F), and Extreme Danger (above 125 F). Heat cramps and heat exhaustion are possible above 90 F; heat stroke is possible above 103 F and likely above 125 F.
Does shade affect the heat index?
The NWS notes that exposure to full sunshine can increase the heat index by up to 15 F above the calculated value. The heat index is calculated for shaded conditions with light wind. Direct sun exposure significantly increases heat stress beyond what the formula shows.
Official sources
- National Weather Service: Heat Index Safety.
- NWS Heat Index: Heat Index Equation Reference.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.