Heat Index Calculator
Find how hot it actually feels using the National Weather Service Rothfusz heat index regression, plus a danger classification.
🥵 What is the Heat Index?
The heat index, sometimes called the "feels-like" temperature, combines air temperature and relative humidity into a single number describing how hot the human body actually perceives the weather to be. High humidity slows the evaporation of sweat, which is the body's main cooling mechanism, so humid heat feels considerably more oppressive than dry heat at the same thermometer reading.
The National Weather Service issues heat advisories and excessive heat warnings based on the heat index, since it tracks heat-related illness risk far better than temperature alone. Employers with outdoor workers use it to schedule breaks and hydration during summer shifts. Sports coaches and school athletic programs use heat index thresholds to decide whether to modify or cancel outdoor practice. Public health officials use it to warn vulnerable populations, such as the elderly and people with chronic illness, about dangerous conditions.
A common misconception is that heat index is simply "how it feels" in a vague sense. It is actually a specific, formula-derived apparent temperature calibrated to typical shaded, lightly breezy conditions, direct sunlight can add up to 15 degrees F to the effective heat index actually experienced outdoors.
This calculator applies the official National Weather Service Rothfusz regression, accepts temperature in Fahrenheit or Celsius, and classifies the result into the NWS heat index danger bands from Caution to Extreme Danger.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — A Humid Summer Afternoon
T = 90°F, RH = 50%
Example 2 — A Dangerous Heat Wave
T = 100°F, RH = 55%
Example 3 — Metric Units
T = 35°C, RH = 60%
Example 4 — Hot but Dry Air
T = 95°F, RH = 10% (low humidity adjustment applies)
❓ Frequently Asked Questions
🔗 Related Calculators
What is the formula for the heat index?
The NWS uses the Rothfusz regression, a multi-term polynomial fit to Steadman's original apparent-temperature model: HI = -42.379 + 2.04901523T + 10.14333127RH - 0.22475541*T*RH - 0.00683783*T^2 - 0.05481717*RH^2 + 0.00122874*T^2*RH + 0.00085282*T*RH^2 - 0.00000199*T^2*RH^2, with T in degrees Fahrenheit and RH as a percentage.
Are there any adjustments to the basic formula?
Yes. When RH is below 13% and T is between 80 and 112 degrees F, the NWS subtracts a correction term for very dry air. When RH is above 85% and T is between 80 and 87 degrees F, the NWS adds a correction term for near-saturated air. Both adjustments account for how humidity extremes affect sweat evaporation.
What heat index counts as dangerous?
The NWS heat index chart uses four bands: 80 to 90 degrees F is Caution (fatigue possible), 90 to 103 is Extreme Caution (heat cramps and exhaustion possible), 103 to 124 is Danger (heat exhaustion likely, heat stroke possible), and 125 degrees F or higher is Extreme Danger (heat stroke highly likely).
How hot does 90 degrees F feel at 50% humidity?
Using the NWS Rothfusz regression, 90 degrees F at 50% relative humidity produces a heat index of about 94.6 degrees F, which falls in the Extreme Caution band where heat cramps and heat exhaustion become possible with prolonged exposure or activity.
Why does the heat index formula only apply above about 80 degrees F?
The Rothfusz regression is a curve fit to Steadman's apparent-temperature tables over a specific range, roughly 80 to 110 degrees F and 40% to 100% relative humidity. Outside that range the polynomial can produce unrealistic values, so results below 80 degrees F or below 40% humidity should be treated as rough approximations.
Is the heat index the same as the actual air temperature?
No. The heat index is an apparent, or feels-like, temperature that combines actual air temperature with relative humidity to estimate how hot the body perceives conditions to be, based on how effectively sweat can evaporate for cooling. The thermometer reading itself does not change.
Does direct sunlight change the heat index?
The standard NWS heat index assumes shady conditions with light wind. Full sunshine can add up to about 15 degrees F to the effective heat index actually experienced, which is why heat advisories often note conditions apply in the shade and can feel worse in direct sun.
Can this calculator use Celsius input?
Yes. Switch the temperature unit to Celsius using the dropdown selector, and the calculator converts the input to Fahrenheit internally before applying the Rothfusz regression, then displays the heat index in both units.
Who created the heat index formula?
The heat index is based on a 1979 apparent-temperature model by Robert Steadman. In 1990, Lans Rothfusz developed a simpler polynomial regression that approximates Steadman's model, and the National Weather Service adopted the Rothfusz regression as its official operational heat index formula.
Is heat index the opposite of wind chill?
In a practical sense, yes. Heat index estimates how much hotter warm, humid air feels because high humidity slows sweat evaporation, while wind chill estimates how much colder cold air feels because wind accelerates heat loss from skin. Both are feels-like temperatures built for opposite ends of the weather spectrum.