Heat Index Calculator
Find the "feels like" temperature from air temperature and relative humidity, using the NOAA Rothfusz regression.
Written by Magesh | Financial Tools & Calculation Specialist · Last updated August 01, 2026
In two sentences: A heat index calculator combines air temperature and relative humidity into a single “feels-like” number using the National Weather Service’s Rothfusz regression, the official U.S. formula for apparent temperature. This guide breaks down the exact NOAA equation, its adjustment rules for extreme humidity, and worked examples so you can understand exactly why a hot, humid day feels more dangerous than the thermometer alone suggests.
What Is a Heat Index Calculator?
A heat index calculator estimates how hot the weather actually feels to the human body by combining measured air temperature with relative humidity. The heat index calculator combines air temperature and relative humidity into the apparent temperature your body actually feels using the NWS Rothfusz regression. This matters because humidity slows the evaporation of sweat, which is the body’s main cooling mechanism in hot weather, so if sweat cannot evaporate efficiently, the body retains more heat and the apparent temperature rises even if the actual air temperature stays the same.
The Heat Index Formula: NOAA’s Rothfusz Regression
The computation used for the heat index is a refinement of a result obtained by multiple regression analysis carried out by Lans P. Rothfusz and described in a 1990 National Weather Service Technical Attachment (SR 90-23), based on tabulated results from Steadman’s 1979 study in the Journal of Applied Meteorology.
HI = c₁ + c₂T + c₃R + c₄TR + c₅T² + c₆R² + c₇T²R + c₈TR² + c₉T²R²
The full set of coefficients is: HI = -42.379 + 2.04901523T + 10.14333127RH – 0.22475541TRH – 0.00683783T² – 0.05481717RH² + 0.00122874T²RH + 0.00085282TRH² – 0.00000199T²RH², where T is air temperature in degrees Fahrenheit and RH is relative humidity as a percentage.
When the full regression applies — and when it doesn’t
A simplified formula is computed first: HIs = 0.5 × (T + 61 + 1.2(T − 68) + 0.094RH). If the average of this simplified result and the actual temperature is 80°F or lower, that simplified value is the final answer. The Rothfusz regression is not appropriate when conditions of temperature and humidity warrant a heat index value below about 80 degrees F — in those cases, a simpler formula is applied to calculate values consistent with Steadman’s results.
Adjustments for extreme humidity conditions
The full regression needs two corrections in edge cases:
- If relative humidity is below 13% and temperature is between 80°F and 112°F, subtract an adjustment: (13 − RH)/4 × √((17 − |T − 95|)/17).
- If relative humidity is above 85% and temperature is between 80°F and 87°F, add an adjustment of [(RH − 85)/10] × [(87 − T)/5].
The Rothfusz regression is not valid for extreme temperature and relative humidity conditions beyond the range of data considered by Steadman, so any heat index calculator should flag results outside its validated range as less reliable.
Worked Examples
Example 1: A hot, moderately humid day
Temperature = 95°F, Relative Humidity = 50%. Plugging into the Rothfusz regression:
HI = -42.379 + 2.049(95) + 10.143(50) − 0.2248(95)(50) − 0.00684(95²)
− 0.0548(50²) + 0.001229(95²)(50) + 0.000853(95)(50²) − 0.00000199(95²)(50²)
HI ≈ 107°F
At 95°F actual temperature, the heat index climbs to roughly 107°F — solidly in the Danger category.
Example 2: The low-humidity adjustment in action
Temperature = 100°F, Relative Humidity = 10% (below the 13% threshold). The base Rothfusz regression gives a starting heat index, but because RH is under 13% and temperature falls between 80–112°F, <cite index=”8-1″>the calculator must subtract (13 − 10)/4 × √((17 − |100−95|)/17) from the raw result</cite> — a small but real correction that keeps low-humidity heat index values from overstating actual heat stress.
Example 3: A cooler, humid day where the simplified formula applies
Temperature = 78°F, Relative Humidity = 60%. Since this combination produces an averaged result below 80°F, the simplified Steadman-consistent formula is used instead of the full nine-term regression, producing a heat index close to the actual air temperature since humidity has less physiological impact at cooler temperatures.
Step-by-Step: How to Use a Heat Index Calculator
- Enter the current air temperature in Fahrenheit.
- Enter the current relative humidity as a percentage.
- The calculator applies the simplified formula first and checks whether the averaged result is above or below 80°F.
- If above 80°F, the full Rothfusz regression runs, with automatic adjustments for very low or very high humidity in specific temperature ranges.
- Compare your result against NOAA’s heat risk categories to understand your actual danger level.
Understanding Heat Index Danger Categories
A heat index above about 105°F falls into the Danger category, where heat cramps, heat exhaustion, and heat stroke become much more likely with prolonged exposure. Values above 130°F are considered Extreme Danger and can become life-threatening quickly. It’s worth noting the published heat index assumes shaded conditions with light wind — direct sunlight and calm air can push the real felt temperature meaningfully higher than the calculated heat index alone suggests.
When to Actually Check a Heat Index Calculator
A heat index calculator is most valuable exactly when a thermometer alone would understate real risk — hot, humid days where the air temperature seems tolerable but the combined effect on the body is dangerous. Outdoor workers, athletes, event organizers, and anyone with young children or elderly family members benefit from checking a heat index calculator before extended time outside, not just glancing at the raw temperature.
Practical situations where a heat index calculator matters most
- Outdoor sports and events — coaches and event organizers increasingly use heat index thresholds to decide whether to modify or cancel outdoor activity, since heat illness risk rises sharply above certain heat index bands.
- Construction and agricultural work — physically demanding outdoor labor in high heat index conditions carries meaningfully elevated heat-stress risk compared to the same air temperature at lower humidity.
- Vulnerable populations — a heat index calculator result in the Danger category should prompt extra caution for infants, older adults, and anyone with cardiovascular or respiratory conditions.
- Travel planning — checking a heat index calculator for your destination, not just the forecasted temperature, gives a more honest picture of what conditions will actually feel like.
Running actual numbers through a heat index calculator, rather than relying on a gut sense of “it’s hot but not that hot,” is a simple habit that meaningfully reduces heat-related health risk.
Frequently Asked Questions
Because the human body cools itself primarily through sweat evaporation, and high humidity slows that evaporation. The same air temperature can feel dramatically different — and pose very different health risks — depending on how much moisture is already in the air.
Generally yes — most weather apps use the NOAA heat index formula (or a close variant) for their summer “feels-like” temperature, while switching to wind chill in cold weather.
No. The published NOAA heat index assumes shaded conditions with light wind; full sun exposure can add several degrees to how conditions actually feel.
Because the full Rothfusz regression was fit specifically to Steadman’s data at higher temperature and humidity combinations, and produces unreliable results outside that validated range — the simplified formula keeps low-heat-index values physically realistic.
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In summary, a heat index calculator built on NOAA’s official Rothfusz regression gives you the same “feels-like” temperature the National Weather Service itself uses, and understanding the formula’s structure — including when it switches to a simplified equation — helps explain why humidity can turn a warm day into a genuinely dangerous one.
About the author: Magesh is a Financial Tools & Calculation Specialist focused on building and fact-checking online calculators across finance, weather, and everyday measurement topics. Figures and formulas in this article are sourced directly from NOAA’s National Weather Service and reviewed for accuracy.
Note: This calculator and article are provided for general educational and informational purposes only and do not constitute medical or safety advice. If you are experiencing symptoms of heat-related illness, seek medical attention immediately. Always follow official heat advisories issued by the National Weather Service for your area.