Heat Index Calculator
Estimate the heat index from air temperature and relative humidity.
Estimate how hot it actually feels ("apparent temperature") by combining air temperature with relative humidity, using the U.S. National Weather Service heat index formula.
How Heat Index Calculator Works
High humidity slows the evaporation of sweat from your skin, which is your body's main cooling mechanism in heat — so the same air temperature feels noticeably hotter as humidity rises, because your body struggles to cool itself as effectively.
The calculator applies the Rothfusz regression, a multi-term polynomial formula (using temperature in °F and relative humidity as a percentage) that the National Weather Service developed to closely match how hot conditions actually feel to the human body across a wide range of temperature and humidity combinations.
This formula is most accurate for temperatures at or above roughly 80°F with humidity at or above 40% — the range where heat and humidity combine to meaningfully raise the apparent temperature above the thermometer reading.
See It In Action
Who Uses Heat Index Calculator and Why
- Checking how dangerous outdoor conditions are before scheduling strenuous work or exercise on a hot, humid day.
- Understanding a weather forecast's "feels like" or "real feel" temperature figure in summer conditions.
- Assessing heat exhaustion or heat stroke risk for an outdoor event based on forecast temperature and humidity.
- Comparing how much worse humid heat feels compared to dry heat at the same thermometer reading.
Mistakes to Avoid
- Using the calculator outside its valid range — the Rothfusz regression is most accurate at or above roughly 80°F with humidity at or above 40%; results outside that range are less reliable.
- Confusing relative humidity with absolute humidity or dew point when entering the humidity value — the formula specifically expects relative humidity as a percentage.
- Ignoring the heat index result when planning strenuous outdoor activity — a 16-degree-plus gap between actual and apparent temperature (as in a typical hot, humid day) meaningfully raises heat exhaustion risk even if the thermometer reading alone doesn't seem extreme.
Tips for Best Results
- Treat a heat index of 90-103°F as a caution zone for heat exhaustion with prolonged exposure, 103-124°F as a danger zone, and 125°F+ as extreme danger, per National Weather Service guidance.
- Remember heat index is calculated for shaded conditions — direct sun exposure can add even more to the perceived temperature than the formula accounts for.
Fixing Common Problems
My heat index result seems unrealistically high or low. — Confirm you entered relative humidity as a percentage (like 70, not 0.70) and temperature in °F — a decimal-form humidity value is a common input mistake that skews the result.
Terms Explained
Heat index: The "apparent temperature" combining air temperature and relative humidity to reflect how hot conditions actually feel to the human body.
Rothfusz regression: The multi-term polynomial formula the National Weather Service developed to calculate heat index from temperature and relative humidity.