Heat Index Calculator

Calculate the NWS heat index from air temperature and relative humidity. You can also enter dew point; that option first derives relative humidity with the separate approximation used by an NWS meteorological calculator.

Inputs and method

Enter air temperature and either relative humidity or dew point. Temperature is converted exactly to Fahrenheit. For dew point, both temperatures are converted to Celsius before relative humidity is calculated as 100 × ((112 − 0.1T + Td) / (112 + 0.9T))⁸; dew point must not exceed air temperature.

The NWS procedure first calculates its simple formula, then averages that result with air temperature. If the average is below 80°F, the calculator reports that decision average and states that the full regression does not apply. At 80°F or above, it applies the nine-term Rothfusz regression. It subtracts the low-humidity adjustment only when humidity is below 13% and temperature is from 80°F through 112°F. It adds the high-humidity adjustment only when humidity is above 85% and temperature is from 80°F through 87°F.

Intermediate values retain full precision. The displayed heat index is rounded to the nearest whole degree Fahrenheit.

Interpreting the estimate

The result is an environmental apparent temperature for shaded, light-wind conditions. Full sunshine can raise heat-index values, while strong wind with very hot, dry air can also be hazardous.

Assumptions and limits

The page does not assign a risk category or make a personalized health interpretation. The NWS source warns that the Rothfusz regression is not valid for extreme temperature and humidity combinations outside the underlying data, but it does not publish a numeric outer rectangle. The NWS dew-point approximation specifies Celsius inputs but no numerical validation range, so this page treats it only as an input transformation and rejects singular or impossible results.

The calculator rejects unsupported units or modes, invalid numeric inputs, temperatures below absolute zero, humidity outside 0%–100%, impossible dew-point relationships, singular dew-point calculations, and invalid intermediate or final results.

For public guidance rather than a personalized forecast, see NWS Heat Safety. Check current NWS watches, warnings, and advisories separately; this calculator does not monitor conditions or alerts.

Frequently asked questions

Why can a value below 80°F appear?

The NWS procedure uses the average of air temperature and its simple-formula result to decide whether to run the full Rothfusz regression. When that average is below 80°F, the page labels the shown result as the decision average and says the full regression does not apply.

Is the dew-point equation part of the Rothfusz regression?

No. It is a separate input transformation used to derive relative humidity. The heat-index procedure then uses temperature in Fahrenheit and relative humidity in percentage points.

Sources

• The Heat Index Equation — NOAA/NWS Weather Prediction Center — simple formula, average decision, Rothfusz coefficients, adjustments, and model limitation.
• Heat Forecast Tools — National Weather Service — interpretation and shaded, light-wind limitation.
• Meteorological Calculator: Additional Information — NWS Binghamton — dew-point-to-relative-humidity equation and required Celsius inputs.
• Heat Safety — National Weather Service — public heat-safety guidance.
• Watches, Warnings, and Advisories — National Weather Service — current public NWS alerts.
• SI Units: Temperature — NIST — exact temperature conversions and absolute zero.