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Heat Index Calculator

Estimate heat index from air temperature, relative humidity, temperature unit with the Heat Index Calculator.

Choose humidity inputCalculate heat index from relative humidity or derive humidity from dew point.
2 options
HEAT INDEX99.677718 °F

NOAA/NWS decision procedure at 60.00% relative humidity; simple formula below the regression threshold with official humidity adjustments.

Estimate based on the values shown
MethodNOAA heat-index regression; dew point is converted to relative humidity with the Magnus equation. Temperature inputs and results may independently use °F or °C.

Before you use the result

Check the entered units and assumptions before using the result for purchasing, construction, engineering, or safety decisions.

Sources

EDITORIAL NOTES

Using the Heat Index Calculator responsibly

Page-specific guidance based on this calculator’s implemented method, inputs, and known limits.

What this result is useful for

Heat index combines air temperature and humidity to estimate apparent heat under the supported weather-service model. It is a screening indicator for environmental heat stress, not a measurement of an individual’s body temperature.

Assumptions and limits

The standard relationship is designed for shaded, light-wind conditions and a limited input range. Direct sun, exertion, clothing, medications, age, pregnancy, illness, acclimatization, and access to cooling can increase personal risk.

How to check the result

Confirm temperature and humidity units, then compare the result with current official local advisories. Move to cooling and seek appropriate help for concerning symptoms regardless of the displayed value; emergency symptoms should not wait for a calculator.

Content review: August 30, 2026
CALCULATOR GUIDE

How to use the Heat Index Calculator

Estimate heat index from air temperature, relative humidity, temperature unit with the Heat Index Calculator. It includes Use Relative Humidity, Use Dew Point Temperature workflows, so choose the one that matches the values you actually know. Use it to check a practical estimate, paying close attention to the selected units and assumptions.

Step by step

  1. For the Heat Index example, select Use Relative Humidity and enter air temperature, relative humidity, temperature unit, result unit. Keep each value in the unit displayed beside its field.
  2. Review the selected Heat Index workflow and inputs, then read heat index in the result panel.
  3. Change one Heat Index assumption at a time to compare scenarios. Use Reset to restore this tool's worked example values.

How the calculation works

NOAA heat-index regression; dew point is converted to relative humidity with the Magnus equation. Temperature inputs and results may independently use °F or °C.

Worked example

To reproduce the Heat Index worked example, reset the calculator. Select the Use Relative Humidity workflow. Use air temperature 90 °F; relative humidity 60 %; temperature unit °F; result unit °F. The result is heat index of 99.677718 °F. NOAA/NWS decision procedure at 60.00% relative humidity. Review the displayed units before comparing the result.

Understanding the result

The Heat Index reports heat index. Use it to check a practical estimate, paying close attention to the selected units and assumptions. Check the entered units and assumptions before using the result for purchasing, construction, engineering, or safety decisions.

How to check the answer

Check the Heat Index result by reversing the conversion or recomputing heat index from air temperature, relative humidity, temperature unit. For construction, electrical, weather, or purchasing decisions, compare the result with the applicable specification or professional guidance.

Common questions

What does the Heat Index calculate?

Estimate heat index from air temperature, relative humidity, temperature unit with the Heat Index Calculator.

Which inputs does the Heat Index example use?

The worked example uses air temperature, relative humidity, temperature unit, result unit in the Use Relative Humidity workflow. Other workflows can show different fields. Keep the chosen workflow and units with the result when comparing calculations.

Is the Heat Index result exact?

Check the entered units and assumptions before using the result for purchasing, construction, engineering, or safety decisions. Recheck air temperature, relative humidity, temperature unit and use heat index only for a decision where that level of precision is appropriate.

Formula tests, limits, and sources reviewed September 1, 2026. Guide examples updated October 12, 2026.
INTERPRET THE RESULTRead the other guideCheck assumptions, limitations and comparison methods →
KEEP GOING

Useful next steps

Dew PointEstimate dew point from air temperature, temperature unit, relative humidity with the Dew Point Calculator.BTUEstimate general heating or cooling load from area, btu factor with the BTU Calculator.Wind ChillEstimate wind chill from air temperature, temperature unit, wind speed with the Wind Chill Calculator.