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Heat Index Calculator

Use this heat index calculator to review an educational health calculation for heat index.

Primary answer
Heat index
Inputs to verify
Air temperature, Temperature unit, and Relative humidity
Use type
Use as an estimate that depends on assumptions.
Keyword intent
heat index calculator

Calculator

Heat Index Calculator

Calculates heat index from air temperature, temperature unit, relative humidity. Defaults are filled in so you can review a working example before changing inputs.

selected

Enter the air temperature; it must convert to the supported 40 F through 112 F range.

Choose Fahrenheit or Celsius for the air temperature input.

%

Enter relative humidity as a percent from 0 to 100.

Result

Result reflects the current submitted inputs.

  • Risk B
  • Reviewed 2026-05-26
  • 3 sources
Heat index106 F
Heat index41.1 C
Heat stress categoryDanger
Formula methodRothfusz regression
Air temperature used90 F
Relative humidity used70%

Breakdown

Air temperature used
90 F
Relative humidity used
70%
Formula method
Rothfusz regression
  • Temperature inputs are converted to Fahrenheit before calculation.
  • Relative humidity is entered as a percent from 0 to 100.
  • The implementation supports air temperatures that convert to 40 F through 112 F.
  • The Rothfusz regression is applied only when the simple apparent-temperature approximation is at least 80 F.
  • Low-humidity and high-humidity adjustments follow the NOAA/NWS Weather Prediction Center equation notes.
  • Heat index values are for shady locations; direct sunlight can make conditions feel hotter.
  • Results are safety estimates only and are not medical, occupational safety, or local weather warning advice.
  • Intermediate values are not rounded; raw outputs are rounded to 10 decimal places for stability.

Accuracy notes

Risk level
B
Reviewed
2026-05-26
Sources
3
Primary result
Heat index

Formula logic is kept in a pure calculator module with fixtures, source notes, and page-visible assumptions.

Categories

What the result means

Heat index is the number to carry forward from this heat index calculation. NOAA/NWS heat index apparent temperature in Fahrenheit. Treat the result as educational context, then read the limitations and assumptions before discussing it with a professional. Use heat stress category, formula method, and air temperature used to explain why heat index moved when an input changed. Record the inputs you used and verify any health concern with a qualified professional.

Heat indexNOAA/NWS heat index apparent temperature in Fahrenheit.
Heat indexHeat index apparent temperature converted to Celsius.
Heat stress categoryNWS heat index category based on the displayed whole-degree Fahrenheit heat index.
Formula methodFormula path used for the final heat index value.

Use the result this way

  1. Start with Heat index, then use supporting outputs only to explain the primary answer.
  2. Verify air temperature, temperature unit, and relative humidity before copying the result.
  3. Choose the mode or method first because it can change which formula is applied, keep units consistent with the labels shown in the form, and stay within the documented minimum and maximum ranges.
  4. Record the inputs you used and verify any health concern with a qualified professional.

User job

How to use this calculator

Use Heat Index Calculator when you need heat index, then use heat index and heat stress category to check the context for general wellness education and preparing for a professional conversation.

Best for

  • Understanding a formula-based estimate
  • Seeing which inputs drive a health-related output
  • Reviewing a default example before entering your own air temperature and temperature unit.

Check before relying

  • Health formulas can be population-specific; do not use the result as diagnosis or treatment advice.
  • Temperature inputs are converted to Fahrenheit before calculation.
  • Relative humidity is entered as a percent from 0 to 100.
  • Source context: NOAA Weather Prediction Center / National Weather Service, reviewed 2026-05-26.

Next useful step

  • Dew Point CalculatorUse next when you need dew point from air temperature and temperature unit after checking heat index.
  • Wind Chill CalculatorUse next when you need wind chill from air temperature and temperature unit after checking heat index.
  • One Rep Max CalculatorUse next when you need estimated 1RM from weight lifted and weight unit after checking heat index.

Formula

Compute the NWS simple apparent-temperature approximation first. If it is 80 F or higher, apply the Rothfusz regression with the NWS low- or high-humidity adjustment when applicable. Key assumptions: Temperature inputs are converted to Fahrenheit before calculation. Relative humidity is entered as a percent from 0 to 100. The implementation supports air temperatures that convert to 40 F through 112 F.

  • Compute the NWS simple apparent-temperature approximation first. If it is 80 F or higher, apply the Rothfusz regression with the NWS low- or high-humidity adjustment when applicable.
  • Temperature inputs are converted to Fahrenheit before calculation.
  • Relative humidity is entered as a percent from 0 to 100.
  • Primary source context: NOAA Weather Prediction Center / National Weather Service.

Inputs

Enter air temperature, temperature unit, and relative humidity for general wellness education and preparation for a professional conversation. Before calculating, choose the mode or method first because it can change which formula is applied, keep units consistent with the labels shown in the form, and stay within the documented minimum and maximum ranges. Air temperature: Enter the air temperature; it must convert to the supported 40 F through 112 F range. Temperature unit: Choose Fahrenheit or Celsius for the air temperature input. Relative humidity: Enter relative humidity as a percent from 0 to 100.

Air temperatureEnter the air temperature; it must convert to the supported 40 F through 112 F range.
Temperature unitChoose Fahrenheit or Celsius for the air temperature input.
Relative humidityEnter relative humidity as a percent from 0 to 100.

Example

Using the default inputs, Heat Index Calculator returns heat index of 106. Adjust air temperature, temperature unit, and relative humidity to match your own scenario.

FAQ

How is heat index calculated here?

Compute the NWS simple apparent-temperature approximation first. If it is 80 F or higher, apply the Rothfusz regression with the NWS low- or high-humidity adjustment when applicable. The first assumption to check is: Temperature inputs are converted to Fahrenheit before calculation.

What does Heat index mean for heat index?

Treat the result as educational context, then read the limitations and assumptions before discussing it with a professional. Secondary values such as heat stress category, formula method, and air temperature used are there to explain the primary answer, not to replace it.

What should I enter for Air temperature?

Enter the air temperature; it must convert to the supported 40 F through 112 F range. Use selected for this field. Choose the mode or method first because it can change which formula is applied, keep units consistent with the labels shown in the form, and stay within the documented minimum and maximum ranges.

How does Temperature unit change heat index?

Choose Fahrenheit or Celsius for the air temperature input. Changing it can alter heat index because the formula uses the submitted inputs together. Also compare measurement method, formula population, age range, units, sex or activity assumptions, and clinical context.

Why does the heat index example show 106 for heat index?

The default inputs produce 106 for heat index. Treat that as a format and scale check, then replace every default value with your own inputs.

Can the heat index result diagnose or prescribe anything?

No. Use it as educational context only. Health formulas can depend on population, measurement method, age range, and clinical context.

Sources

Last reviewed: 2026-05-26

  • Reviewed 2026-05-26
    The Heat Index EquationNOAA Weather Prediction Center / National Weather Service. Rothfusz regression coefficients, low-humidity adjustment, high-humidity adjustment, simple formula path below about 80 F, and warning that the regression is not valid beyond the data range considered by Steadman.
    Scope
    Official WPC/NWS heat index equation notes.
    Supports
    Rothfusz regression coefficients, low-humidity adjustment, high-humidity adjustment, simple formula path below about 80 F, and warning that the regression is not valid beyond the data range considered by Steadman.
  • Reviewed 2026-05-26
    What is the heat index?National Weather Service / NOAA. Official examples for 100 F at 55% RH and 15% RH, heat-stress category ranges, shaded-location assumption, direct-sun caveat, and Rothfusz formula context.
    Scope
    NWS public heat index explanation, examples, category table, and shade/sun caveat.
    Supports
    Official examples for 100 F at 55% RH and 15% RH, heat-stress category ranges, shaded-location assumption, direct-sun caveat, and Rothfusz formula context.
  • Reviewed 2026-05-26
    Heat Index Probability ForecastsNOAA Weather Prediction Center / National Weather Service. Formula lineage from Steadman's table and Rothfusz regression, and limitation that regression equations should not be applied outside the studied temperature and relative humidity range.
    Scope
    WPC technical details for heat index forecast calculations.
    Supports
    Formula lineage from Steadman's table and Rothfusz regression, and limitation that regression equations should not be applied outside the studied temperature and relative humidity range.

Disclaimer

This health calculator is for educational estimates only. It does not diagnose, treat, or replace advice from a qualified health professional.