Quick answer
The heat index combines temperature and humidity using the Rothfusz regression. At 90 °F and 70% relative humidity, the heat index is 105.9 °F, what the air feels like to the body.
Enter the air temperature in °F and the relative humidity. The calculator returns the heat index.
Related tools: Dew Point Calculator: Dew point from temperature and relative humidity. Wind Chill Calculator: Wind chill temperature (°F, mph). Specific Heat Calculator: Specific heat capacity or heat required from Q = m c ΔT.
How to use the Heat Index Calculator
- Enter air temperature (°F) (between 80 and 110).
- Enter relative humidity (%) (between 40 and 100).
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Air temperature | A number, in °F | ≥ 80, ≤ 110 |
| Relative humidity | A number, in % | ≥ 40, ≤ 100 |
Formula and method
Formula
HI = −42.379 + 2.04901523T + 10.14333127RH − 0.22475541·T·RH − 0.00683783T² − 0.05481717RH² + 0.00122874T²·RH + 0.00085282T·RH² − 0.00000199T²·RH²
IEEE-754 double precision; shown to at most 6 decimal places.
Rothfusz regression used by the U.S. National Weather Service; valid for T ≥ 80 °F and RH ≥ 40%.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Air temperature = 90; Relative humidity = 70.
| Result | Value |
|---|---|
| Heat index | 105.9 °F |
Example 2
Inputs: Air temperature = 80; Relative humidity = 40.
| Result | Value |
|---|---|
| Heat index | 79.9 °F |
The first line is the headline result (heat index). IEEE-754 double precision; shown to at most 6 decimal places.
Danger levels
The US National Weather Service gives 80 to 90 °F as caution, 90 to 103 extreme caution, 103 to 124 danger and 125 °F or more extreme danger. 105.9 °F falls in the danger band, where heat cramps and exhaustion are likely.
Validity
The regression is valid only for 80 to 110 °F and 40 to 100% relative humidity. Direct sunlight can raise the effective value by up to 15 °F. Drink water, rest in shade and check on vulnerable people.
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Air temperature: entering "70" shows "Air temperature must be at least 80." beside that field and no result.
- Relative humidity: entering "20" shows "Relative humidity must be at least 40." beside that field and no result.
Choosing between the Heat Index Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Dew Point Calculator](/science/dew-point/) | air temperature and relative humidity | Dew point |
| [Wind Chill Calculator](/science/wind-chill/) | air temperature and wind speed | Wind chill |
| [Specific Heat Calculator](/science/specific-heat-calculator/) | solve for, mass and temperature change | Specific heat |
| [Heat Transfer Calculator](/science/heat-transfer-calculator/) | thermal conductivity, area and temperature difference | Heat flow |
| [Heat Pump COP Calculator](/science/heat-pump-cop-calculator/) | use | COP |
Background: environment and acoustics
Decibels are a logarithmic scale, so sound levels cannot be added arithmetically: two equal sources raise the level by about 3 dB, not double it. Environmental and emissions results depend on the factors behind them (for example the US EPA's equivalency factors), which are averages for a country, fuel and year.
Use the result to compare scenarios rather than as a measured value.
Assumptions and limitations
- Outside 80–110 °F and 40–100% RH the regression is not valid
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for air temperature and relative humidity to confirm this.
Frequently asked questions
Why does humidity matter?
Sweat evaporates more slowly.
What is the wet-bulb globe temperature?
A more complete heat-stress measure.
Does wind help?
Yes, by aiding evaporation.
What does the Heat Index Calculator calculate?
Apparent temperature from air temperature and relative humidity (°F). It returns Heat index.
What formula does the Heat Index Calculator use?
HI = −42.379 + 2.04901523T + 10.14333127RH − 0.22475541·T·RH − 0.00683783T² − 0.05481717RH² + 0.00122874T²·RH + 0.00085282T·RH² − 0.00000199T²·RH². Rothfusz regression used by the U.S.
What do I need to enter in the heat Index Calculator?
You enter Air temperature (°F) and Relative humidity (%).
How is the result rounded or limited?
IEEE-754 double precision; shown to at most 6 decimal places. Outside 80–110 °F and 40–100% RH the regression is not valid.
What is an example of a heat Index result?
With air temperature 90 and relative humidity 70, the heat index is 105.9 °F.
Where does the method come from?
The method follows: Rothfusz LP, NWS Technical Attachment SR 90-23 (1990); NOAA/NWS Heat Index Equation.
Related calculators
- Dew Point Calculator — Dew point from temperature and relative humidity.
- Wind Chill Calculator — Wind chill temperature (°F, mph).
- Specific Heat Calculator — Specific heat capacity or heat required from Q = m c ΔT.
- Heat Transfer Calculator — Conductive heat flow through a flat wall.
- Heat Pump COP Calculator — Coefficient of performance from heat output and power input, or the Carnot limit.
Sources
- Rothfusz LP, NWS Technical Attachment SR 90-23 (1990)
- NOAA/NWS Heat Index Equation
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.