How to use the heat index calculator
Estimate the apparent temperature from heat and humidity. Enter air temperature, relative humidity in the fields above; the heat index are computed instantly and plotted on the chart so you can see how the answer changes.
Common searches this calculator answers include “heat index calculator”, “heat index converter”, “feels like temperature”, “how hot does it feel”, “humid heat danger”.
Formula
Variables
- T — Air temperature (°F)
- RH — Relative humidity (%)
Worked example
With air temperature = 90 °F, relative humidity = 65 %, the calculator gives heat index ≈ 102.6544 °F; heat index ≈ 39.25246 °C.
Tips for accurate results
- Use shade air temperature and relative humidity; direct sun can make conditions feel hotter.
- The NWS equation is normally applied at air temperatures of about 80 °F (27 °C) and above.
- Check the result against local heat safety guidance during outdoor work or exercise.
Common mistakes
- Using the heat index in cold weather, where wind chill is the relevant measure.
- Applying the equation outside its usual range, where the result is less reliable.
- Entering a Celsius temperature in the Fahrenheit field.
Frequently asked questions
What is the heat index calculator used for?
Estimate the apparent temperature from heat and humidity. It helps with questions such as “heat index calculator”, “heat index converter”, “feels like temperature”.
What is the formula for the heat index calculation?
Heat index (°F) = -42.379+2.04901523 × T+10.14333127 × RH-0.22475541 × T × RH-0.00683783 × T × T-0.05481717 × RH × RH+0.00122874 × T × T × RH+0.00085282 × T × RH × RH-0.00000199 × T × T × RH × RH. Heat index (°C) = ([Heat index]-32) × 5 / 9. Here T is air temperature (°F), RH is relative humidity (%).
What inputs do I need for the heat index calculator?
You need air temperature in °F, relative humidity in %. Each field is pre-filled with a typical example so you can see how the result responds as you edit the numbers.
How do I use the heat index calculator?
Type values into the input fields. Results and the chart update instantly. Use Reset to restore the example values, Copy results to paste them elsewhere, or Share to copy a link that preserves your inputs.
Is the heat index result exact?
Calculations use standard formulas and double-precision arithmetic, then display about seven significant figures. Real-world measurements carry uncertainty, so treat results as estimates unless your inputs are exact.