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, heat index are computed instantly and plotted on the chart so you can see how the answer changes.
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.
Frequently asked questions
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 this 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 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.