The calculation from the NWS: https://www.weather.gov/epz/wxcalc_heatindex (see the linked PDF[1])
Given an air temperature (T) in °F and a relative humidity (rh).
Indexheat = − 42.379 + (2.04901523 × T) + (10.14333127 × rh) − (0.22475541 × T × rh) − (6.83783×10−3×T^2) − (5.481717 × 10−2 × rh^2) + (1.22874 × 10−3 × T^2 × rh) + (8.5282×10−4 × T × rh^2) − (1.99×10−6 × T^2 × rh^2)