Here is the 16 element equation used to convert dry bulb temperature (T) and relative humidity (RH) into the Heat Index. This equation works at dry bulb temperatures of 70°F and higher. It is configured as I use it in Excel: The symbol  "^"   means  "raised to the power of".

=16.923+((1.85212*10^-1)*T)+(5.37941*RH)-((1.00254*10^-1)*T*RH)
+((9.41695*10^-3)*T^2)+((7.28898*10^-3)*RH^2)+((3.45372*10^-4)*T^2*RH)
-((8.14971*10^-4)*T*RH^2)+((1.02102*10^-5)*T^2*RH^2)-((3.8646*10^-5)*T^3)
+((2.91583*10^-5)*RH^3)+((1.42721*10^-6)*T^3*RH)
+((1.97483*10^-7)*T*RH^3)-((2.18429*10^-8)*T^3*RH^2)
+((8.43296*10^-10)*T^2*RH^3)-((4.81975*10^-11)*T^3*RH^3)