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 70F 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)