What fees are included in the lender's APR calculation?

[Krystian Sosinski]

I'm building a spreadsheet mortgage calculator for my toolkit in excel. It encompasses many variables and can be used for fixed FHA and conventional loans. I'm almost finished with this project but I'm having trouble building a formula for APR as I am unsure what fees it incorporates. I looked at several blogs/websites but their information varies. Any lenders or smart professionals out there who could clarify what fees are included and excluded in an APR formula for mortgages.