Your principal amount is the original sum you borrowed, which will be available on your mortgage documents. You can also subtract your down payment amount from your home’s sale price to determine how much you borrowed to purchase your home.
Principal = home sale price - down payment
Otherwise, you can adjust the simple interest formula to determine the principal amount if you know your interest rate, number of payments and interest paid. When you divide (interest rate x number of payments) from each side, you isolate the principal variable to create this formula:
Principal = (simple interest) / (interest rate x time)