This example uses the PPmt function to calculate how much of a payment for a specific period is principal when all the payments are of equal value. Given are the interest percentage rate per period (APR / 12), the payment period for which the principal portion is desired (Period), the total number of payments (TotPmts), the present value or principal of the loan (PVal), the future value of the loan (FVal), and a number that indicates whether the payment is due at the beginning or end of the payment period (PayType).
PPmt([APR] / 12, [Period], [TotPmts], -[PVal], [FVal], [PayType])