Monthly payment, total interest and an amortization schedule from principal, annual rate and term. Annuity is the usual amortizing loan (fixed payment, interest charged on the remaining balance at rate ÷ 12); flat splits principal × rate × years evenly and also reports the equivalent nominal annual rate so flat quotes can be compared. An extra monthly payment (annuity only) shortens the term. Rows are rounded like a bank statement and the final payment clears the balance exactly. No fees, insurance or taxes — arithmetic only, not financial advice. Everything runs locally.