How to Audit a Mortgage Amortization Schedule Row by Row

Most borrowers and analysts accept mortgage calculator outputs without verification, but a structured audit approach can reveal rounding errors, compounding bugs, and rate-encoding mistakes. Every amortization row should contain five values: payment number, payment amount, interest portion, principal portion, and remaining balance, and these must reconcile precisely across rows. Three quick tests — checking that interest matches the monthly rate times the prior balance, that principal plus interest equals the scheduled payment, and that the final row closes at exactly zero — can catch the majority of calculation errors. Any mortgage calculator worth trusting must expose per-row principal and interest breakdowns that are exportable and reproducible from the original inputs. These auditing principles apply equally to standard fixed-rate loans and more complex structures involving extra payments, variable rates, or biweekly schedules.