What an amortization schedule actually shows#
A fixed-rate loan keeps the same scheduled payment every month, but the mix inside that payment changes. Early on, most of each installment is interest charged on the outstanding balance; later, most of it is principal. An amortization schedule is the month-by-month ledger of that shift: payment number, how much of that payment went to principal, how much went to interest, and the remaining balance after the payment posts.
Seeing the table — not only the monthly payment — is what makes trade-offs concrete. A 30-year mortgage and a 15-year mortgage can share the same principal and rate while producing wildly different total interest, because the longer term keeps a large balance alive for more years. The schedule makes that visible payment by payment.
How the fixed monthly payment is calculated#
This tool uses the standard fixed-payment amortization formula: given principal P, monthly rate r (annual rate ÷ 12), and n months, the level payment is P · r · (1 + r)^n / ((1 + r)^n − 1). At 0% interest the payment is simply principal divided by the number of months. That payment is the contractual installment; interest each month is balance × r, and the rest of the installment reduces principal.
Amounts in the table are rounded to the nearest cent per row so the schedule matches how people expect a payment ledger to look. The final installment may be a few cents smaller so the balance lands exactly on zero instead of leaving a leftover penny.
Why extra monthly payments change everything#
An extra payment goes entirely to principal (after that month’s interest is covered). Because future interest is charged only on the remaining balance, every extra dollar permanently removes interest that would have accrued in later months. The scheduled contractual payment stays the same; the loan simply reaches zero sooner, so the schedule has fewer rows and a lower total interest figure.
That is why “amortization schedule calculator with extra payments” is a common search: people want to see how many months disappear and how much interest is avoided if they add $50, $100 or $200 on top of the fixed monthly payment — not just a vague “you will save money” claim.
Mortgages, car loans and what this does not include#
The same math covers mortgages, auto loans and other fixed-rate installment debt. Enter the term in years for a classic 15/30-year mortgage, or in months for a 36/60/72-month car loan. The schedule is the core loan amortization only: it does not add property taxes, homeowners insurance, PMI, HOA fees or other escrow items sometimes rolled into a mortgage bill.
Long schedules (360 months is common) are truncated on screen so the page stays usable; download the CSV for the complete ledger. Nothing you enter leaves the browser — the calculation and CSV export run entirely client-side.