About the Amortization Schedule Builder
This tool builds a complete payment-by-payment ledger for any fixed-rate, declining-balance loan — the same method used for the overwhelming majority of personal, auto, student, and mortgage loans. Every row shows exactly how that month's payment splits between interest (calculated on the remaining balance) and principal (what actually reduces what you owe), plus the balance left afterward. Add an optional extra monthly payment to see it applied entirely toward principal, which shortens the loan and reduces total interest — the schedule and summary update to reflect the faster payoff. You can download the full table as a CSV file to open in a spreadsheet. Everything is calculated locally in your browser.
How Amortization Works
Declining balanceInterest each month is calculated only on what you still owe, not the original loan amount — as the balance shrinks, so does the interest portion of each payment.
Fixed payment, shifting splitYour total payment stays the same every month, but early payments are mostly interest and later payments are mostly principal — the split gradually flips over the life of the loan.
Why extra payments help so muchEvery extra dollar goes straight to principal, which reduces the balance interest is calculated on for every remaining month of the loan — not just the current one. That's why even modest extra payments meaningfully cut total interest.
Last payment adjustmentThe final payment in a schedule is often a few cents different from the rest — it's sized to bring the balance to exactly zero rather than slightly negative.
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