Build and read your amortization schedule
An amortization schedule is the payment-by-payment story of a loan. Enter the amount you borrowed, the interest rate and the term in years and months, and the calculator lists every payment with the interest charged, the principal repaid and the balance still owed. It opens in a monthly view, which is the level of detail most borrowers want when they are checking a lender statement, and you can switch to a summary by loan year at any time without recalculating. If you add extra payments, a second switch lets you compare the schedule with and without them.
The example loads a 200,000 loan at 6% over 15 years. The scheduled payment is 1,687.71 a month for 180 payments, and the total interest is 103,788.46.
What each column means
The payment column is the contractual principal-and-interest payment. The extra column is any additional principal you scheduled. Interest is what the loan charged that month on the opening balance, and principal is everything paid beyond interest, scheduled plus extra. The balance is what remains after the payment. In the loan-year view, each row adds up twelve consecutive payments and shows the balance after the last one, so the annual and monthly views always agree.
People use amortization schedules for several different jobs. A borrower checks a lender statement against the expected balance. Someone planning extra payments sees how much interest they would avoid and when the loan would end. A buyer compares two terms or two rates by their total interest instead of by the monthly payment alone. The same schedule also answers a practical question: how much do I still owe after a given number of payments? Because it applies to any fixed-rate installment loan, the same tool serves a car loan, a personal loan or a student loan as well as a mortgage.
How an amortizing payment is calculated
For loan amount P, monthly rate r (the annual rate divided by 1,200) and n monthly payments, the level payment is M = P × r ÷ (1 − (1 + r)−n). At a zero rate the formula becomes M = P ÷ n. Each month, interest is the opening balance times r, and the rest of the payment reduces principal. Because the balance is highest at the start, the share of each payment that goes to interest is highest at the start and shrinks as the balance falls.
For the example, r is 0.5% and M is 1,687.71. Payment one has interest of 1,000.00 and principal of 687.71, leaving a balance of 199,312.29. Payment two charges interest of 996.56 on that smaller balance, so principal rises to 691.15. The principal repaid across all payments equals the loan exactly, and the final payment is adjusted by a few cents of rounding so the balance ends at zero.
The shape of the schedule explains why early extras are powerful. In loan year one of the example, 11,769.23 of the 20,252.56 paid is interest, about 58% of the payments. In the final loan year, interest is only 643.13 of 20,252.56, about 3%. An extra dollar paid in year one removes a balance on which interest would have accrued for the whole remaining term, whereas an extra dollar in the last year avoids only a few months of interest. That asymmetry is the reason the same extra payment saves more interest the earlier it is made.
What extra payments change
Switch on extra payments to add a monthly amount, a yearly amount and up to eleven one-time payments, each with its own month. Each month the scheduled payment is applied first and the extra principal after it, capped at the remaining balance. The contractual payment does not change, so extras shorten the loan rather than shrink the payment. Adding 100 every month from the first payment brings the example from 180 to 165 payments and saves 10,028.47 of interest, and the extra principal actually applied is 16,400.00.
Timing matters as much as size. The table compares three ways of paying extra on the same loan; the one-time 100 and the yearly 1,200 are not the same as 100 every month.
| Extra payment pattern | Payments | Interest saved |
|---|---|---|
| 100 once, in payment 1 | 180 | 144.19 |
| 1,200 each year, first in payment 12 | 166 | 9,338.49 |
| 100 every month from payment 1 | 165 | 10,028.47 |
Extra payments that coincide are added before the cap, and one that would overpay the loan simply clears it, with the unused remainder reported rather than charged. An extra dated outside the loan (before the first payment or after the last scheduled one) is rejected with an explanation. If the loan is a mortgage you already hold and you want a plan from its current balance, plan payoff on an existing mortgage. A zero-rate check shows the mechanics: 1,200 over a year with 100 extra a month is paid in 6 payments with no interest.
A sensible way to use extras is to decide the amount first, date it, and then read the result as a planning aid. Choose a figure you can sustain, calculate, and look at the payoff month and the interest saved. If the answer is not worth the commitment, change the amount or the timing and run it again. Before you act, confirm that your lender applies additional money to principal rather than treating it as an advance on a future payment, and check the loan documents for any prepayment terms. The calculator assumes extra money reduces principal on the date you give it.
Monthly schedules, annual totals and CSV rounding
The calculator keeps full precision internally and rounds only for display, so totals are computed from exact values rather than from rounded rows. That means the sum of the cent-rounded rows can differ from the rounded total by a few cents; the CSV states this in its header. The export holds the scenario you are viewing, with scheduled payment and extra principal in separate columns, never as one unlabeled amount, and a total row at the end. Principal across the file adds up to the loan and the last balance is zero.
Dates follow a strict convention. Payment one falls in the first payment month you enter, so the final month is that month plus the number of payments minus one. The 180-payment example starting in October 2026 ends in September 2041, and with the 100 monthly extra the 165th payment falls in June 2040. This is a payment-calendar label, not a prediction of when your loan originated, and the date does not affect the arithmetic. If you leave it blank, the schedule is numbered without dates, but dated extras need it.
Opening the CSV in a spreadsheet is a good way to audit the result. The file lists each payment with the scheduled amount and the extra principal in separate columns, so you can sum a column, filter by date or draw your own chart. Check that the principal column adds up to the loan amount and that the last balance is zero; both hold to within a cent of rounding. If you save the file for each scenario, you can compare, for example, a 15-year and a 20-year term, or the loan with and without a yearly bonus, line by line.
Loan amortization versus accounting amortization
The word has two meanings. In lending, amortization is the repayment of a loan through scheduled payments that combine interest and principal, which is what this tool calculates. In accounting it describes spreading the cost of an asset across several periods, which follows different rules and is not calculated here. Credit cards, interest-only loans and balloon loans are not amortizing in the loan sense, so a fixed schedule does not fit them.
When this schedule will differ from your lender
Real loans can differ in several ways. Some lenders accrue interest daily from your last payment date rather than monthly, so the date you pay changes the interest. Payments may be due on different days, fees and escrow may sit alongside the loan, and servicers round differently or apply a payment to fees and interest before principal. Adjustable and variable rates change the payment over time, which a fixed-rate schedule cannot show. For a purchase with property tax, insurance and mortgage insurance, include property taxes and insurance in the mortgage calculator. To estimate a loan payment including an origination fee and the resulting APR, estimate loan payments and included-fee APR. And if rates have fallen since you borrowed, compare refinancing costs before you decide. These are educational estimates, not financial advice; see the disclaimer.
Before relying on a schedule, compare three numbers with your lender: the payment amount, the remaining balance after a recent payment and the interest on the most recent statement. If they agree to within rounding, the monthly model fits your loan and the extra-payment results are a fair guide. If they differ noticeably, the cause is usually daily interest, a different due date, a fee or a variable rate, and your servicer can explain which applies.
Amortization calculator FAQs
What is an amortization schedule?
It lists each modeled payment, the interest charged, the principal repaid and the balance remaining. Annual rows summarize the same monthly ledger.
How do I use an amortization calculator?
Enter the financed amount, note rate and remaining term. Choose the first payment month, add any extra principal and calculate before viewing or downloading the schedule.
Why do early payments contain more interest?
Interest is calculated on the opening balance. That balance is largest at the start, so less of a fixed payment initially reduces principal.
Can I add annual and one-time extra payments?
Yes. Give each extra payment a date. The calculator applies scheduled interest and payment first, then caps extra principal at the amount still owed.
Why might my lender's schedule differ?
Actual loans may use daily accrual, different due dates, fees or different rounding. This estimate uses the monthly convention stated above.
Does this calculate accounting amortization?
No. The numerical model repays a fixed-rate loan. Accounting amortization spreads an asset cost across periods and follows different rules.