How Loan Amortization Works (With Examples)
Why your early payments are mostly interest, how the split shifts over time, and what an amortization schedule actually tells you.
Amortization is the process of paying off a debt with a series of equal, scheduled payments. Each payment covers the interest that has accrued since the last one, and whatever is left over reduces the principal — the amount you still owe. Because the payment is fixed but the balance falls every month, the share going to interest shrinks over time while the share going to principal grows.
The formula behind the fixed payment
The payment on a standard amortizing loan is set so that the balance reaches exactly zero on the final payment. It depends on three inputs: the amount borrowed, the periodic interest rate, and the number of payments.
Here P is the principal, r is the monthly interest rate (the annual rate divided by 12), and n is the total number of monthly payments. You can run any loan through the Loan Payment Calculator to get this number instantly.
A worked example
Take a $25,000 loan at 6.5% over 5 years. The fixed payment is $489.15. Here is how the first few and last few payments break down:
| Payment | Interest | Principal | Balance after |
|---|---|---|---|
| 1 | $135.42 | $353.73 | $24,646.27 |
| 2 | $133.50 | $355.65 | $24,290.62 |
| 30 | $78.20 | $410.95 | $14,024 |
| 60 | $2.64 | $486.51 | $0.00 |
The payment is always $489.15. In month 1, 28% of it is interest; by month 60, less than 1% is. Over the whole loan you pay $4,349.22 in interest — about 17 cents on every dollar borrowed.
Why early payments barely move the balance
Interest is charged on the current balance, which is largest at the start. So early on, most of your payment is just covering the interest bill and only a sliver reduces principal. This effect is dramatic on long loans: on a 30-year mortgage, it can take close to two decades before more than half of each payment goes to principal. See the full picture on the Loan Amortization Calculator.
What this means for extra payments
Any extra money you put toward principal skips the interest line entirely and permanently removes that amount from every future interest calculation. A dollar of extra principal in year 1 of a long loan can save several dollars of interest over the life of the loan; the same dollar in the final year saves almost nothing. That is why accelerating a loan early is so powerful — you can model it with the Extra Payment Calculator or the Early Payoff Calculator.
How to read an amortization schedule
- Payment number / date — which installment the row represents.
- Interest — the lender's charge on that month's opening balance.
- Principal — the part that actually reduces your debt.
- Ending balance — what you still owe after the payment.
- Cumulative interest — total interest paid so far; useful for seeing the true running cost.
Amortized vs. other structures
Not every loan amortizes. Interest-only loans keep the balance flat until a later lump sum or reset. Add-on (simple) interest loans calculate all the interest up front on the original balance — see APR vs. interest rate and the Simple Interest Calculator. Credit cards use revolving balances with no fixed end date. Amortization is the structure behind most mortgages, auto loans, personal loans, and student loans.