OraHow loan amortization works
Amortization divides each repayment between interest and principal. On a standard fixed-rate repayment loan, the payment stays broadly level while the falling balance shifts more of it toward principal.
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One payment, two different jobs
Interest is the charge for the period's outstanding balance. Principal repayment reduces the amount still owed. A payment of 304.22 is not a 304.22 reduction in debt when some of it covers interest.
With a constant rate and regular payments, a larger balance produces more interest at the start of the schedule. As principal is repaid, the next period's interest gets smaller. More of a level payment can then reduce principal. The CFPB explanation of mortgage repayment describes this changing split.
Calculate the level payment
For amount borrowed P, monthly rate i and number of monthly payments n, payment M = P × i ÷ [1 − (1 + i)−n]. For a nominal annual rate of 6%, i = 0.06 ÷ 12 = 0.005. A zero-rate loan uses P ÷ n.
This formula assumes payments at the end of equal monthly periods and a constant rate. The Microsoft PMT reference documents the same type of constant-payment model. Neither the formula nor its payment result includes every possible fee or tax.
Daily-interest contracts, odd first periods, balloon balances and rate changes need a schedule that matches their actual terms.
Follow the balance for three months
Take an illustrative 10,000.00 loan at 6% nominal annually for 36 monthly payments. The formula gives 304.219374… per month, rounded to 304.22. This example rounds each month's interest to cents before applying payment.
On a narrow screen, scroll the table horizontally to see all columns.
| Month | Payment | Interest | Principal | Closing balance |
|---|---|---|---|---|
| 1 | 304.22 | 50.00 | 254.22 | 9,745.78 |
| 2 | 304.22 | 48.73 | 255.49 | 9,490.29 |
| 3 | 304.22 | 47.45 | 256.77 | 9,233.52 |
Read a row from left to right: payment covers interest first, the remaining portion reduces principal, and the closing balance becomes the next month's opening balance.
Why the last payment can differ
Rounding a theoretical payment to cents introduces a small difference every month. Rounding interest also changes a schedule slightly. A final payment adjusted to clear the remaining balance is therefore normal in a modeled schedule.
For the example above, 35 payments of 304.22 and a final payment of 304.18 total 10,951.88. Principal is 10,000.00, so total interest is 951.88. Multiplying the unrounded theoretical payment by 36 gives 951.89748… of interest instead. Both calculations must be labeled with their conventions; mixing them creates a false discrepancy.
A lender using actual payment dates or a different rounding policy can produce a different schedule.
How an extra payment changes later interest
If an extra amount immediately reduces principal, the following period starts with a smaller balance. Under the same fixed rate, that produces less interest. Keeping the normal payment unchanged can therefore shorten the schedule.
Adding 50.00 monthly to this example clears the modeled loan in 31 months with total interest of 807.29. The 144.59 reduction is interest saved, not money magically created by the extra payments. The extra money repays principal earlier.
Check the agreement before overpaying. The CFPB prepayment-penalty explanation notes that some mortgage contracts can charge for early repayment.
Use the schedule to ask better questions
Compare the amount financed, rate basis, first payment date, repayment term and fees with the actual offer. A mortgage statement can also collect taxes or insurance, making the amount due greater than its principal-and-interest portion.
The loan repayment calculator provides a transparent fixed-rate illustration. Use the overpayment comparison to isolate regular extra principal. Neither makes a credit-approval decision or replaces a lender's account statement.
Common questions
Why is interest higher at the start?
The outstanding balance is larger. At the same periodic rate, a larger balance generates more interest.
Is amortization the same as simple interest?
Amortization describes repayment of the balance over time. A loan may use a simple daily interest method while still amortizing; its exact dates and terms determine the schedule.
Does a long term always have a lower total cost?
No. A longer term may reduce each payment but allow interest to accrue for more periods.
Educational estimate using the stated assumptions. Fees, taxes, changing rates and individual circumstances can change an outcome. This is not personalized financial advice. Read the estimates disclaimer.