What Is Loan Amortization?

Loan amortization is the process of paying off a debt through a series of regular payments over time. Each payment is split between two components: interest on the remaining balance and a reduction of the principal. Over the life of the loan, the interest portion shrinks and the principal portion grows — until the final payment retires the debt completely.

Why your balance does not drop in a straight line

Interest is charged as a percentage of the outstanding balance — not the original loan amount. Early in the loan, the balance is high, so the interest portion of each payment is large and the principal reduction is small. As the balance falls, less interest accrues per period, leaving more of each fixed payment to go toward principal.

Example — $200,000 mortgage at 6% for 30 years (monthly payment $1,199):

Year	Annual interest paid	Annual principal paid	Balance remaining
1	$11,933	$2,455	$197,545
5	$11,534	$2,854	$186,016
10	$10,900	$3,488	$168,685
20	$8,778	$5,610	$127,622
30 (final)	$701	$12,155	$0

After 10 years of payments on a 30-year mortgage, you have repaid only ~16% of the principal. After 20 years, only ~36%. This is why early extra payments are so powerful — they reduce the balance that generates future interest charges.

The amortization formula

The split for each period is:

Interest payment = Outstanding balance × monthly rate
Principal payment = Total EMI − Interest payment
New balance = Outstanding balance − Principal payment

Fixed-rate vs adjustable-rate amortization

Fixed-rate loans: The total payment stays constant. Interest and principal shares shift, but the payment amount never changes.

Adjustable-rate loans (ARMs): When the rate resets, the lender recalculates the payment to fully amortise the remaining balance over the remaining term at the new rate. This can cause payment shock if rates rise significantly.

The power of extra payments

Because interest compounds on the outstanding balance, extra principal payments made early have an outsized effect:

• An extra $100/month on a $200,000 30-year mortgage at 6% saves approximately $49,000 in total interest and shortens the loan by 5+ years
• The same extra $100/month applied starting in year 20 saves only about $2,500

See How Extra Payments Reduce Loan Interest for a deeper walkthrough.

Negative amortization

If a payment is smaller than the interest owed for that period, the shortfall is added to the principal — the loan balance actually grows. This is called negative amortization and can occur with some interest-only loans, certain ARMs with payment caps, or deferred-interest products. It is a warning sign for any loan.

Model home loans with the Mortgage Calculator, view the full payment-by-payment breakdown with the Amortization Schedule Calculator, and see the impact of extra payments with the Extra Payment Payoff Calculator.