What Is Amortization? A Complete Beginner Guide to How Loans Work

Amortization means paying off a loan in scheduled, equal installments so that the balance reaches exactly zero on the last payment. Each installment is split in two: first it covers the interest that built up since the previous payment, and whatever is left reduces the amount you owe. Because the interest is charged on the balance still outstanding, the interest part gets smaller and the principal part gets larger every month while the total payment stays the same. This guide uses one small loan so you can follow every number.

Four words you need

• Principal: the amount you still owe, not counting interest. It starts at the amount you borrowed.
• Interest: the price of using the lender's money, charged each period on the current principal. This guide uses a monthly charge of one-twelfth of the annual rate.
• Term: the number of payments, for example 36 for a three-year loan paid monthly.
• Amortization schedule: the table listing each payment and how it splits between interest and principal, with the balance left afterwards.

A small example, payment by payment

Assumptions: borrow $10,000 at a fixed 5% annual rate and repay over 36 monthly payments. The payment is $299.71 (rounded), which comes from the standard payment formula. Month 1 interest is $10,000 x 0.05 / 12 = $41.67, so $258.04 of the payment reduces the balance to $9,741.96. Month 2 interest is charged on that smaller balance: $40.59, leaving $259.12 for principal. Selected rows:

Over the 36 payments the borrower pays $10,789.52 in total, so the interest cost is $789.52. Of that, $428.04 is paid in the first 12 payments, $265.94 in the second year and $95.54 in the third. The loan is short and the rate is modest, so interest is a small slice from the start. On a 30-year mortgage the same mechanics produce a payment that is mostly interest for years.

Why the split changes

There is no scheduling trick here, only arithmetic. Interest in any month equals the balance times the monthly rate. The payment is fixed, so principal is the payment minus that interest. When the balance falls, next month's interest falls, so more of the same payment is left for principal, which lowers the balance further. The loan gets cheaper to carry as it shrinks. This is why paying extra principal early saves more than the same extra amount paid late, and why the first years of a long loan reduce the balance so slowly.

Loans that are not fully amortized

Not every loan works this way. Comparing three alternatives on the same $10,000 at 5% for 36 months shows what the amortizing payment actually buys:

Interest-only and balloon structures are sometimes used on purpose, for example for short-term financing, but they leave a large amount due later. Negative amortization usually happens only where a loan contract allows payments below the interest charged, and the unpaid interest is added to the balance. Credit cards are revolving rather than amortized: there is no fixed schedule, and the minimum payment is set by the card agreement, so the payoff time depends on how much you pay.

What amortization does not tell you

• The real cost of borrowing. The rate used in a schedule is the interest rate. Upfront fees raise the cost, which is what an APR measures.
• Other things in your bill. A mortgage payment may include property tax and insurance collected by the lender, and these are not part of the amortization schedule.
• What you will earn elsewhere. A schedule shows the cost of the loan, not whether it is better to repay early or invest.
• The effect of rate changes. A schedule is built for a fixed rate. On an adjustable-rate loan the payment is recalculated when the rate changes.

All figures are illustrative calculations from stated assumptions, not a loan offer. Payments here are rounded to the cent for display; the underlying calculation carries the unrounded value.