How Loan Amortization Works: Why Early Payments Are Mostly Interest

Most car loans, personal loans and fixed-rate mortgages are amortizing loans. You pay the same amount every month, and by the final payment the balance is exactly zero. What changes from month to month is how each payment is split between interest and principal. That split explains why a loan balance can seem to barely move for years, and then fall quickly near the end.

The monthly cycle

Every month on an amortizing loan follows the same three steps:

1. Interest is charged on the balance you still owe. The monthly rate is the annual rate divided by 12. At 6% a year, that’s 0.5% a month.
2. Your payment covers that interest first.
3. Whatever is left over reduces the principal. The new, lower balance is what next month’s interest is charged on.

The payment itself comes from a standard formula:

M = P × r(1 + r)^n ÷ ((1 + r)^n − 1)

where P is the amount borrowed, r is the monthly rate and n is the number of payments. It finds the one fixed payment that, run through the cycle above n times, leaves a balance of exactly zero.

Following one loan from start to finish

Take a $250,000 mortgage at 6% over 30 years. The monthly principal and interest payment is $1,498.88.

Month 1. The balance is $250,000, so interest is 0.5% of that: $1,250.00. Only $248.88 of the payment goes toward principal. The balance falls to $249,751.12.

Month 2. Interest is now charged on the slightly lower balance, so it’s a little smaller, and the principal portion is a little larger. This repeats every month, and the change speeds up as the balance shrinks.

Here’s how the loan looks at a few points along the way:

After	Remaining balance	Interest paid so far
5 years	$232,635.89	$72,568.47
10 years	$209,214.31	$139,079.47
15 years	$177,622.11	$197,419.85
20 years	$135,008.97	$244,739.28
30 years	$0.00	$289,595.47

A few things stand out:

• The first five years are about 81% interest. Of roughly $89,900 paid, only about $17,400 reduced the balance.
• Halfway through the term, more than 70% of the loan is still owed. After 15 years, the balance is $177,622.11.
• The crossover comes late. The principal portion of the payment doesn’t exceed the interest portion until month 223, more than 18 years in.
• The last payment is almost all principal. In month 360, interest is just $7.46 and $1,491.42 clears the balance.

Over the full term, the borrower pays $289,595.47 in interest, more than the amount borrowed. Nothing is hidden here. It is simply 6% a year charged on a large balance that takes a long time to pay down.

You can produce this schedule for any loan with the loan calculator, or for a home purchase, including taxes and insurance, with the mortgage calculator. Both show principal paid, interest paid and the remaining balance for each year.

What changes the total cost

The term. The same $250,000 at 6% over 15 years has a payment of $2,109.64, about $611 more a month. But total interest falls to $129,735.57, less than half the 30-year figure. A shorter term means a higher payment but much less interest, because the balance comes down faster and is charged interest for fewer months.

The rate. Because interest is charged on the balance every month for the whole term, small rate differences add up. The effect is largest on long loans.

The amount borrowed. Payment and interest scale in direct proportion. Borrow 10% less and both fall by 10%.

Extra payments. On a standard amortizing loan, any extra amount paid toward principal lowers the balance immediately, and every later month’s interest is charged on that lower balance. On the 30-year example, adding $100 a month toward principal would pay the loan off in 25 years and 6 months and cut total interest by about $51,600. The loan and mortgage calculators show the standard schedule without extra payments. Before paying extra, check that your lender applies it to principal and that the loan has no prepayment penalty.

Why this matters in practice

Equity builds slowly at first. If you sell a home or a car a few years into the loan, you may owe more than you expect, because early payments mostly went to interest. With a car, whose value usually falls quickly, this can leave you owing more than the car is worth.

Refinancing restarts the clock. A new 30-year loan begins again with interest-heavy payments, even if the rate is lower. Compare the total remaining cost, not just the monthly payment.

Longer terms cost more overall. The CFPB notes that a longer loan term lowers the monthly payment but increases the total interest paid. Seeing the full schedule makes that trade-off concrete.

When a loan doesn’t amortize normally

Not every loan follows this pattern. Interest-only loans charge only interest for a set period, so the balance doesn’t fall at all during that time. With negative amortization, payments don’t cover the interest due, and the unpaid interest is added to the balance, so you owe more over time. Some loans use precomputed interest, where the total interest is fixed at the start. If a loan offer describes any of these features, ask the lender for its full payment schedule. To learn how interest that builds on itself differs from interest on the principal alone, see simple vs. compound interest.

Sources

1. How does paying down a mortgage work? — Consumer Financial Protection Bureau. Checked October 4, 2026.
2. What is amortization and how could it affect my auto loan? — Consumer Financial Protection Bureau. Checked October 4, 2026.