Loan Amortization

What Is It

Amortization is the process of paying off a loan through fixed periodic payments, where each payment splits between interest — the cost of borrowing the money still outstanding — and principal, which actually reduces what you owe. The interest portion of any given payment is simply the remaining balance multiplied by the periodic interest rate. Since the balance is highest at the very start of the loan, the interest portion is highest then too — which is why early payments on any installment loan barely dent the balance, even though the payment amount never changes.

Why It Matters

This isn't a mortgage-only phenomenon, even though mortgage amortization gets most of the attention — the same math runs underneath every fixed-payment installment loan: auto loans, personal loans, and student loans included. Two practical consequences follow directly from it. First, extra principal payments save the most interest when made early, since that's when the largest share of each payment would otherwise have gone to interest instead. Second, a loan statement showing you still owe nearly the full amount after a year or two of on-time payments isn't a sign anything went wrong — it's exactly what the math predicts, since the front of the schedule is interest-heavy by design.

Quick Example

A $30,000 auto loan at 6% APR over 5 years carries a payment of $579.98 a month. In month one, $150.00 of that payment is interest and only $429.98 reduces the balance. By month 30 — halfway through the loan — the split has shifted to $83.09 interest and $496.90 principal, simply because the balance has already dropped. Over the full 5 years, total interest paid comes to $4,799.04 — nearly 16% of the amount borrowed, on a rate that sounds modest.

Try It Yourself

See the full month-by-month interest-vs-principal split for your own loan amount, rate, and term.

Related Concepts