How Loan Amortization Actually Works
Finance - 3 min read - Published 2026-08-27 - Updated 2026-10-19
Most car loans, personal loans and mortgages are amortizing loans: you pay the same amount every month, and by the final payment the loan is fully repaid. What surprises many borrowers is how that fixed payment is split. In the early years, most of it is interest; only later does most of it reduce what you owe. Understanding why makes it much easier to compare loans and to decide whether extra payments are worth it.
Interest is charged on what you still owe
Each month, the lender charges interest on the remaining balance. With an annual rate of 7%, the monthly rate is 7% ÷ 12 = 0.5833%. Whatever part of your payment isn't needed for that month's interest goes toward the balance (the principal).
At the start, the balance is at its highest, so the interest charge is too, and less of the payment is left for principal. As the balance falls, each month's interest falls with it, and more of the same fixed payment goes to principal. The payment never changes; only the split does.
The formula behind the payment
The fixed monthly payment M for a loan of P at a monthly rate r over n months is: M = P × r × (1 + r)^n ÷ ((1 + r)^n − 1). It's the one payment that, applied every month, brings the balance to exactly zero at the end.
Worked example: a $20,000 car loan at 7% for 60 months. The payment comes to $396.02. In month one, interest is $20,000 × 0.5833% = $116.67, so $279.36 goes to principal. By the last payment, almost all of it is principal. Over five years you pay $23,761 in total: the $20,000 borrowed plus $3,761 of interest.
Why a mortgage feels so slow at first
The effect is far stronger on long loans. Take a $300,000 mortgage at 6.5% over 30 years. The monthly payment (principal and interest only, before taxes and insurance) is $1,896.20. In the first month, interest is $1,625.00 and only $271.20 reduces the balance - about 14% of the payment.
Over 30 years, total interest comes to about $382,633, more than the amount borrowed. That isn't a hidden fee; it's the cost of borrowing a large sum for a long time, and it's exactly what the formula above produces.
What extra payments do
Any amount you pay beyond the required payment usually goes straight to principal, as long as the lender applies it that way - check your loan terms, and some lenders ask you to mark the extra as "principal only". Because the balance is lower, every later month's interest is lower too, so the saving compounds.
On the car loan, paying an extra $100 a month ($496.02 in total) clears it in 47 months instead of 60, and total interest falls from $3,761 to about $2,868, a saving of nearly $900.
On the mortgage, an extra $200 a month clears it in about 277 months (just over 23 years) instead of 360, and cuts total interest from about $382,633 to about $279,185, a saving of over $100,000. Extra payments early in the loan save the most, because they remove principal that would otherwise have built interest for decades.
Shorter terms: higher payment, much less interest
The same $300,000 at 6.5% over 15 years has a monthly payment of $2,613.32, about $717 more than the 30-year loan. But total interest is about $170,398, less than half. Shorter loans often come with slightly lower interest rates as well, which widens the gap further.
The right choice depends on whether the higher payment fits comfortably in your budget. A longer loan with voluntary extra payments keeps flexibility: you can pay more in good months and fall back to the required payment if money gets tight.
Before you overpay
Check whether your loan has a prepayment penalty; many don't, but some do, especially in the first years. Keep an emergency fund first, since money paid into a loan is hard to get back quickly. And compare rates: paying off a 4% loan early saves 4% a year, while clearing a 22% credit card balance saves far more.
An amortization schedule, a month-by-month table of interest, principal and balance, makes all of this visible. A loan calculator can produce one in seconds, and trying different extra payments shows exactly what each option saves.