Loan amortization explained: why early payments are mostly interest

The mechanism in one paragraph

An amortizing loan has a fixed payment that covers both interest and principal. The CFPB explains that in an amortizing loan a percentage of each monthly payment goes to principal and to interest, and that at the beginning of the term more of the payment generally goes to interest than principal. [1] The reason is arithmetic, not a lender trick: interest is charged on the balance still owed. The CFPB's mortgage explainer says the same thing in reverse: as you pay down principal, you owe less interest each month because the balance is lower. [2]

The formula in plain text

Monthly payment = P x r / (1 - (1 + r) ^ -n), where P is the amount borrowed, r is the annual rate divided by 12, and n is the number of monthly payments. Then each month: interest = current balance x r; principal = payment - interest; new balance = old balance - principal.

Our example uses $20,000 for 60 months at 11.90 percent, the rate the Federal Reserve's G.19 release dated October 7, 2026 reports for 24-month personal loans at commercial banks in August 2026 (preliminary). [3] It is an anchor, not a quote; five-year loans and other lenders can price differently. Here r = 0.119 / 12 = 0.0099167, so the payment works out to $443.88. In month one, interest is $20,000 x 0.0099167 = $198.33, and principal is $443.88 - $198.33 = $245.55. By month 30, interest is down to $116.98 and principal is up to $326.90. In the final month, interest is $4.36.

The schedule by year

Rolled up by year, the shift is clear. In year one, interest takes about 42 percent of what you pay. In year five it takes about 6 percent. Total interest over the five years is about $6,633, so the loan costs about $26,633 in all. Figures are rounded to the nearest dollar and assume on-time payments and no fees.

What an extra $100 a month does

Adding $100 to every payment, applied to principal, pulls the balance down faster, which shrinks every later interest charge. On this loan the result is a payoff in 46 payments instead of 60, the last one partial, and total interest of about $5,001 instead of about $6,633. That is about $1,632 saved, and the loan ends 14 months early.

Timing matters as much as amount. An extra $100 in month one avoids interest on that $100 for the remaining life of the loan; an extra $100 in month 55 saves very little because almost no interest is left. If you can only spare a lump sum once, earlier is worth more. The same logic is why a longer term costs more: the CFPB says the longer the loan term, the lower your monthly payments but the more you pay in interest over the life of the loan. [1]

Check for prepayment penalties first

Extra payments only help if the lender applies them to principal and does not charge you for paying early. For closed-end credit such as installment loans, Regulation Z requires the disclosure to state whether a charge may be imposed for paying all or part of the principal early or, for loans whose finance charge is not computed on the unpaid balance, whether you are entitled to a rebate of any finance charge if you prepay. [4] Those disclosures must be given in writing before consummation, in a form you can keep. [5]

The CFPB's explainer on prepayment penalties is written for mortgages, but its advice transfers: read the fine print carefully, understand exactly when you would have to pay and how much, and ask for a quote on a similar loan without a penalty so you can compare total costs. It notes penalties typically apply to paying off the whole balance early rather than small extra principal payments, but says it is always a good idea to check with the lender. [6]

Checklist: making extra payments count

For checking the amount you actually receive versus the amount you borrow, see our loan offer fine-print guide.

• Find the prepayment line in your loan disclosure: is there a penalty, and is there a finance charge rebate? [4]
• Ask the servicer how to mark a payment as extra principal, and confirm on the next statement that the balance dropped by the full extra amount.
• Make sure an extra payment does not just advance your next due date while sitting unapplied; ask how the servicer treats it.
• Recompute your payoff date with the formula above or a spreadsheet after any lump sum.
• Compare against your other debts: an extra $100 to a credit card at a higher rate usually saves more than $100 to this loan.
• Keep an emergency buffer before prepaying; money paid into a loan is not easy to get back.