The Complete Guide to Loan Payments and Amortization
By Arshi Khan, Founder of Just4Marketing · Last reviewed October 2026
Almost every fixed-rate installment loan — personal loans, auto loans, student loans, and mortgages — is repaid the same way: one equal payment each month that slowly shifts from mostly interest to mostly principal. This guide explains the formula behind that payment, how amortization works, and the levers that change what a loan really costs you.
The loan payment formula
A fixed-rate loan payment depends on three things: how much you borrow (P), the monthly interest rate (r = APR ÷ 12), and how many monthly payments you make (n). The formula is payment = P × r ÷ (1 − (1 + r)^−n). When the rate is 0%, it simplifies to the amount borrowed divided by the number of months.
For example, borrowing $25,000 at 8.5% for five years gives r = 0.085 ÷ 12 ≈ 0.00708 and n = 60. The payment works out to about $512.91 a month, or roughly $30,775 in total — meaning about $5,775 of the total is interest.
How amortization works
Each month, the lender first charges interest on the balance you still owe, and the rest of your payment reduces that balance. Because the payment stays the same while the balance falls, the interest share keeps shrinking and the principal share keeps growing. The year-by-year table under the calculator shows exactly this shift.
| $25,000 at 8.5%, 5 years | Principal paid | Interest paid |
|---|---|---|
| Year 1 | ≈ $4,190 | ≈ $1,965 |
| Year 3 | ≈ $4,965 | ≈ $1,190 |
| Year 5 | ≈ $5,880 | ≈ $275 |
What changes the total cost of a loan
The interest rate
Even one percentage point matters. On the $25,000, 5-year example, 7.5% instead of 8.5% saves roughly $700 in interest.
The term
A longer term lowers the monthly payment but increases total interest, because the balance stays higher for longer.
Fees
Origination fees are often deducted from the amount you receive, so you pay interest on money you never got. Compare APRs, not just rates.
Extra payments
Rounding up your payment or adding a lump sum goes straight to principal and shortens the loan.
Frequently asked questions
How is a monthly loan payment calculated?
Lenders use the amortization formula: payment = P × 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. It produces the single fixed payment that brings the balance to exactly zero on the last month.
Is APR the same as the interest rate?
Not quite. The interest rate is the cost of borrowing the principal. APR (annual percentage rate) also folds in certain upfront fees, such as origination fees, so it's usually a little higher. When comparing offers, APR is the fairer number to line up side by side.
Does paying extra each month save money?
Yes, on a standard amortizing loan. Any amount above the scheduled payment goes straight to principal, which lowers the balance that future interest is charged on. That shortens the loan and cuts total interest. Check first that your lender doesn't charge a prepayment penalty.
Why is so much of my early payment interest?
Interest each month is charged on the balance still owed. At the start the balance is at its highest, so the interest portion is largest. As the balance falls, the interest part shrinks and more of the same fixed payment goes to principal.