How the Loan Payment Calculator works
Most installment loans — mortgages, auto loans, and personal loans among them — use a fixed monthly payment that stays exactly the same for the entire term. That payment is calculated so that each month's interest is covered first, with the remainder going toward your principal balance, until the balance reaches zero on the final payment.
The math behind that fixed payment is the standard amortization formula. It only needs three inputs: how much you're borrowing, your annual interest rate, and how many months you'll be paying it back.
Here, P is your loan amount, r is your monthly interest rate (your annual rate divided by 12), and n is the total number of monthly payments. Once you know the payment, you can also see the total interest you'll pay over the life of the loan and roughly when you'll be debt-free.
A worked example
Say you borrow $25,000 at a 6.5% fixed annual rate over 5 years (60 monthly payments). Your monthly interest rate is 6.5% ÷ 12 = 0.5417%. Plugging those numbers into the amortization formula gives a fixed payment of $489.15 per month.
| Figure | Amount |
|---|---|
| Monthly payment | $489.15 |
| Total of 60 payments | $29,349.22 |
| Total interest paid | $4,349.22 |
| Interest as a share of what you borrowed | about 17% |
In the first month, roughly $135 of that $489.15 goes to interest and the rest to principal. By the final year, almost the entire payment is principal — the payment never changes, but its makeup shifts steadily as the balance falls.
How to read your results
Monthly payment is the fixed amount of principal and interest you owe each month. Total interest paid is the extra you hand the lender on top of the amount borrowed — the true price of the loan. Total cost of loan is principal plus interest combined. Estimated payoff date assumes you start this month and never miss or prepay a payment.
When to use this calculator
Use it before you sign anything: to check that a lender's quoted payment matches the rate and term you were promised, to see how a longer or shorter term changes the monthly cost, or to compare what you'd pay at two different interest rates. It covers any fixed-rate installment loan — personal loans, auto loans, student loans, and the principal-and-interest portion of a mortgage.
Common mistakes to avoid
- Confusing the rate with the APR. This tool uses your nominal interest rate. If your loan has origination fees, the APR — and your real cost — is higher. Use the APR Calculator for that.
- Forgetting taxes and insurance. On a mortgage or car loan, your actual bill often includes escrow, property tax, or insurance that this calculator does not add.
- Choosing a term by monthly payment alone. Stretching a loan from 3 to 5 years lowers the payment but can add hundreds or thousands in interest.
- Assuming a variable rate stays put. If your rate can adjust, re-run the numbers at a higher rate to see your worst-case payment.