How to Calculate Loan Payments
Lenders use a standard amortization formula to calculate fixed monthly payments on installment loans. The formula is:
M = P × r × (1 + r)^n / ((1 + r)^n − 1)- M — monthly payment
- P — loan amount (principal)
- r — monthly interest rate (annual rate ÷ 12)
- n — total number of monthly payments
Example: A $250,000 mortgage at 6.5% APR over 30 years gives a monthly payment of about $1,580. Over the full term you'll pay roughly $319,000 in interest — more than the original loan amount.
What Affects Your Loan Payments
Four main inputs determine your monthly payment and total interest cost:
- Interest rate. Even a 1% difference can mean tens of thousands of dollars over a 30-year mortgage. Shop around and improve your credit score before applying.
- Loan term. Longer terms reduce monthly payments but dramatically increase total interest. A 15-year mortgage costs much less in interest than a 30-year one.
- Principal (loan amount). Borrowing less is always cheaper. A bigger down payment lowers your principal and your monthly payment.
- Extra payments. Paying more than the minimum each month reduces principal faster, slashing total interest and shortening the loan.
Amortization Explained
Amortization is how a loan is paid off through equal monthly payments. Each payment is split between interest and principal, but the proportions change every month:
- Early payments are mostly interest because the balance — and therefore the interest charge — is highest at the start.
- Later payments are mostly principal because the balance has shrunk and less interest accrues.
The amortization schedule above shows this breakdown for every single month of your loan. Open it to see exactly how much interest vs principal you pay each month.
How Extra Payments Save Money
Every extra dollar you pay goes directly to principal. That reduces the balance interest is calculated on for every remaining month — so the savings compound.
Example: On a $250,000 mortgage at 6.5% over 30 years, paying just $200 extra per month saves about $95,000 in interest and pays the loan off roughly 7 years early.
Use the "Extra Monthly Payment" field above to model your own scenario — the calculator will show your interest savings instantly.
Frequently Asked Questions
How is a loan payment calculated?
Loan payments use the amortization formula M = P × r × (1+r)ⁿ / ((1+r)ⁿ − 1), where P is the principal, r is the monthly rate, and n is the total number of payments. Each payment covers interest first, with the remainder reducing the principal balance.
What is amortization?
Amortization is the process of paying off a loan in equal monthly installments. Early payments are mostly interest; later payments are mostly principal. The amortization schedule shows the breakdown for every month.
How can I reduce my loan interest?
Choose a shorter term, get a lower rate, make a larger down payment, or add extra principal each month. Even small overpayments can save thousands over the life of the loan.
What happens if I make extra payments?
Extra payments reduce the principal directly, which lowers future interest charges and shortens the loan. Use the extra payment field above to see the exact impact.
Is this loan calculator accurate?
Yes — it uses the standard amortization formula used by banks. Actual payments may differ slightly due to taxes, insurance, escrow, or fees not included here.
Does this work for mortgages, auto loans, and personal loans?
Yes. The math is the same for any fixed-rate installment loan — mortgages, car loans, personal loans, or student loans. Just enter your amount, rate, and term.
What is APR vs interest rate?
The interest rate is the cost of borrowing the principal. APR includes interest plus fees and reflects the true annual cost of the loan. This calculator uses the interest rate.