The basic formula and what each number means

Your monthly loan payment depends on three things: how much you borrowed, the interest rate, and how long you have to pay it back. Banks and lenders use a standard formula to calculate this, and you can do the same math yourself with a calculator or a spreadsheet.

The formula is: M = P × [r(1 + r)^n] / [(1 + r)^n − 1]. Here, M is your monthly payment, P is the principal (the amount you borrowed), r is your monthly interest rate (the annual rate divided by 12), and n is the total number of payments you'll make.

The reason the formula looks complicated is that it accounts for how interest compounds—meaning you pay interest on the interest you already owe. Early payments go mostly toward interest; later payments go mostly toward principal. The formula spreads this out so your payment stays the same every month.

Key Takeaways

  • Your monthly payment is determined by the loan amount, the annual interest rate, and the number of months you have to repay it.
  • You can calculate your payment using the standard amortization formula, a spreadsheet function like PMT(), or an online calculator.
  • A higher interest rate or shorter repayment period will increase your monthly payment; a lower rate or longer period will decrease it.
  • Your actual payment may differ slightly from the calculated amount because of rounding, fees, or variable interest rates.
  • Knowing how to calculate payments helps you compare loan offers and understand how much interest you'll pay over the life of the loan.

Using a spreadsheet to calculate the payment

Most spreadsheet programs—Excel, Google Sheets, LibreOffice—have a built-in PMT function that does the math for you. You enter the monthly interest rate, the number of payments, and the loan amount, and the function returns your payment.

In Excel or Google Sheets, the syntax is: =PMT(rate, nper, pv). The rate is your monthly interest rate (annual rate divided by 12), nper is the number of payments, and pv is the loan amount as a negative number. For example, if you borrowed $20,000 at 6% annual interest over 60 months, you would type: =PMT(0.06/12, 60, -20000). The result will be your monthly payment.

Google Sheets also has a LOAN_PAYMENT function that works the same way. If you prefer not to use formulas, many lenders and financial websites offer payment calculators where you enter the three numbers and get the result when ready.

What happens when you change the loan terms

The relationship between loan terms and your payment is direct and predictable. If you increase the loan amount, your payment goes up proportionally. If you borrow $40,000 instead of $20,000 at the same rate and term, your payment doubles.

Interest rate changes have a larger effect than most people expect. A 1% increase in the annual rate can add $100 or more to your monthly payment on a large loan. On a $200,000 mortgage over 30 years, the difference between 6% and 7% is roughly $133 per month—or about $48,000 over the life of the loan.

Extending the repayment period lowers your monthly payment but increases the total interest you pay. A 60-month car loan at 5% costs less per month than a 36-month loan at the same rate, but you'll pay more interest overall because you're borrowing the money for longer.

The difference between calculated and actual payments

The formula gives you the base payment, but your actual bill may be slightly different. Lenders often round payments to the nearest dollar or nearest five dollars. Some loans include fees—origination fees, insurance, or servicing fees—that get added to your monthly bill. Variable-rate loans recalculate the payment when the interest rate changes.

Property taxes and homeowners insurance are not part of the loan payment itself, but if you have an escrow account, your lender may collect money for these each month and include it in your bill. Your loan documents will specify what is and isn't included in the payment amount.

If you want to know exactly what you'll pay, ask your lender for an amortization schedule—a month-by-month breakdown showing how much of each payment goes to principal and how much goes to interest. This document shows the exact payment amount and how your balance decreases over time.

Comparing loans with different terms

When you're choosing between loan offers, calculating the payment for each one lets you see the real cost of each option. A lower interest rate might seem better, but if it comes with a much longer repayment period, you could end up paying more interest overall.

Create a straightforward comparison table: list the loan amount, annual interest rate, term in months, and calculated monthly payment for each offer. Then multiply the monthly payment by the number of months to see the total amount you'll pay. Subtract the original loan amount to see how much interest you'll pay. This shows you not just which payment fits your budget, but which loan actually costs you less.

Some lenders quote an annual percentage rate (APR) instead of just the interest rate. The APR includes certain fees and costs, so it's often higher than the stated interest rate. Always use the APR when comparing loans, because it gives you a more complete picture of what the loan will cost.

Why the payment stays the same even though interest changes

Early in the loan, most of your payment goes to interest because you owe a large balance. As you pay down the principal, the interest portion shrinks and the principal portion grows. But the total payment stays the same every month.

On a $200,000 mortgage at 6% over 30 years, your first payment might be roughly $1,199. Of that, about $1,000 goes to interest and $199 goes to principal. By payment 300 (near the end), the split might be $50 to interest and $1,149 to principal. The payment itself never changes, but where the money goes shifts dramatically.

This is why paying extra toward principal early in the loan saves you so much money. Every dollar you pay toward principal reduces the balance that interest is calculated on, which means less interest in future months.

Frequently Asked Questions

What if the interest rate changes during the loan?

With a fixed-rate loan, your payment never changes—the rate is locked in from the start. With a variable-rate or adjustable-rate loan, the payment recalculates when the rate changes, usually on a set schedule. Your lender will send you a new payment amount and an updated amortization schedule when this happens.

Does the payment formula work for all types of loans?

The standard amortization formula works for any loan where you make equal payments over a fixed period at a fixed interest rate. This includes car loans, personal loans, and mortgages. Some loans, like credit cards or lines of credit, don't work this way because the balance and payment change month to month.

How do I know if my lender calculated my payment correctly?

Use the PMT formula or an online calculator with the exact loan amount, annual interest rate, and number of months from your loan documents. Your calculated payment should match the lender's payment within a dollar or two. If it's significantly different, ask your lender to explain what's included in their payment amount.

Can I use this formula to figure out how long it will take to pay off a loan?

No—this formula calculates the payment when you already know the term. To find the term when you know the payment, you need a different formula or a financial calculator. Most lenders can tell you how many months remain on your loan if you ask.

What's the difference between principal and interest in my payment?

Principal is the portion of your payment that reduces what you owe. Interest is the cost of borrowing the money. Early payments are mostly interest; later payments are mostly principal. Your amortization schedule shows the exact split for each payment.