The formula: multiply your balance by the monthly rate, then divide by what you don't pay down
The standard way to calculate a monthly payment is the amortization formula. You need three numbers: your loan balance (the amount you borrowed), your annual percentage rate or APR, and the number of months you have to repay it. The formula tells you what payment keeps you on track to owe zero at the end.
Here is the formula written out:
Monthly Payment = (Balance × Monthly Rate) / (1 − (1 + Monthly Rate)^−Number of Months)
The monthly rate is your APR divided by 12. So if your APR is 6%, your monthly rate is 0.06 ÷ 12 = 0.005. The exponent (the small number) means you raise (1 + monthly rate) to the power of negative months — this accounts for the fact that each payment shrinks what you owe, so later payments cover less interest.
Most people do not calculate this by hand. A loan calculator, a spreadsheet, or your lender's website will do it for you. But understanding what the numbers mean helps you spot errors and compare offers.
Key Takeaways
- Monthly payment depends on three things: how much you borrowed, your APR, and how many months you have to pay it back.
- The monthly rate is your APR divided by 12; a 6% APR becomes a 0.5% monthly rate.
- Early payments cover more interest than principal; later payments cover more principal than interest.
- A loan calculator or spreadsheet will compute the payment faster and more accurately than doing it by hand.
- The same balance, APR, and term will always produce the same monthly payment, regardless of lender.
Why the payment is higher than balance divided by months
If you borrowed $10,000 over 60 months, you might think the payment is $10,000 ÷ 60 = $166.67. But that ignores interest. With a 6% APR, your actual payment is about $193. The difference is the interest the lender charges for letting you borrow the money.
Interest accrues on the balance that remains. In month one, you owe the full $10,000, so interest is high. In month 60, you owe very little, so interest is low. The monthly payment is set so that it covers all the interest that will accrue over the life of the loan and also pays down the principal to zero.
This is why paying extra principal early saves you money: you reduce the balance faster, so less interest accrues in later months. If you pay $200 instead of $193 in month one, that extra $7 goes straight to principal, and you owe less interest in month two.
How to use a spreadsheet to calculate the payment
Open a spreadsheet (Excel, Google Sheets, or similar). In one cell, enter your loan balance. In another, enter your APR as a decimal (so 6% becomes 0.06). In a third, enter the number of months.
Then use the PMT function. In Excel or Google Sheets, the syntax is:
=PMT(rate, nper, pv)
Where rate is the monthly rate (APR ÷ 12), nper is the number of months, and pv is the loan balance as a negative number. For a $10,000 loan at 6% APR over 60 months:
=PMT(0.06/12, 60, -10000)
The result will be approximately $193.33. The negative sign in front of the balance tells the spreadsheet you are borrowing money; the result will be positive, representing what you pay each month.
What changes the monthly payment
Three things and only three things determine your monthly payment: the balance, the APR, and the term (number of months). Nothing else matters.
If you increase the balance, the payment goes up. If you increase the APR, the payment goes up. If you increase the term, the payment goes down — you are spreading the same debt over more months. If you decrease the term, the payment goes up — you are paying off the same debt in fewer months.
The lender's name does not matter. The day you took out the loan does not matter. Whether you make extra payments does not matter to the calculation of the standard payment (though it does matter to how much you actually owe later). Fees that are added to the balance at the start will increase the payment; fees charged separately will not.
How to read an amortization schedule
An amortization schedule is a table that shows, for each month, how much of your payment goes to interest and how much goes to principal. Most lenders provide this when you sign the loan, and you can generate one using a spreadsheet or online calculator.
The pattern is always the same: early payments are mostly interest, later payments are mostly principal. In month one of a $10,000 loan at 6% APR over 60 months, about $50 goes to interest and $143 goes to principal. By month 60, almost all of the $193 payment goes to principal because so little is left to accrue interest on.
The schedule also shows your remaining balance after each payment. This is useful if you want to know what you owe at any point, or if you want to calculate how much interest you will pay if you stick to the standard payment. For the $10,000 example, total interest over 60 months is about $5,600 — you pay back $15,600 for a $10,000 loan.
The difference between APR and interest rate
APR includes not just the interest rate but also certain fees the lender charges. For credit cards and personal loans, APR is the number you use in the payment formula. For mortgages, the APR may include origination fees, discount points, or other closing costs spread across the loan term.
Your lender is required to disclose the APR before you sign. It is the number to use when comparing loans, because two loans with the same interest rate but different fees will have different APRs and therefore different monthly payments.
Some loans have a variable APR, meaning the rate changes over time. If your APR changes, your monthly payment may change too. The formula still works the same way — you just use the new rate to recalculate.
What to do if your payment does not match the lender's statement
Calculate the payment yourself using the formula or a spreadsheet. If it does not match what the lender says you owe, ask the lender to explain the difference. Common reasons include:
- The lender rounded the payment up or down to a whole dollar amount.
- Fees were added to the balance at the start, increasing the amount you are paying off.
- The lender uses a different day-count method (some count 360 days per year, others count 365).
- You made a payment that changed the balance, and the lender recalculated based on the new balance and remaining term.
- The APR you used does not match the APR on your loan documents.
The lender's payment is the one you owe, but you have the right to understand how it was calculated. If the difference is large or unexplained, contact the lender in writing and ask for a detailed breakdown.
Frequently Asked Questions
Can I calculate the payment if my APR changes partway through the loan?
Yes, but you have to recalculate. Use the original formula with the new APR and the remaining balance and remaining months. For example, if you have 30 months left and your APR drops from 6% to 5%, recalculate the payment using the new 5% rate, the balance you currently owe, and 30 months. Your new payment will be lower.
Why does my first payment seem to cover mostly interest?
Because you owe the full balance on day one, so interest accrues on the full amount. As you pay down the principal, the balance shrinks, and interest accrues on a smaller amount. This is why the interest portion of each payment gets smaller over time.
If I pay extra principal, does my monthly payment go down?
Not automatically. Your standard monthly payment stays the same. But paying extra principal reduces the balance, so you owe less interest in future months and the loan ends sooner. Some lenders let you recalculate the payment based on the new balance and remaining term, which would lower your payment; others let you keep paying the same amount and just finish early.
What if the term is in years instead of months?
Convert years to months by multiplying by 12. A 5-year loan is 60 months. Use 60 in the formula, not 5.
Does the payment formula work for credit cards?
The formula works, but credit card payments are usually calculated differently. Most credit cards require only a minimum payment (often 1–3% of the balance), not a fixed payment that pays off the balance in a set time. If you want to know what fixed payment would pay off a credit card balance in a specific number of months, the formula will tell you.