The basic formula for monthly loan payments
The monthly payment on a loan 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 amount you borrowed (called the principal), r is your monthly interest rate (your annual rate divided by 12), and n is the total number of months you have to repay the loan.
This formula assumes you make equal payments every month and that the interest rate does not change. Most personal loans, car loans, and mortgages work this way.
Key Takeaways
- Your monthly payment depends on the loan amount, the annual interest rate, and the number of months you have to repay it.
- You can calculate your payment by hand using the standard loan formula, or use an online calculator or spreadsheet to do the math faster.
- The interest rate matters more than you might think — a 1% difference in rate can change your monthly payment and total cost significantly.
- Your loan documents will show you the exact monthly payment amount, so you do not have to calculate it yourself unless you are comparing loan offers.
Breaking down the formula with a real example
Let's say you borrow $10,000 at 6% annual interest over 5 years (60 months). First, convert the annual rate to a monthly rate: 6% ÷ 12 = 0.5% per month, or 0.005 as a decimal. Then plug the numbers in: P = $10,000, r = 0.005, n = 60.
Working through the formula step by step: (1 + 0.005)^60 = 1.3489. Then multiply: 0.005 × 1.3489 = 0.006745. Divide: 0.006745 ÷ (1.3489 − 1) = 0.006745 ÷ 0.3489 = 0.01933. Finally, multiply by the principal: $10,000 × 0.01933 = $193.33 per month.
Over 60 months, you will pay $193.33 × 60 = $11,599.80 total. The difference between what you borrowed and what you paid back ($1,599.80) is the interest the lender earned.
Using a spreadsheet or calculator instead
If the formula feels overwhelming, you have easier options. Most spreadsheet programs (Excel, Google Sheets) have a built-in PMT function that does this calculation for you. In Google Sheets, the formula is =PMT(rate, nper, pv), where rate is your monthly interest rate, nper is the number of months, and pv is the loan amount as a negative number.
For the example above, you would type: =PMT(0.005, 60, -10000). The spreadsheet returns 193.33, matching the hand calculation.
Online loan calculators are even simpler — you enter the loan amount, annual interest rate, and loan term in years, and the calculator shows your monthly payment when ready. Many banks and lender websites have free calculators you can use without signing up or providing personal information.
How interest rate changes affect your payment
The interest rate has a large effect on what you pay each month. Using the same $10,000 loan over 5 years, a 4% rate gives a monthly payment of $184.59, while a 8% rate gives $202.76. That is an $18 difference per month, or $1,080 over the life of the loan.
This is why comparing interest rates before you borrow matters. Even a difference of 0.5% or 1% can cost you hundreds of dollars. When a lender offers you a rate, ask what rate other lenders are offering for the same loan amount and term.
How loan length affects your payment
A longer loan term lowers your monthly payment but raises your total cost. For a $10,000 loan at 6%, a 3-year term (36 months) costs $299.71 per month, while a 5-year term (60 months) costs $193.33 per month. The 5-year loan is $106 cheaper each month, but you pay $11,599.80 total instead of $10,789.56 — an extra $810 in interest.
When you are deciding on a loan term, think about what monthly payment you can afford, not just the lowest payment available. A longer term feels easier now but costs more later.
What your loan documents will tell you
You do not have to calculate your payment yourself. Your loan agreement or disclosure document will show the exact monthly payment amount, the interest rate, the loan term, and the total amount you will pay over the life of the loan. In the United States, lenders are required to provide this information before you sign.
Look for a document called a Truth in Lending disclosure or Loan Estimate (for mortgages). This document lists the payment amount, the annual percentage rate (APR), and the finance charge — the total interest you will pay. Read this before you commit to the loan.
When you might want to calculate the payment yourself
You do not need to do this math for a loan you have already taken out. But calculating payments is useful when you are comparing offers from different lenders or deciding whether to borrow more or less. If you are thinking about a $15,000 car loan instead of $12,000, or wondering whether a 4-year or 5-year term makes sense for your budget, doing the math helps you see the real cost before you decide.
Some people also calculate payments to understand how much extra they would save by paying off a loan early. If you pay $250 per month instead of the required $193.33, you will finish the loan in less than 5 years and pay less total interest.
Frequently Asked Questions
What is the difference between APR and interest rate?
The interest rate is the cost of borrowing the money itself. The APR (annual percentage rate) includes the interest rate plus other costs like origination fees or insurance. For calculating monthly payments, use the interest rate, not the APR. Your loan documents will show both.
Do variable-rate loans use the same formula?
No. Variable-rate loans have an interest rate that changes over time, so the formula only works for the current rate period. Your lender will tell you when the rate changes and what the new payment will be. For planning purposes, ask your lender what the highest possible rate could be.
Can I use this formula for credit cards?
The formula works if you have a fixed balance and a fixed interest rate, but credit cards are different — your balance changes as you spend and pay, and the interest compounds daily, not monthly. Use a credit card calculator instead, or ask your card issuer for a payoff estimate.
What if I want to pay off the loan early?
You can pay more than the required monthly payment at any time, and the extra goes toward the principal. This shortens the loan term and reduces the total interest you pay. Check your loan documents to make sure there is no penalty for early repayment.
Why does my actual payment differ slightly from what I calculated?
Rounding differences in the formula, fees added to the payment, or a slightly different interest calculation method can cause small differences. Your lender's official payment amount is what you owe — use that instead of your calculation.