To calculate a monthly loan payment, you need three numbers: the loan amount, the interest rate, and the loan term. With those inputs and a standard formula, you can figure out exactly what you’ll owe each month on a car loan, mortgage, personal loan, or any other installment debt. Here’s how the math works and how to apply it.
The Standard Loan Payment Formula
Most consumer loans use what’s called amortization, where you make equal monthly payments that gradually pay down both the interest and the principal (the original amount you borrowed). The formula for calculating that fixed monthly payment is:
Monthly Payment = Loan Amount × [ i × (1 + i)^n ] / [ (1 + i)^n − 1 ]
There are two variables to plug in:
- i = your monthly interest rate (annual rate divided by 12)
- n = the total number of monthly payments
To get your monthly interest rate, take your annual rate and divide by 12. If your annual rate is 6%, your monthly rate is 0.5%, or 0.005 as a decimal. For the total number of payments, multiply the loan term in years by 12. A 5-year loan has 60 payments; a 30-year mortgage has 360.
A Step-by-Step Example
Say you’re borrowing $25,000 for a car at 6% annual interest over 5 years. Here’s how to work through the formula:
First, convert the interest rate: 6% ÷ 12 = 0.005 per month. Next, count the payments: 5 years × 12 = 60 payments. Now plug those into the formula:
Monthly Payment = $25,000 × [ 0.005 × (1.005)^60 ] / [ (1.005)^60 − 1 ]
Start by calculating (1.005)^60. That equals roughly 1.3489. Then the numerator inside the brackets becomes 0.005 × 1.3489 = 0.006745. The denominator becomes 1.3489 − 1 = 0.3489. Divide: 0.006745 / 0.3489 = 0.01933. Multiply by the loan amount: $25,000 × 0.01933 = $483.32 per month.
Over the full 60 months, you’d pay $483.32 × 60 = $28,999, meaning roughly $3,999 goes toward interest and $25,000 repays the principal.
How Each Payment Gets Split
Even though your monthly payment stays the same, the portion going to interest versus principal shifts over time. Early in the loan, most of each payment covers interest because the outstanding balance is still large. As you pay down the balance, less interest accrues each month, so more of each payment chips away at principal.
Using the example above, your first payment of $483.32 would include $125 in interest (0.005 × $25,000) and $358.32 toward principal. By payment 50, the remaining balance is much smaller, so nearly all of the $483.32 goes to principal. This schedule of shifting allocations is called an amortization schedule, and most lenders will provide one when you close on a loan. You can also generate one with a spreadsheet by recalculating the interest on the remaining balance each month.
Simple Interest vs. Compound Interest
The formula above assumes the standard method used for most car loans, mortgages, and personal loans: interest is calculated on the remaining principal balance each period. This is sometimes called simple interest in the context of installment loans because you’re not charged interest on accumulated interest.
Compound interest works differently. It charges interest on both the principal and any previously accrued interest that hasn’t been paid. Credit cards are the most common example. If you carry a $1,000 balance at 20% APR and make no payments, after one month you owe about $16.67 in interest. The next month, interest is calculated on $1,016.67, not the original $1,000. Over time, compounding makes debt grow faster than simple interest would.
For a standard installment loan where you make regular monthly payments, the amortization formula above is what you need. The compounding effect matters more for revolving debt like credit cards or for savings accounts where interest accumulates.
What Changes the Payment Amount
Three levers control your monthly payment, and understanding them helps you shop smarter:
- Loan amount: Borrowing more means a higher payment. Putting more money down on a car or home directly reduces the financed amount.
- Interest rate: Even small rate differences add up. On a $250,000 mortgage over 30 years, the difference between 6% and 7% is roughly $165 per month, or nearly $60,000 in total interest over the life of the loan.
- Loan term: Stretching the term lowers your monthly payment but increases the total interest you pay. A $25,000 car loan at 6% costs $483 per month over 5 years but only $402 over 7 years. The catch is you’d pay about $8,600 in total interest over 7 years instead of $4,000 over 5.
Extra Costs Beyond Principal and Interest
The formula gives you the principal-and-interest portion of your payment, but your actual monthly obligation may be higher depending on the loan type. Mortgages are the biggest example. Lenders typically collect four components in a single monthly payment, often abbreviated as PITI: principal, interest, taxes, and insurance.
Property taxes get divided into monthly installments and deposited into an escrow account your lender manages, then the lender pays the tax bill on your behalf. Homeowners insurance works the same way. These two items alone can add several hundred dollars a month. If your down payment is less than 20% on a conventional mortgage, you’ll also pay private mortgage insurance, which protects the lender if you default.
Your PITI payment can change over time even on a fixed-rate mortgage. Property tax assessments can rise if your home’s value increases, and insurance premiums can go up as well. The principal-and-interest portion stays locked, but the escrow portion adjusts.
For car loans and personal loans, the formula amount is usually the full payment. Some car loans require separate insurance coverage, but that’s paid to an insurer, not rolled into the loan payment itself.
Faster Ways to Calculate
You don’t have to do the math by hand. Spreadsheet programs have a built-in function called PMT that does this calculation instantly. In Excel or Google Sheets, type: =PMT(0.005, 60, -25000). The first argument is the monthly rate, the second is the number of payments, and the third is the loan amount (entered as a negative number so the result comes back positive). The cell will display $483.32.
Online loan calculators work the same way. Plug in the loan amount, rate, and term, and you get the monthly payment along with a full amortization schedule. These tools are useful for comparing scenarios quickly, like seeing how a half-point rate reduction or a shorter term would change your payment before you commit.