The Basic Formula for a Monthly Car Payment
To calculate your monthly car payment with interest, you need four numbers: the loan amount (principal), the annual interest rate, the loan term in months, and the monthly interest rate. The formula is:
Monthly Payment = P × [r(1 + r)^n] / [(1 + r)^n − 1]
In this formula, P is your principal (the amount you borrow), r is your monthly interest rate (annual rate divided by 12), and n is the total number of payments. This is called an amortizing loan calculation, and it's what every car loan uses.
The reason the formula looks complicated is that it accounts for how interest compounds over time. Early payments cover more interest than principal; later payments cover more principal than interest. The formula spreads the total interest across all your payments so each one is the same amount.
Key Takeaways
- Your monthly payment depends on three things: how much you borrow, your interest rate, and how many months you have to pay it back.
- To use the formula, convert your annual interest rate to a monthly rate by dividing by 12, then convert that to a decimal (5% becomes 0.05).
- A higher interest rate or shorter loan term raises your monthly payment; a lower rate or longer term lowers it.
- You can calculate this yourself with a spreadsheet, use an online calculator, or ask your lender for the exact payment before you sign.
Breaking Down Each Number You Need
The principal is the amount of money you actually borrow. If you buy a car for $25,000 and put down $5,000, your principal is $20,000. Trade-in value, rebates, and down payments all reduce the principal.
The annual interest rate is what your lender charges you to borrow the money, expressed as a percentage per year. A 6% annual rate means you pay 6% of the loan amount in interest over 12 months (though the actual interest you pay depends on how fast you pay down the principal). Your lender tells you this rate before you sign the loan agreement; it's called the APR or annual percentage rate.
The loan term is how many months you have to repay the loan. Common terms are 36, 48, 60, or 72 months. A longer term means a lower monthly payment but more total interest paid over the life of the loan.
The monthly interest rate is your annual rate divided by 12. If your annual rate is 6%, your monthly rate is 0.5% (6 ÷ 12 = 0.5). When you plug this into the formula, convert it to a decimal: 0.5% becomes 0.005.
Working Through a Real Example
Say you borrow $20,000 at 6% annual interest for 60 months. Here's how to calculate your payment:
First, convert the annual rate to a monthly decimal: 6% ÷ 12 = 0.5% = 0.005.
Then plug into the formula: P = $20,000, r = 0.005, n = 60.
Monthly Payment = $20,000 × [0.005(1.005)^60] / [(1.005)^60 − 1]
Working through the exponent: (1.005)^60 = 1.34885
Monthly Payment = $20,000 × [0.005 × 1.34885] / [1.34885 − 1]
Monthly Payment = $20,000 × [0.00674425] / [0.34885]
Monthly Payment = $20,000 × 0.019333 = $386.66
Over 60 months, you pay $386.66 each month. Your total payments add up to $23,199.60, which means you paid $3,199.60 in interest.
How Interest Rate Changes Affect Your Payment
A small change in interest rate creates a real difference in what you pay each month. Using the same $20,000 loan over 60 months:
| Annual Interest Rate | Monthly Payment | Total Interest Paid |
|---|---|---|
| 3% | $359.37 | $1,562.20 |
| 6% | $386.66 | $3,199.60 |
| 9% | $415.17 | $4,910.20 |
| 12% | $444.89 | $6,693.40 |
The difference between 3% and 12% is $85.52 per month—more than $5,000 over the life of the loan. This is why your credit score and down payment matter: they determine what interest rate you get offered.
Using a Spreadsheet or Calculator Instead
You don't have to do this math by hand. Most spreadsheet programs have a built-in function for this. In Excel or Google Sheets, use the PMT function:
=PMT(rate, nper, pv)
For the example above, you'd type: =PMT(0.005, 60, -20000). The rate is your monthly rate as a decimal, nper is the number of periods (months), and pv is the present value (the loan amount, entered as negative). The result is $386.66.
Online car payment calculators work the same way—you enter the loan amount, interest rate, and term, and the calculator does the math. These are faster and less error-prone than doing it by hand, and they're free to use.
What Happens When You Pay Extra or Pay Early
The formula above assumes you make the same payment every month for the full term. If you pay extra toward principal, you reduce the total interest you pay and shorten the loan.
For example, if you paid $450 instead of $386.66 on that $20,000 loan at 6%, you'd pay off the loan in about 45 months instead of 60, and you'd pay roughly $2,400 in interest instead of $3,200. The extra $63.34 per month saves you nearly $800 in interest.
Some lenders charge a prepayment penalty if you pay off the loan early, though this is less common with car loans than with mortgages. Check your loan agreement to see if there's a penalty before you decide to pay extra.
Why Your Actual Payment Might Differ From the Formula
The formula gives you the principal and interest portion of your payment. Your actual monthly payment may also include property tax, insurance, and registration fees, depending on how your lender structures the loan. Ask your lender for an amortization schedule—a month-by-month breakdown of how much of each payment goes to principal, interest, and other costs.
Some lenders also charge origination fees, documentation fees, or dealer fees that get rolled into the loan amount. These increase your principal and therefore your monthly payment. Make sure you know the total amount being financed, not just the car's price.
Frequently Asked Questions
What's the difference between APR and interest rate?
APR (annual percentage rate) includes the interest rate plus any fees the lender charges. For car loans, the APR and interest rate are often the same, but not always. Your lender must disclose the APR in writing before you sign. Use the APR in the formula, not a lower advertised rate.
Does a longer loan term always mean I pay more interest?
Yes. A 72-month loan at the same interest rate will cost you more in total interest than a 60-month loan, because you're borrowing the money for longer. However, your monthly payment will be lower. The trade-off is between affordability now and total cost over time.
Can I negotiate the interest rate my lender offers?
Yes. Your credit score, down payment, and the lender you choose all affect the rate you're offered. Getting pre-approved by a bank or credit union before you go to the dealership gives you a rate to compare against the dealer's offer. You can also shop rates from multiple lenders.
What if my interest rate changes during the loan?
Car loans almost always have a fixed interest rate, meaning it stays the same for the entire loan term. The payment you calculate at the start is the payment you make every month until the loan is paid off. If your lender offers a variable rate (rare for cars), the payment can change when the rate changes.
How do I know if my monthly payment is reasonable?
A common rule is that your total monthly car payment (including insurance and fuel) should not exceed 15% to 20% of your gross monthly income. Calculate what the payment will be before you commit to the loan, and make sure it fits your budget.