How Your Car Payment Is Calculated
Your loan amount is the price, minus your down payment and trade-in, plus sales tax and any fees you roll in. If you still owe money on your trade-in, that balance is added too. The payment then follows the standard loan formula: the same amount every month, with more going to interest early on and more to principal at the end.
Most states tax the price after your trade-in is subtracted, which is a real savings on a trade. A few states tax the full price. Check the box in step 3 to match your state.
The 20/4/10 Rule
A simple way to keep a car from straining your budget:
- 20% down, so you don't start the loan owing more than the car is worth.
- 4 years (48 months) or less, so you aren't paying interest for years on a car that keeps losing value.
- 10% of gross monthly income or less for the car payment. Some versions include insurance and gas in the 10%, which is stricter.
Not every purchase will hit all three. Treat any miss as a warning sign worth a second look, not an automatic no.
Getting the Most for Your Trade-In
Dealers often make up for a good price on the new car by offering less for your trade, or the reverse. Treat them as two separate deals. Look up your car's trade-in value on Kelley Blue Book or Edmunds, then get one or two free instant offers (CarMax and Carvana make offers online). Walk in with a real number, and if the dealer offers less, you can sell to the higher bidder instead.
If you still owe money on your car, call your lender for the payoff amount. That's often slightly higher than the balance on your statement. If you owe more than the car is worth, that "negative equity" gets rolled into your new loan, and the calculator shows how much it adds.
When Refinancing a Car Loan Makes Sense
Refinancing pays off when your new rate is meaningfully lower: your credit score improved since you bought, rates dropped, or you financed at the dealer without shopping around. Watch the term. Stretching 36 remaining months into a new 60-month loan can lower the payment but raise the total you pay. The calculator shows both numbers so you can see the trade-off.
0% APR vs. Cash Rebate
Automakers often let you pick either special financing or a cash rebate, not both. 0% looks unbeatable, but a large rebate plus a decent rate from your bank or credit union can cost less overall. The only way to know is to compare the total paid both ways, which is what the third tab does. Get a pre-approval before you go to the dealer so you have a real outside rate to compare. Read the full guide with worked examples โ