ClutchCalcs

Math

Car Payment Calculator

Sitting at the dealer with a quote in front of you trying to figure out if the monthly payment number is real or padded? This calculator runs the real auto-loan math: vehicle price + sales tax (minus trade-in credit, since most states tax only the difference) minus down payment = amount financed. Then term + APR = monthly payment + total interest paid. Plug in your numbers before you sign anything. Most dealers have already done this math and shaved 3-5% to make the monthly look smaller — know what the actual payment should be so you can spot the games.

The math the dealer ran (but didn't show you)

  1. Taxable amount = vehicle price – trade-in (in most states; check your state's rules).
  2. Total cost with tax = taxable amount × (1 + tax rate).
  3. Amount financed = total cost with tax – down payment.
  4. Monthly payment = amortizing loan formula: M = P×i×(1+i)^n / ((1+i)^n – 1), where P = financed amount, i = APR/12, n = months.

Worked example: $35,000 vehicle, 7% sales tax, $5,000 down, no trade-in, 7.5% APR, 60 months. Taxable: $35,000. With tax: $37,450. Financed: $32,450. Monthly: $650. Total interest paid: $6,548 over 5 years.

Term tradeoffs

  • 36 months: highest payment, lowest total interest, fastest equity build. Best for borrowers with strong cash flow who don't want to be underwater.
  • 48 months: standard for many people. Manageable payment, reasonable interest.
  • 60 months: most common new-car term. Acceptable balance of payment and interest. Stay within this if possible.
  • 72 months: lower payment but you'll be underwater for the first 3-4 years — the car depreciates faster than you pay off. Risky if you might need to sell early.
  • 84 months: long enough that you'll be replacing the car before you finish paying for it. Avoid. The monthly looks tempting; the math is brutal.

Industry rule: never finance longer than the car's useful life. If you plan to keep a car 10+ years, even an 84-month loan technically works. If you upgrade every 5-7 years, 60 months max.

How to use this calculator

  1. Vehicle price: out-the-door before tax.
  2. Sales tax %: your state's combined rate (verify with the dealer's documentation — they apply state + sometimes local).
  3. Down payment: cash you're putting down.
  4. Trade-in value: what the dealer credited you for trade-in.
  5. APR: the actual quoted rate, not just the dealer's pitched rate.
  6. Term: 36-84 months.
  7. Output: monthly payment, amount financed, total interest.
  8. If the dealer's number differs by $20+/month, ask them to break down the math — they may have added GAP insurance, an extended warranty, or other line items.

Common scenarios

$30K Honda Civic, 7% tax, $3K down, 7% APR, 60 months. Taxable $30K, with tax $32,100, financed $29,100. Monthly: $577. Total interest: $5,481. Reasonable for an entry-level car.

$50K Ford F-150, 7% tax, $8K down, $10K trade-in, 6.5% APR, 72 months. Taxable $40K, with tax $42,800, financed $24,800. Monthly: $416. Total interest: $5,196. Long term + big trade-in keeps the payment palatable.

$45K Tesla Model 3, 7% tax, $0 down, 6.99% APR, 84 months. Taxable $45K, with tax $48,150, financed $48,150. Monthly: $727. Total interest: $12,940. The 84-month loan adds ~$5,000 in interest vs 60 months — stretch the payment and pay way more.

FAQ

84-month loans — yes or no? +
Avoid. You'll owe more than the car is worth for the first 3-4 years (negative equity / "upside-down"). If you need to sell or wreck the car mid-loan, the gap between payoff and vehicle value comes out of your pocket. Stick to 60 months max for new cars; 48 max for used.
Does the dealer tax the trade-in credit? +
In most states, sales tax is on (price – trade-in). A $5K trade-in on a $30K car saves ~$350 in sales tax at 7% rate, plus the $5K direct equity. Some states (CA, MI, OK, others) tax full price without trade-in deduction. Confirm with your state's tax rules.
What about GAP insurance? +
GAP (Guaranteed Asset Protection) covers the difference between what you owe and what the car's worth if it's totaled. Useful if you're underwater (long term, low down payment). Skip if you put 20%+ down on a 48-month or shorter loan. Dealers mark up GAP 2-3x; buy from your auto insurance carrier for less.
Should I take dealer financing or my credit union? +
Get pre-approved at a credit union or your bank BEFORE going to the dealer. Then use the dealer's offer as a check — sometimes they beat your bank's rate (manufacturer subsidized loans). If the dealer rate is higher, finance through your bank. Always know your alternative.
How much should I put down? +
20% is the historical recommendation; 10-15% is more common today. Putting down less than 10% leaves you immediately underwater on most new cars. Putting down 20%+ keeps you in positive equity throughout the loan — you can sell anytime without paying out of pocket to escape.
Should I pay cash if I can? +
If your alternative is investing the money: depends on rates. Auto loan at 6%: definitely pay cash (you can't reliably earn 6% after-tax in safe investments). Auto loan at 1.9% (manufacturer subsidized): probably keep the cash and invest it. Run the math vs your investing return.
What's the rule of 1/10 / 20/4/10? +
1/10: monthly car payment shouldn't exceed 10% of gross income. 20/4/10: 20% down, 4-year max term, total transportation costs (payment + insurance + gas + maintenance) under 10% of gross income. Useful sanity check on whether you're buying "too much car."
Should I refinance my auto loan? +
If rates have dropped 1.5%+ since your original loan, refinancing can save $50-200/month with a modest closing fee ($0-200). Most worthwhile in the first 1-3 years; later in the loan you're mostly paying down principal anyway. Credit unions are typically the best refinance source.