How auto loan interest works

What an auto loan estimate is modeling

An educational auto loan calculator estimates how much is financed, what a fixed-rate monthly payment might be, and how interest accumulates over the term — based on price, down payment, trade-in, tax assumption, fees, APR, and term you enter.

It is not a dealer offer or lender quote.

Explore scenarios in the auto loan calculator. If you are comparing payoff order across several debts, use the debt payoff calculator.

Building the amount financed

In the documented MVP:

financedAmount =
  vehiclePrice
  − downPayment
  − tradeInValue
  + tradeInLoanBalance
  + salesTax
  + fees

Sales tax (MVP assumption):

taxableAmount = vehiclePrice − downPayment − tradeInValue
salesTax = taxableAmount × (salesTaxRate ÷ 100)

If that taxable amount would be negative under this assumption, the tool stops rather than inventing an alternate jurisdiction rule. Trade-in loan payoff increases the amount financed when entered.

How interest appears in the payment

Monthly payment and the schedule use fixed-rate amortization. With monthly rate r (APR ÷ 100 ÷ 12) and n months:

M = P × r × (1 + r)^n / ((1 + r)^n − 1)

If APR is 0%, payment is principal ÷ months. Each schedule row splits a payment into interest and principal; total interest is the sum of interest portions over the loan.

Assumptions

• Fixed-rate, fully amortizing loan with monthly payments
• Fees and estimated sales tax are included in the amount financed
• Taxes are not assumed to already be inside vehicle price
• Tax treatment is simplified; jurisdictions differ

Limitations

• Does not reproduce every state or local vehicle tax rule
• Does not invent lender rates, rebates, or dealer financing offers
• Total vehicle cost shown is an estimate from your inputs and the MVP tax assumption

Takeaways

Auto loan “interest” in this model is the cost of amortizing the constructed principal at the APR and term you entered. Change financed amount, rate, or term and the interest path changes — without the tool claiming those inputs match a real offer.