Auto Loan Calculator accuracy and testing

Formula source

• This calculator determines the 'Net Loan Amount' first by taking the Vehicle Price, adding Sales Tax (often calculated on the price minus trade-in difference), subtracting the Down Payment and Trade-In, and then amortizing that final amount over the loan months.

Test cases

Known limitations

• This calculator provides an estimate. Actual dealership offers may vary based on credit score tiers, documentation fees ($85-$500 depending on state), and title/registration costs which are not included here.

Assumptions

• Sales tax is estimated from the vehicle price less trade-in value, which matches many but not all state rules.
• The loan uses fixed monthly payments and does not model dealer fees, registration, or optional add-ons.
• Trade-in and down payment are applied immediately to reduce the financed amount.

Unit handling

• Vehicle Price accepts values in $.
• Trade-in Value accepts values in $.
• Down Payment accepts values in $.
• Interest Rate accepts values in %.
• Loan Term accepts one of the configured options.
• Sales Tax accepts values in %.
• Estimated Monthly Payment is displayed as currency.
• Financed Amount is displayed as currency.

Rounding rules

• Number displays use up to 2 decimal places in the calculator UI.
• Currency outputs are displayed with standard US dollar formatting.
• Input precision follows configured step values where provided.

Version history

Change log