Auto Loan Calculator

This tool calculates auto loan costs by applying the standard fixed-rate amortization model to user-supplied terms. It targets borrowers planning finance through banks, credit unions, or dealership desks, as well as analysts benchmarking offers. Given loan_amount, annual_interest_rate, term_years, and optional down_payment, the calculator derives the monthly payment using the formula P = L * r / (1 - (1 + r)^-n), where r is monthly_rate and n is total payments. Values are presented with two-decimal currency precision for clear budgeting.

Inputs support both essential and advanced scenarios. Required: loan_amount (USD), annual_interest_rate (percent), term_years (years). Optional: down_payment, taxes_and_fees_monthly. Advanced optional inputs include trade_in_value and extra_monthly_payment to reduce principal or accelerate payoff. Outputs include monthly_payment, total_interest, total_cost, and payoff_date. The tool can also indicate whether an amortization schedule is available for export. It handles zero down and zero-interest edge cases by adjusting calculations without failure and reports zero payments when principal is zero.

Error handling ensures invalid or missing inputs produce clear results: negative values or non-numeric entries trigger validation and prevention of calculation until corrected. If inputs imply negative principal after down_payment and trade-in, the system clamps results to zero and returns zero monthly payments. Localization uses USD currency formatting and YYYY-MM dates. Assumptions include fixed rate for the loan term and no additional unmodeled costs beyond taxes/fees input. Exclusions include variable-rate loans, escrowed costs not provided, or promotional offers. This tool emphasizes transparency, scenario comparison, and budgeting support for planning and negotiation.