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Car Loan Calculator

Computes the amount financed for a vehicle purchase from price, sales tax, fees, down payment and trade-in, then the level monthly payment, total interest and total cost of the car over the loan.

When to use

You are buying a car with an installment loan and want the monthly payment or the full cost including sales tax, dealer fees and a trade-in credit.

Do not use when: You are leasing rather than buying (use car-lease), or you only have a loan amount and rate (use loan-payment).

Formula

taxable = vehicle_price − trade_in_value (or vehicle_price if trade_in_reduces_tax = false); sales_tax = taxable × sales_tax_percent/100; amount_financed = vehicle_price + sales_tax + fees − down_payment − trade_in_value; i = annual_rate_percent/1200; monthly_payment = amount_financed × i / (1 − (1 + i)^−term_months); total_cost = vehicle_price + sales_tax + fees + total_interest

Inputs

Parameter	Type	Unit	Required	Description
`vehicle_price`	number		yes	Negotiated purchase price before tax and fees. Range: > 0, ≤ 100000000
`down_payment`	number		default 0	Cash paid at signing. Range: ≥ 0, ≤ 100000000
`trade_in_value`	number		default 0	Credit for the vehicle traded in (net of any loan still owed on it). Range: ≥ 0, ≤ 100000000
`sales_tax_percent`	number	%	default 0	Sales tax rate applied to the taxable price. Range: ≥ 0, ≤ 30
`annual_rate_percent`	number	%	yes	Loan APR in percent; the monthly rate is this / 12. Range: ≥ 0, ≤ 100
`term_months`	integer	months	default 60	Loan term in months (36, 48, 60, 72 and 84 are common). Range: ≥ 1, ≤ 120
`fees`	number		default 0	Documentation, title and registration fees rolled into the loan. Range: ≥ 0, ≤ 1000000
`trade_in_reduces_tax`	boolean		default true	Most US states tax vehicle_price − trade_in_value; set false where tax is charged on the full price (e.g. California, Virginia).

Outputs

Output	Type	Description
`sales_tax`	number	Taxable price × sales_tax_percent / 100.
`amount_financed`	number	vehicle_price + sales_tax + fees − down_payment − trade_in_value.
`monthly_payment`	number	Level payment from the amortization formula.
`total_interest`	number	monthly_payment × term_months − amount_financed.
`total_of_payments`	number	monthly_payment × term_months.
`total_cost`	number	vehicle_price + sales_tax + fees + total_interest (down payment and trade-in are part of this, not additional).

Example

30,000 car, 5,000 down, 6 % tax, 7 % APR, 60 months: {"vehicle_price":30000,"down_payment":5000,"sales_tax_percent":6,"annual_rate_percent":7,"term_months":60} → {"sales_tax":1800,"amount_financed":26800,"monthly_payment":530.67,"total_interest":5040.33,"total_of_payments":31840.33,"total_cost":36840.33}

45,000 car, 10,000 trade-in, 3,000 down, 8.25 % tax, 500 fees, 4.9 % for 72 months: {"vehicle_price":45000,"down_payment":3000,"trade_in_value":10000,"sales_tax_percent":8.25,"annual_rate_percent":4.9,"term_months":72,"fees":500} → {"sales_tax":2887.5,"amount_financed":35387.5,"monthly_payment":568.27,"total_interest":5528.18,"total_cost":53915.68}

GET https://tttkmbb.com/api/v1/calculate/car-loan?vehicle_price=30000&down_payment=5000&sales_tax_percent=6&annual_rate_percent=7&term_months=60

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/car-loan (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/car-loan · Markdown: https://tttkmbb.com/finance/car-loan.md · JSON definition: https://tttkmbb.com/finance/car-loan.json
MCP: server https://tttkmbb.com/mcp, tool run_calculator with calculator_id="car-loan"
OpenAPI operationId: calculate_car_loan
Freshness: static. Authentication: none. Rate limit: fair use (see rate limits).

Sources

CFPB – Auto loans: shopping for a loan and comparing costs (government)
FTC – Financing or Leasing a Car (government)
Wikipedia – Amortization calculator (annuity payment formula) (reference)

FAQ

Is sales tax charged before or after the trade-in?

In most US states the trade-in value is deducted before tax is calculated; a few states (California, Virginia, Michigan above a cap, and others) tax the full price. Set trade_in_reduces_tax accordingly.

Does a longer term save money?

It lowers the monthly payment but raises total interest: 26,800 at 7 % costs 5,040 interest over 60 months but about 6,100 over 72 months.

Related calculators

Loan Payment Calculator — Plain amortization for any loan amount.
Car Lease Calculator — Compare with leasing the same vehicle.
Sales Tax Calculator — Sales tax on its own.