# 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.

- Calculator id: `car-loan` · Category: Finance (`finance`) · Tool name: `calculate_car_loan`
- Canonical page: https://tttkmbb.com/finance/car-loan · This document: https://tttkmbb.com/finance/car-loan.md · JSON definition: https://tttkmbb.com/finance/car-loan.json

## Purpose

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.

**Use when:** 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).

## Input

| Parameter | Type | Unit | Required | Description |
| --- | --- | --- | --- | --- |
| `vehicle_price` | number |  | required | Negotiated purchase price before tax and fees. (> 0, max 100000000) |
| `down_payment` | number |  | optional, default 0 | Cash paid at signing. (min 0, max 100000000) |
| `trade_in_value` | number |  | optional, default 0 | Credit for the vehicle traded in (net of any loan still owed on it). (min 0, max 100000000) |
| `sales_tax_percent` | number | % | optional, default 0 | Sales tax rate applied to the taxable price. (min 0, max 30) |
| `annual_rate_percent` | number | % | required | Loan APR in percent; the monthly rate is this / 12. (min 0, max 100) |
| `term_months` | integer | months | optional, default 60 | Loan term in months (36, 48, 60, 72 and 84 are common). (min 1, max 120) |
| `fees` | number |  | optional, default 0 | Documentation, title and registration fees rolled into the loan. (min 0, max 1000000) |
| `trade_in_reduces_tax` | boolean |  | optional, 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). |

## Output

| Field | Type | Unit | 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). |

## 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`

## Data Sources

- CFPB – Auto loans: shopping for a loan and comparing costs — https://www.consumerfinance.gov/consumer-tools/auto-loans/ (government, retrieved 2026-09-24)
- FTC – Financing or Leasing a Car — https://consumer.ftc.gov/articles/financing-or-leasing-car (government, retrieved 2026-09-24)
- Wikipedia – Amortization calculator (annuity payment formula) — https://en.wikipedia.org/wiki/Amortization_calculator (reference, retrieved 2026-09-24)

Data freshness: `static`. Deterministic formula with fixed constants; results never go stale. Inputs supplied by the caller determine the output.

## API

- `GET https://tttkmbb.com/api/v1/calculate/car-loan?vehicle_price=…&annual_rate_percent=…`
- `POST https://tttkmbb.com/api/v1/calculate/car-loan` with JSON body `{"inputs": {…}}`
- Response: unified envelope (`success`, `request`, `result.values`, `result.units`, `sources`, `freshness`, `timestamp`, `next_actions`, `links`); see https://tttkmbb.com/docs/response-format.md
- Schema: https://tttkmbb.com/api/v1/calculators/car-loan · OpenAPI operationId `calculate_car_loan` in https://tttkmbb.com/openapi.json
- Authentication: none. Rate limit: fair use, see https://tttkmbb.com/docs/rate-limits.md.

## MCP

- Server: `https://tttkmbb.com/mcp` (Streamable HTTP, JSON-RPC 2.0, no auth)
- Tool:  `run_calculator` with `{"calculator_id": "car-loan", "inputs": {…}}`

## Example

- 30,000 car, 5,000 down, 6 % tax, 7 % APR, 60 months: inputs `{"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: inputs `{"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
```

## Limitations

You are leasing rather than buying (use car-lease), or you only have a loan amount and rate (use loan-payment). All values are computed from the formula above; no measurement or live data is involved.

## 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

- [Loan Payment Calculator](https://tttkmbb.com/finance/loan-payment.md) — Plain amortization for any loan amount.
- [Car Lease Calculator](https://tttkmbb.com/finance/car-lease.md) — Compare with leasing the same vehicle.
- [Sales Tax Calculator](https://tttkmbb.com/finance/sales-tax.md) — Sales tax on its own.