Auto Loan Calculator API

What comes back

amountFinanced (principal after down payment, rebate and trade equity, plus any financed fees), monthlyPayment, totalInterest, totalPaid, interestByYear (interest per 12-month block, summing to totalInterest, with a partial final year as a stub), tradeInEquity (value minus payoff, negative when underwater), an echo of apr and months, source "carscrape" and the standard meta envelope with coverageStatus "complete" — the one endpoint on the API that can honestly claim complete coverage, because it computed everything it returned rather than observing it.

curl -sS -G https://api.vehicles.dev/v1/vehicles/loan \
  -H "Authorization: Bearer $VEHICLES_API_KEY" \
  -d price=35000 -d down_payment=5000 -d apr=0.069 -d months=60

{
  "amountFinanced": 30000.0,
  "apr": 0.069,
  "interestByYear": [1907.47, 1536.82, 1139.77, 714.43, 258.81],
  "monthlyPayment": 592.62,
  "months": 60,
  "source": "carscrape",
  "totalInterest": 5557.29,
  "totalPaid": 35557.29,
  "tradeInEquity": 0.0
}

The complete parameter table, the response-field dictionary and the per-endpoint error codes live in the reference: full parameter reference for /v1/vehicles/loan.

What it is actually built on

The amount financed is the price, plus any fees you roll in, minus the down payment, minus the rebate, minus trade-in equity — where trade-in equity is the trade's value minus its payoff and is allowed to be negative. That principal is then amortized at the nominal monthly rate over the term using the standard level-payment formula, with a zero-rate loan handled as a straight-line split rather than a divide by zero. Interest is accumulated one period at a time rather than from a closed form, which is what lets the per-year split and the total reconcile exactly, and the final period absorbs accumulated rounding so the balance lands on zero instead of a stray cent. Nothing is cached because nothing is fetched; the whole call is arithmetic.

Data source

Nothing. This is deterministic arithmetic over the values you send: no vehicle lookup, no rate shopping, no upstream call.

Route budget

5 s

Billed per

calculation, successful responses only

MCP tool

get_loan_payments

Limits: what this endpoint will not do

There is no data here at all, and that is the point: this is deterministic arithmetic over the values you send. It does not look up the vehicle, it does not shop rates, and it will not tell you what APR a buyer qualifies for — you must supply the rate you were quoted. apr is the nominal annual rate compounded monthly, which is how US auto loans quote and amortize; it is not an APR in the Regulation Z sense that folds in finance charges, so a loan with financed fees will have a true disclosed APR above the rate you pass. The schedule assumes a level monthly payment, a fixed rate for the whole term, no prepayment and no balloon. Terms are capped at 120 months and prices at $10,000,000. Excluded by design: gap insurance, extended service contracts, lender-specific fees, and any tax or registration cost — pass those through fees if you want them financed, or use the tax endpoint to size them first. Because nothing is looked up, there is no not-found case and no upstream to fail. The endpoint is available now and successful responses use the published catalog rate. Standard plans include no SLA.

What teams build with it

Integrating it properly

The reference ships the bare curl. This is the shape worth deploying: a typed client that branches on the RFC 9457 code slug rather than the human-readable title, and retries only the codes this endpoint marks retryable.

const API = "https://api.vehicles.dev";

// Only these codes are marked retryable for this endpoint. Everything else is terminal.
const RETRYABLE = new Set([]);

export async function getLoanPayments(params: Record<string, string>, attempt = 0): Promise<GetLoanPaymentsResponse> {
  const response = await fetch(
    `${API}/v1/vehicles/loan?${new URLSearchParams(params).toString()}`,
    {
      headers: {
        Accept: "application/json",
        Authorization: `Bearer ${process.env.VEHICLES_API_KEY ?? ""}`
      },
      // The route budget is 5 s; allow it plus headroom.
      signal: AbortSignal.timeout(10_000)
    }
  );

  if (response.ok) return (await response.json()) as GetLoanPaymentsResponse;

  // Failures are application/problem+json. Branch on `code`, never on the human-readable title.
  const problem = (await response.json()) as { code: string; detail?: string };
  if (RETRYABLE.has(problem.code) && attempt < 3) {
    await new Promise((resolve) => setTimeout(resolve, 250 * 2 ** attempt));
    return getLoanPayments(params, attempt + 1);
  }
  throw new Error(`${response.status} ${problem.code}: ${problem.detail ?? ""}`);
}

How to check it is behaving

The route enforces a 5-second upstream budget, so a slow dependency returns a problem document instead of holding your request open. Billing is success-only: failed calls release their reservation, so you can measure error rates and hold-out accuracy against your own data without paying for the failures. Usage is visible per endpoint in the dashboard.

What this one endpoint costs

Priced per calculation. The platform fee buys throughput and endpoint access; the unit price covers the data work. Worked at 250,000 successful calls a month so you can see where the plans cross over — the full price matrix is in the reference.

Eligible for the Starter plan's included monthly calls; past those it draws on your credit balance. Data fees are prepaid from credits on every plan: when included calls and credits cannot cover a call you get 402 insufficient_credits rather than an overage bill. Published rates are active and apply only to successful responses. Standard plans include no SLA.

Questions engineers actually ask

Where to go next

Most integrations chain two or three of these. The ones that pair with auto loan calculator api most often:

• Car Sales Tax APIPurchase sales or use tax by state, including the per-state trade-in credit rule, plus title, registration and documentation fees.
• Total Cost of Ownership APIFive-year cost of ownership across depreciation, fuel, upkeep, tax and financing — with every component labelled by how much of it we actually measured.
• Vehicle Valuation APIAn ML-predicted current asking price in whole USD for a year/make/model, with the model's 3.7% median holdout error reported on every response.

The reference entry for this endpoint — Loan & Payments in the API documentation — carries the parameter constraints, defaults, response fields and the full error table with retryable flags. Agents can reach the same endpoint as the get_loan_payments tool on the vehicles-dev-mcp MCP server, and the machine-readable contract is published at https://api.vehicles.dev/openapi.json.

Browse the rest of the catalog on the Vehicle data APIs hub.