Drive the calculators by URL — parameters & JSON API

Flagship parameters — Lease vs Buy Calculator

This table is generated from the same parameter codec the calculator itself uses, so it can never drift from the code. Values travel in human units: percent-like parameters are percent numbers (tax=7 means 7%), the money factor travels raw (mf=0.0025), and money and distance are plain numbers with no symbols or separators.

Parameters for the other calculators

The same conventions apply everywhere: percent-like parameters travel as percent numbers (apr=7 means 7%), flags are 1 or 0, omitted parameters use the documented defaults, and unknown parameters are ignored.

JSON API

Prefer numbers over HTML? The comparison engine is exposed directly at https://leasevsbuycar.com/api/calculate.

• GET — accepts exactly the flagship parameters documented above (including taxbasis), e.g. ?horizon=8&price=38000. Human units, same as the calculator URLs.
• POST — send a JSON body containing a partial ComparisonInputs object; it is merged over the documented defaults. Note the body uses engine units: rates are decimals ("salesTaxRate": 0.07 means 7%), unlike GET parameters.

Both methods respond with the full ComparisonResult: the lease quote and monthly payments, net costs and ending positions for each path, the break-even month, the verdict with its drivers, and the month-by-month series behind the chart.

Example request

curl "https://leasevsbuycar.com/api/calculate?horizon=6"

Example response (abbreviated, defaults with a 6-year horizon)

Comments and ellipses added for readability; the payment figures ($640.84 lease, $870.76 buy) match the worked example on the methodology page.

{
  "verdict": "buy",                  // "buy" | "lease" | "even"
  "savings": 7200,                   // net-cost gap at the horizon (rounded here)
  "breakEvenMonths": 43,             // null = leasing still ahead at the horizon
  "leaseQuote": {
    "monthlyPayment": 640.84,        // tax-inclusive monthly lease payment
    "dueAtSigning": 2640.84,
    "aprEquivalent": 0.06            // money factor × 24, as a decimal APR
    // …adjustedCapCost, residualValue, fee breakdown…
  },
  "buyMonthlyPayment": 870.76,
  "resaleValueAtHorizon": 15086,
  "lease": { … },                    // net cost / net position, lease path
  "buy": { … },                      // net cost / net position, buy path
  "series": [ … ]                    // one point per month, for charting
}

POST body example

POST https://leasevsbuycar.com/api/calculate
Content-Type: application/json

{
  "horizonYears": 8,
  "salesTaxRate": 0.07,
  "lease": { "moneyFactor": 0.0018 }
}

CORS is open — call the API from any origin, in the browser or server-side.

For AI agents

You don't need the API to read results. Every calculator page renders its outcome in a machine-readable container — fetch a deep link and read the data attributes:

<section id="calculator-result"
  data-verdict="buy"
  data-savings="7200"
  data-break-even-months="43">

• A site guide for language models lives at the site root: https://leasevsbuycar.com/llms.txt.
• Major AI crawlers are allowed in https://leasevsbuycar.com/robots.txt.
• Definitions for every term used in the parameter tables are on the glossary; the lease math itself is explained in how car lease math works.