Car calculators that show their work

Every calculator here shows the formula it uses and a worked example with real numbers, so you can check the math yourself — or hand it to a dealer. No account needed. Assumptions are yours to edit, not ours to hide.

Buy

• Car Loan Calculator: monthly payment, total interest, and payoff from the real amount financed, tax and fees included.
• Out-the-Door Price Calculator: the full drive-away price with tax, doc fee, add-ons and government fees itemized line by line.
• Car Sales Tax by State: the vehicle tax rate, trade-in rules and caps for your state, sourced from official state revenue and DMV pages.
• Car Loan Payoff Calculator: what extra monthly payments or a lump sum do to your payoff date and total interest.
• Car Refinance Calculator: the month a refinance breaks even, and whether a lower payment quietly costs more interest.
• How Much Car Can I Afford: coming
• Pay Cash vs Finance Calculator: coming

Lease

• Money Factor Calculator: convert a lease money factor to its APR equivalent (approx.) and back, or work the money factor out of a quoted payment when the dealer will not state it.
• Adjusted Cap Cost Calculator: the worksheet that builds the number a lease is priced on, line by line.
• Car Lease Calculator: the monthly payment split into depreciation and finance charge, with tax.
• Lease Mileage Calculator: mileage overage charges vs buying the miles upfront.
• Lease vs Buy Calculator: coming
• Lease Buyout Calculator: coming

In the finance office

Everything pitched after the price is agreed, in the finance office (the box). Each of these is planned as a tool where a tool works, and straight information where it does not.

• Dealer Add-On Cost Calculator: what any add-on really costs once it is financed, line by line (coming)
• Dealer Add-On Price Check: look up the name on your deal sheet (ResistAll, Simoniz, etch, nitrogen) and see what the product actually is and what it costs in the open market (coming)
• Service Contract Refund Calculator: what you get back when you cancel an extended warranty, worked out by the contract's own pro-rata formula (coming)
• Warranty Overlap Checker: how many years of a service contract duplicate the factory warranty the car already has (coming)
• Service Contract Markup Check: what dealers pay for extended warranties vs what they charge, from documented cost data (coming)
• GAP Insurance Calculator: the dealer's GAP price vs the same coverage from an insurer or a credit union (coming)
• Tire and Wheel Break-Even Calculator: the plan price vs the likely cost of the repairs it covers (coming)
• Prepaid Maintenance Calculator: the plan price vs paying for the same services as you go (coming)
• Payment Packing and Rate Markup: how to spot a payment padded with products, or a rate marked up above the approval, on a deal sheet (coming)

Decide

• Repair or Replace Calculator: coming