The Car Buying Calculator Pack — Seven linked calculators, every formula checked by recalculation

Seven calculators for the numbers a car purchase actually turns on. Start with sheet 1 for the payment and total interest, then sheet 7 for what the car costs a month once insurance, fuel and upkeep are in -- on the worked example the payment is only 61% of it. Sheet 3 handles extra payments exactly, down to the smaller final payment. Every formula was recalculated and checked against an independent implementation before this shipped; see the Verification sheet.

What is in it

7 calculators, one per sheet. Overwrite the blue cells with your own numbers and everything else follows — nothing is locked, there are no macros, and it opens in Excel, Numbers, LibreOffice and Google Sheets.

• Loan payment — What is the monthly payment, and what does the loan cost?
• Interest by term length — What does stretching the loan from 36 to 72 months cost?
• Extra payment payoff — What does an extra amount each month save in months and interest?
• Down payment effect — What does putting more down save each month and in total?
• Lease versus buy — Over the same period, which costs less: the lease or the loan?
• Negative equity rollover — What does rolling an upside-down loan into the next one cost?
• Total cost per month — What does the car really cost a month, not just the payment?

Every formula was checked

This pack contains 53 formulas. Before it was published, every one of them was recalculated by a spreadsheet engine and compared against an independent implementation of the same arithmetic written in Python. All 53 agreed to within a cent, and the workbook carries a Verification sheet saying so.

That is an unusual thing to be able to claim about a spreadsheet, and it is the main reason to buy this one rather than download a free template. A formula that looks right and is wrong is worse than no formula at all, because you will act on it.

What each calculator assumes

There is a sheet listing this, because a calculator that hides its assumptions is just a number generator:

• A fixed rate, payments on schedule, no fees beyond what is included in the price.
• Each term's rate is what you enter; the example uses one rate for all four so the term effect is shown on its own.
• The extra goes to principal every month. Check with your lender that it is applied that way and that there is no prepayment penalty.
• The same rate and term either way. Some lenders change the rate with the down payment; if yours does, run each case on sheet 1.
• No credit for interest the down payment could have earned, no lease mileage or wear charges, and the bought car sold at the resale figure entered.
• The same rate applies with or without the rollover. Some lenders charge more on a loan above the car's value.
• Costs spread evenly by month. Depreciation is not a monthly cash cost and is left out here; sheet 5 counts it through the resale value.

What this is not

Not financial, tax or legal advice, and not a recommendation to buy, sell, borrow or sign anything. These are calculators: they show what a choice costs, not which choice is right for you. Where a published rate or threshold is used it is the one current when the pack was built — those change, the arithmetic does not.

Honest about the format

One .xlsx workbook, 7 calculator sheets plus Start Here, Assumptions and Verification. Not a PDF, not a physical product, not a Notion template. Available immediately after purchase; nothing ships.

Refunds

If this isn't what you expected, email me within 30 days and I'll refund you in full — no need to explain. Because it's an instant download I can't take the file back, so I'd rather you just had your money back.