How the tools work

What they do and don’t do

Each tool adds up the amounts you enter, applies the tax rate you enter, and shows every step. None of them looks up a tax rate, knows a state’s fee schedule, values a car, or contacts a dealer or lender. State-specific rules are not built in yet. When one is added, it will name its official source and the date it was last checked, and the result will say when it was used.

Money and rounding

• Amounts are kept as whole cents, so totals never drift by fractions of a cent.
• Rates are kept to four decimal places of a percent (8.875% is exact).
• Tax is rounded once, to the nearest cent, with half a cent rounded up. Your state or dealer may round differently.

“Not sure” answers

Whether a trade-in, rebate or fee lowers or raises the taxed amount depends on the state and sometimes on the paperwork. When you answer “Not sure”, the estimate uses the answer that costs more and lists how much less tax you’d pay if the cheaper answer is right. A guess never quietly makes the total look smaller.

What the results say about themselves

• An estimate was calculated from everything the tool needs, all of it entered by you. It appears on a green sign.
• Incomplete means something needed for a full total was left blank. The total leaves it out and names it; a blank is never treated as zero.
• Not covered means the situation is outside what the tool handles. It says so and points to an official source instead of guessing.
• Anything to check, such as a warning or a quote that doesn’t add up, is black on yellow.

Loans

Monthly payments assume a simple fixed-rate loan with equal payments: payment = principal × r ÷ (1 − (1 + r)⁻ⁿ), where r is the APR divided by 12 and n the number of months; at 0% it is principal ÷ n. Lenders’ rounding, first-payment dates and anything added to the loan change the real figure.

Worked examples

The examples on each page use fictional rates and fees, and are calculated by the same code as the tools when the site is built, so the example always matches the calculator.

Testing

Every formula has automated tests: worked fixtures, edge cases such as trade-ins worth more than the car, negative equity and expired quotes, and checks that the tools agree with each other (the trade-in tool’s tax saving equals the difference between two out-the-door calculations, for example).

Mistakes

If something looks wrong, please say so through the note form on keepreading.org. Corrections are made in the code and its tests, and the review date changes only when the content is actually reviewed. Last reviewed October 6, 2026.