Last reviewed: September 14, 2026
The calculator uses the standard fixed-rate amortization formula for a level monthly payment M on principal P, at monthly interest rate r, over n months:
M = P × r(1+r)ⁿ / ((1+r)ⁿ − 1)
r is your annual percentage rate (APR) divided by 12. If APR is 0% (a promotional/interest-free loan), the payment is simply principal divided by term length.
Amount financed = vehicle price − trade-in value − down payment, plus sales tax and fees if you choose to roll them into the loan (the default). If you uncheck "roll tax & fees into the loan," they're added to your cash due at signing instead and excluded from the financed principal -- meaning you don't pay interest on them.
The calculator assumes the common case: sales tax is charged on the vehicle price net of trade-in value (a "trade-in tax credit"), using the combined state + local rate you enter. A handful of states -- notably California -- tax the full vehicle price regardless of trade-in, so the estimate may overstate your true tax there. Always confirm the rule for your state before relying on the number.
The insurance field is opt-in only -- the calculator never generates or assumes an insurance figure on your behalf. Whatever you enter is simply multiplied by your loan term and added to "true total cost." Leave it blank if you don't have a quote yet; the rest of the numbers will still be accurate.
Negative equity carried over from a trade-in loan (the "still owed on trade-in" field) IS modeled -- it's rolled into the amount financed per the trade-in handling above.
Email royalplanet2009@gmail.com or see the disclaimer for the calculator's limits.