How the payoff date calculator works
The math under the hood
Every month, interest is added (balance × APR ÷ 12) and your payment is subtracted. Early on, most of a payment is interest; late in the loan, most is principal. Solving that recursion for "months until zero" gives the payoff date — the classic n = −ln(1 − rB/P) ÷ ln(1+r) where r is the monthly rate, B the balance, and P the payment.
Why the payment barely covering interest is the cliff
The dangerous zone is a payment within a few dollars of the monthly interest charge. There the balance barely moves, and one late fee or rate bump tips it into negative amortization — the balance grows while you pay. If the calculator refuses your payment, that is the cliff it is protecting you from.
Using the date as motivation
A specific month ("June 2028") is more motivating than a vague "someday." Mark it, then buy the date down: every extra $25–50 per month pulls it earlier, and the calculator's interest figure shows exactly what that acceleration is worth.