"How long until this card is paid off?" has a one-formula answer, and "how much interest will I pay?" has a two-formula answer, as long as you handle the last payment properly. Most quick calculators multiply months × payment and overstate the interest, because the final payment is almost always smaller than the rest. This tutorial builds a two-tab sheet, Settings and Debts , with one row per debt. For each debt it shows the months to pay off, the size of the last payment, the total intere...