About the Debt Payoff Calculator
Knowing exactly how long a debt will take to pay off - and how much interest you'll pay along the way - turns an abstract balance into a concrete plan. Our Debt Payoff Calculator solves for payoff time and total interest from your balance, rate, and monthly payment.
How It Works
The calculator uses logarithmic amortization math to solve for how many months it takes to pay off your balance at your specified monthly payment and interest rate, then multiplies to find total interest paid. If your payment doesn't exceed the monthly interest charge, it flags that the debt can never be paid off at that payment level.
Formula & Methodology
Solving for time-to-payoff from a fixed payment requires the same logarithmic rearrangement of the amortization formula used elsewhere on this site: n = -log(1 - (r×balance)/payment) / log(1+r). The 'never pays off' warning comes directly from this formula's domain restriction - if the payment is less than or equal to the monthly interest charge (balance × monthly rate), the expression inside the logarithm becomes zero or negative, which has no valid real-number solution, mathematically confirming the balance would never shrink at that payment.
Step-by-Step: Calculating It By Hand
- 1Convert the annual interest rate to a monthly rate.
- 2Check that the monthly payment exceeds the monthly interest charge (balance × monthly rate) - if not, payoff is mathematically impossible at that payment.
- 3Apply the logarithmic payoff formula to the balance, monthly rate, and payment to find the number of months to payoff.
- 4Multiply the monthly payment by the number of months and subtract the original balance to find total interest paid.
Examples
Standard payoff
An $8,000 balance at 19.99% APR with a $300 monthly payment takes a bit over 3 years to pay off, with a meaningful chunk of the total going to interest.
Payment too low
The same balance at a $130 monthly payment might not even cover the interest accruing each month, meaning the balance would never shrink - a clear signal the payment needs to increase.
Advantages
- Shows both payoff time and total interest, not just one or the other
- Flags the dangerous case where a payment is too low to make progress at all
- Works for any type of debt with a fixed interest rate
- Makes the real cost of high-interest debt concrete with actual numbers
Common Mistakes
- Making only minimum payments without realizing how slowly (or never) that pays off the balance
- Not comparing this against a debt consolidation loan at a lower rate
- Underestimating how much of each payment goes to interest on a high-APR balance
- Forgetting that any extra payment amount, even small, meaningfully shortens payoff time
Edge Cases to Watch For
- A payment barely above the monthly interest charge technically pays off the debt, but can take an extremely long time and rack up enormous total interest.
- This assumes a fixed interest rate for the full payoff period; a variable-rate debt's real payoff timeline would shift if the rate changes.
- Any additional charges added to the balance during the payoff period (new purchases on a credit card, for example) aren't reflected in this single-balance projection.
- Rounding the final payment to bring the balance to exactly zero is standard and can make the actual final payment slightly different from the regular monthly amount.
Common Use Cases
- Understanding exactly how long a debt will take to pay off at your current payment
- Seeing the real cost of high-interest debt in total dollars
- Deciding how much to increase a payment to hit a target payoff date
- Prioritizing which of several debts to focus on paying down first