About the Loan Payoff Calculator
Any fixed-rate loan - personal, auto, or otherwise - can be paid off faster with extra payments, and the savings are often bigger than people expect. Our Loan Payoff Calculator shows exactly how much time and interest an extra payment amount saves on your specific loan.
How It Works
The calculator uses your current balance, rate, and payment to solve for how many months remain at your current payment amount, then solves again with your extra payment added, using logarithmic amortization math. The difference between the two payoff times is your time saved.
Formula & Methodology
Because the standard amortization formula solves for payment given a fixed number of periods, finding the reverse - the number of periods given a fixed payment - requires rearranging it into a logarithmic form: n = -log(1 - (r×balance)/payment) / log(1+r). This is why solving for time remaining isn't simple division of balance by payment - interest keeps accruing on the shrinking balance throughout, and the logarithm is what accounts for that compounding effect precisely.
Step-by-Step: Calculating It By Hand
- 1Convert your annual interest rate to a monthly rate.
- 2Apply the logarithmic payoff formula to your current balance, rate, and monthly payment to find months remaining at that payment.
- 3Re-run the same formula with your extra amount added to the monthly payment.
- 4Subtract the second result from the first to find the number of months saved.
Examples
Credit card or personal loan
A $15,000 balance at 9% with a $400 monthly payment pays off meaningfully faster with just $100 extra added each month.
When extra payments don't help
If your current payment barely covers the interest accruing each month, the calculator flags that the loan would never pay off at that payment - a sign the payment needs to increase.
Advantages
- Works for any fixed-rate loan, not just mortgages
- Flags the edge case where a payment is too low to ever pay off the balance
- Shows the real payoff timeline, not just a rough guess
- Makes the value of small extra payments concrete with real numbers
Common Mistakes
- Not confirming with your lender that extra payments go to principal immediately, not next month's payment
- Paying extra on a low-interest loan while carrying a higher-interest balance elsewhere
- Assuming all loans allow prepayment without penalty - some do not
- Forgetting that even small extra payments compound in benefit the earlier they start
Edge Cases to Watch For
- If your payment doesn't exceed the monthly interest accruing on the balance, the logarithmic formula has no valid solution - the loan would never pay off at that payment level, no matter how long you waited.
- This assumes a fixed interest rate for the remaining term; a variable-rate loan's actual payoff timeline would shift if the rate changes.
- Extra payments only accelerate payoff if applied to principal immediately - check that your lender doesn't hold extra amounts toward a future scheduled payment instead.
- Any fees for early payoff (rare on personal loans but worth checking) would offset part of the calculated interest savings.
Common Use Cases
- Deciding how much extra to pay toward a personal or auto loan
- Understanding how close a payment is to only covering interest
- Prioritizing which debt to pay off faster when you have several
- Setting a realistic debt-free target date