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What does an extra payment actually buy you?
Enter the balance, the rate and your current payment, then add the extra you are considering.
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How the months are derived
For a fixed-rate balance with a payment above the monthly interest charge, the number of remaining payments follows from the logarithm of the amortisation identity. Adding an extra payment changes the payment size, so the calculator solves the payoff length twice and reports the difference.
Interest is estimated as the total paid minus the original balance. It is an estimate because the final payment is usually smaller than the regular one, and because the balance is assumed to stay at the entered figure.
When the result breaks
If the payment does not exceed the monthly interest, the balance never falls and the formula has no real answer; the output shows a dash instead of a fake payoff date. New charges, variable rates, fees and missed payments also invalidate the estimate.
Frequently asked questions
Should the payment include insurance or fees?
No. Enter only the part that reduces the principal and interest, otherwise the payoff length will be understated.
Why does the interest saved exceed the extra payments?
Because every extra dollar also stops generating interest for every remaining month. That compounding is the whole point of the calculation.
Does it work for a mortgage?
Yes, for a fixed-rate mortgage without offset accounts or redraw mechanics. Those features change how payments interact with the balance.
What if I pay extra only once?
Enter the one-off amount as a temporary extra for one month, then rerun with zero extra to see the new balance. The estimate is built for a recurring extra.