➕ Extra Payment Payoff Calculator
Enter your balance, APR, and minimum payment, then add an extra monthly amount to see how much time and interest you'd save.
One payoff formula, evaluated twice
The closed form is solved for your current payment, again for the payment plus the extra, and the answers subtracted. B is the balance, r the monthly rate:
n(P) = -ln(1 - r x B / P) / ln(1 + r)
interest(P) = n(P) x P - B
time saved = n(P) - n(P + extra)On the defaults - $10,000 at 15%, paying $250 with $100 extra - that is 55.8 months and $3,949 of interest against 35.6 months and $2,448. Adding $100 a month buys back 20.2 months and $1,501.
The first dollars of extra do the heavy lifting
The saving grows with the extra, but not in proportion to it:
| Extra per month | Payoff | Interest saved |
|---|---|---|
| none | 55.8 months | - |
| $25 | 48.8 months | $531 |
| $100 | 35.6 months | $1,501 |
| $250 | 23.2 months | $2,370 |
That first $25 returns about $21 of interest for every dollar added each month; at $250 the last dollars return under $10. A small extra you can sustain beats a large one you abandon.
Frequently asked questions
How much does paying $100 extra a month save?
On $10,000 at 15% APR with a $250 payment, 20 months and $1,501 of interest. The saving scales with the rate: the same extra against a 5% loan returns far less, because there is less interest to avoid.
Should I pay extra on the loan or put the money in savings?
Paying down a 15% debt is a guaranteed 15% return, which almost nothing safe matches. The usual exceptions are a small emergency fund and any employer match you would give up.
Why does the payoff time not halve when I double the payment?
It does better than halve. Doubling the payment to $500 cuts 55.8 months to 23.2, because the extra lands entirely on principal and the interest charged each month falls with it.