Extra Payment Calculator
An extra $100 per month does not feel like much. But on a high-APR balance, that $100 routinely pulls in months sooner and saves hundreds in interest. See exactly what your extra payment is worth.
How to use it
- 01.Enter the balance, APR, and current minimum payment for the debt you want to attack. For a multi-debt portfolio, start with the highest-APR debt — that is where the extra payment has the biggest impact.
- 02.Enter the extra amount you can put toward this debt each month. Be realistic; an extra payment you cannot sustain does not actually save you the interest.
- 03.Read the comparison. The output shows months to payoff with and without the extra, along with the dollar interest saved.
The method, briefly
Extra payments work because credit-card-style debt compounds. Every month, the balance accrues interest at one-twelfth of the APR. The portion of your payment that does not go to interest reduces the principal, which then accrues less interest next month. An extra payment moves more dollars to principal sooner, which cascades through the entire payoff timeline. The result is often dramatic on high-APR debt — paying $50 more per month on a 25% APR card frequently saves more than 12 months and several hundred dollars in interest.
Frequently asked questions
Which debt should I add the extra payment to?
On math alone: the highest-APR debt. On psychology: the smallest balance (so you finish it fastest and the visible win sustains motivation). For a hybrid approach, RealiPlan supports avalanche above 20% APR then snowball on the rest as a built-in option.
What if I cannot sustain the extra payment every month?
Inconsistent extra payments still help, but not as much as the calculator shows. The 'with extra payment' result above assumes the extra is paid every single month. If you can only do it some months, the actual interest savings is somewhere between the two scenarios.
Should I save the extra in an emergency fund first?
If you have no emergency fund and a card has an APR under 8%, save first. If your card APR is above 15%, the interest you avoid by paying down debt typically beats the return on a savings account, even adjusted for the risk of a future emergency. Many users split — half to emergency fund, half to debt — until both have meaningful balances.
Does this work for student loans or mortgages too?
Yes, but the math is less dramatic because of the lower APR. An extra payment on a 6% mortgage saves much less per dollar than an extra payment on a 25% credit card. For most users, attacking credit card debt first is the right call before adding extra to a mortgage.
What is a one-time windfall worth, versus a monthly extra?
A windfall (tax refund, bonus) applied to a high-APR debt can save thousands in interest. The math is exactly the same — every extra dollar removes future interest at the debt's APR. RealiPlan has a dedicated windfall modeling feature on the dashboard that handles one-time payments on top of your regular plan.
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Track your real debts in RealiPlan — freeLast updated 2026-05-26. This calculator runs entirely in your browser. No data is sent to RealiPlan unless you create an account.