Extra Payment Calculator

Two ways to use it

See my savings starts from an extra payment: enter an amount and see how much interest and time it saves. Reach a target starts from a deadline: enter when you want to be debt-free and the calculator works out what it takes — shown two ways, as a higher monthly payment or as a single lump sum today. Your debt details stay the same in both modes; only how you pay it off changes. Because your current payment already sets the slowest you would clear the balance, a target has to be sooner than that.

Assumptions & Limitations

This calculator assumes a debt with a fixed monthly payment and interest compounded monthly — the norm for most personal loans, auto loans and mortgages in the United States. It assumes your payment stays the same until the balance clears, that every extra payment goes entirely toward your principal, and that your lender applies it immediately with no prepayment penalty. Some lenders charge a fee for paying early or apply overpayments only at certain times, so check your loan agreement before relying on these numbers. A lump sum is treated as a single payment made today.

Calculator Inputs

• Balance: What you still owe on the debt today — the figure on your latest statement, not the amount you originally borrowed.
• Monthly payment: What you pay each month right now. This is the baseline your savings are measured against, and the calculator uses it to work out how long the debt would take to clear as things stand. If it's less than the monthly interest the balance never goes down — the calculator flags that, and “Reach a target” mode still shows the payment that would clear it.
• Interest: The annual interest rate on the debt. The higher the rate, the more you stand to save by paying extra.
• Extra per month: An amount you pay on top of your current payment, every month. Even a small amount adds up over the life of the debt.
• One-off payment now: A single lump sum applied to your balance today — for example, from a bonus or tax refund.
• Target payoff time: In “Reach a target” mode, how soon you want the debt gone. It has to be sooner than your current payoff, since a bigger payment or a lump sum can only speed things up.

Understanding Your Results

The summary highlights your two headline savings: how much sooner you would be debt-free, and how much interest you would avoid. Beneath it, a table compares your debt with and without the extra payments:

• Payoff time: How long until the balance is fully repaid in each case.
• Total interest: The interest you pay over the rest of the debt in each case — the gap between the two is your interest saving.
• Monthly payment: Your current payment versus your current payment plus the extra monthly amount.

In “Reach a target” mode the results flip around: instead of savings, you get the two ways to hit your deadline — the extra per month to add to every payment, or the one-off lump sum to pay today at your current payment — each with the interest that route saves. Pick whichever fits your cash flow.

Because the savings come from reducing your balance early, the biggest gains come on higher-rate debts and from paying extra as soon as possible. If your debt has no prepayment penalty, even an occasional overpayment in months you have spare cash will move your payoff date forward.