Extra Payment Calculator
See what overpaying actually buys on any loan, card, or line of credit — a fixed amount each month, one extra payment a year, a single lump sum, or all three — in months off the payoff date and interest avoided.
The debt and the payment
What you owe today on this one loan, card, or line of credit.
Nominal annual rate; interest is charged monthly at one twelfth of it.
What the lender asks for each month, before anything extra.
Applied entirely to the balance after the required payment.
3 years, 9 months instead of 5 years, 1 month, and $1,385.67 less interest.
- Extra actually paid
- $6,600.00
- Interest avoided
- $1,385.67
- Per extra dollar
- $0.21 saved
| Plan | Time to clear | Total interest | Total paid |
|---|---|---|---|
| Required payment only | 5 years, 1 month | $5,069.24 | $30,069.24 |
| With your extra payments | 3 years, 9 months | $3,683.57 | $28,683.57 |
What a monthly extra alone would do
How the two schedules are compared
The same debt is amortized twice under identical interest rules. Each month, interest is charged at one twelfth of the APR, the required payment is applied, and then any extras are applied on top and go entirely to the balance. The baseline run has the extras switched off; the accelerated run has yours switched on. The months and interest of each are reported, and their differences are the saving.
Interest = Balance × (APR ÷ 12); Paid = Required + Extra + Annual + Lump- Extra
- Your fixed amount, every month
- Annual
- Charged in months 12, 24, 36 and so on
- Lump
- Charged once, in the month you choose
Every extra is clipped to whatever balance is actually left, so the final month never reports money you would not have had to hand over. If the required payment does not exceed the first month's interest, the baseline has no payoff date at all — the balance rises every month — and the calculator says so rather than printing one. In that case there is nothing to measure a saving against, and the extra is what gives the debt an ending.
$150 a month against a $25,000 loan at 7.5%
A $25,000 balance at 7.5% APR with a $500 required monthly payment clears in 61 months and costs $5,069.24 in interest. Adding $150 a month clears it in 45 months instead — 16 months sooner — for $3,683.57 of interest, a saving of $1,385.67. The extra payments themselves came to $6,600, so each extra dollar avoided about 21 cents of interest.
The same money in other shapes does less. One extra $500 payment a year finishes the loan five months early and saves $392.92, having paid $2,000 extra. A single $3,000 lump sum in month one finishes it nine months early and saves $1,236.11 — about 41 cents avoided per extra dollar, the best rate of the three, because all of it lands before any of the interest it prevents.
Raising the monthly extra keeps working but with diminishing force: $50 a month saves $567.25 and $300 a month saves $2,167.58. The per-dollar return stays near 21 cents throughout, because on this loan the rate and the remaining term — not the size of the extra — set what a prepaid dollar is worth.
What this calculator assumes
- The APR is fixed for the whole payoff period.
- Interest is charged monthly at one twelfth of the APR. Debts that use a daily periodic rate will differ slightly.
- Every extra payment goes entirely to the balance, not to next month's instalment, and there is no prepayment penalty.
- The yearly extra lands in months divisible by twelve; the lump sum lands once, in the month you set.
- Extras are clipped to the remaining balance, so the reported extra paid is money genuinely handed over.
- No new borrowing, fees, or insurance is added. Money values are rounded to the nearest cent for display.
Understanding the Extra Payment Calculator
Every extra dollar sent to a debt does two things at once: it removes a dollar of principal, and it removes every future month of interest that dollar would have generated. The second effect is the one people underestimate, and it is why a modest extra payment made early usually beats a large one made late. This calculator prices that effect on any loan, card, or line of credit — as a monthly amount, as one extra payment a year, as a single lump sum, or all three together.
It runs the debt twice with the same interest rules: once at the required payment alone, once with your extras applied. The difference in months and the difference in interest are the answers. It also reports how much extra you actually paid, so the saving can be read against its cost rather than in isolation.
Who this calculator is for
- Anyone with a fixed loan paymentauto, student, personal — who wants to know what overpaying actually buys.
- People expecting a tax refund or bonuswho want to see what a one-off lump sum does, and whether timing matters.
- Anyone considering a thirteenth paymentwho want the effect of one extra payment a year without switching to biweekly billing.
- People choosing between two extra amountswho want the months and interest each one buys before committing.
- Anyone whose payment barely moves the balancewho needs to know whether the required payment covers the interest at all.
Why it matters
- It converts an abstract intention — 'I should pay a bit more' — into a payoff date and a dollar figure.
- It handles all three real-world shapes of overpaying at once: a monthly extra, a yearly extra, and a one-off lump sum in the month it lands.
- It shows the interest avoided per extra dollar paid, so you can see the actual return on the money rather than only the headline saving.
- It clips the final payment to the balance that was really left, so the extra it reports is money you would genuinely have handed over.
- It says plainly when the required payment does not cover the interest — the case where the debt has no ending without the extra, and no baseline exists to measure a saving against.
How to use this calculator
- Enter the current balance, the APR, and the payment your lender requires each month.
- Enter what you could add every month. This is the input that does most of the work on most debts.
- Open the one-off and yearly extras if you also expect a bonus, a thirteenth payment, or a refund, and set the month the lump sum lands.
- Read the months saved and the interest avoided, then check the extra actually paid so the two can be weighed against each other.
- Use the comparison cards to see what a monthly extra alone would do at three different amounts.
How to read your result
The headline is how much sooner the debt ends and how much interest that avoids. Beside it sits the interest avoided per extra dollar paid, which is the figure that keeps the result honest: on a 7.5% loan with a few years left, a dollar of prepayment typically buys twenty-odd cents of avoided interest, not a dollar. That is a real return — better than most savings accounts, and guaranteed — but it is not the same as the headline saving, and knowing both stops the number being oversold.
Timing matters more than the calculator's totals suggest at first glance. The same lump sum applied in month one avoids far more interest than the same amount applied three years in, because it removes interest from every month that follows it. If you are choosing when to deploy a windfall, move the lump-sum month back and forth and watch the saving change while the amount paid stays identical.
- Check that extra payments go to principal. Some servicers apply overpayments to next month's instalment instead, which pauses your bill without shortening the loan — ask explicitly and check the statement afterwards.
- Some loans carry prepayment penalties. They are rare on consumer credit but not extinct, and they are not modeled here.
- The APR is assumed fixed. A variable rate moves with the index, and a promotional rate that ends changes the arithmetic entirely.
- Interest is modeled as monthly at one twelfth of the APR. Credit cards and some loans use daily periodic rates, which differ slightly.
- Overpaying a low-rate debt while carrying a high-rate one costs money. Clear the expensive balance first.
- An emergency fund usually beats prepayment on a low-rate loan; money sent to a loan is hard to get back.
- Set the extra as a standing order on payday rather than paying it at month end. The amount is the same and it actually happens.
- If you get a raise, send the difference before your spending adjusts to it. The comparison cards show what each step up buys.
- Send a windfall early rather than saving it for a round-number moment — the same money avoids more interest the sooner it lands.
Frequently asked questions
How much does one extra payment a year really save?
It depends on the rate and how long is left. On a $25,000 balance at 7.5% with a $500 monthly payment, adding $500 once every twelve months finishes the loan five months early and avoids $392.92 of interest, for $2,000 of extra payments. A steady monthly extra usually does more for the same annual outlay, because it starts working immediately.
Does it matter when a lump sum lands?
Considerably. A lump sum removes interest from every month after it, so the earlier it arrives the more it avoids — with the amount paid unchanged. Move the lump-sum month in the calculator and the saving changes while the extra paid stays the same.
What does 'interest avoided per extra dollar' mean?
It is the interest saved divided by the extra actually paid. The extra dollars are not a cost — they were principal you owed anyway — so this is the return the prepayment earned. On short, low-rate loans it is small; on long, high-rate balances it can exceed a dollar per dollar.
What if my required payment does not cover the interest?
Then the balance grows every month and the debt has no payoff date at all. The calculator reports that instead of inventing one, and shows what your extra payments do — which in that situation is the difference between a debt that ends and one that does not.
Should I overpay the loan or invest the money?
Paying down a balance earns its APR with certainty; investing might earn more or less. The pay off debt or invest calculator runs both paths over the same horizon and reports the assumed return at which they tie.
Are my inputs saved or shared?
No. Every calculation runs in your browser; nothing is stored on our servers or sent to analytics. A shareable link only encodes the numbers you choose to share.
Where the extra dollar should go
This page prices overpaying one debt. If you hold several, the snowball vs. avalanche calculator decides which one the extra should attack, and the debt-free date calculator works backwards from a deadline instead. If the debt is a card on minimum payments, the minimum payment trap calculator shows what doing nothing costs. And before committing the money at all, the pay off debt or invest calculator compares prepayment against the alternative use of the same dollars.
Sources and review notes
Methodology last checked Jul 28, 2026. Both amortization runs are covered by deterministic unit tests, including zero-rate debts, lump sums larger than the balance, and required payments that never cover the interest. No financial professional review is claimed yet.