Calculation & Conversion / Calculators
Extra Payment Mortgage Calculator
Extra Payment Mortgage Calculator shows what happens when you pay more than the scheduled amount on a loan: every extra dollar aimed at principal shrinks the balance, so future interest is lower and the loan ends sooner. Nothing is uploaded.
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Input parameters
Keep monthly payment unchanged; shorten the remaining term (saves more interest)
Results
This space is reserved for a sponsor. Every tool on this site stays free and runs locally in your browser.
How to use
- 1
Enter the loan basics
Type the loan amount, the annual interest rate and the original term in years. The calculator builds the standard amortising schedule from these three inputs, splitting each payment into interest on the remaining balance and principal that reduces what you owe.
- 2
Add an extra monthly payment
Enter a fixed amount to add to every regular payment. Because that extra money goes straight to principal, the balance drops faster, which in turn lowers the interest charged next month — a compounding effect that shortens the loan more than the raw extra amount suggests.
- 3
Add a one-off lump sum
Include a single extra payment, such as a bonus or tax refund, on the month it arrives. A lump sum aimed at principal has the same accelerating effect as ongoing extra payments, just applied once; the schedule shows the exact month the loan now ends.
- 4
Compare standard vs accelerated
The result places the original schedule beside the accelerated one, with the interest saved and the months or years removed. The difference is not linear: early extra payments save more than identical payments made late in the loan, because there is more balance left for them to act on.
- 5
Read the month-by-month schedule
Open the schedule to see, for every month, the payment, the interest portion, the principal portion and the remaining balance under both plans. This is the evidence behind the headline numbers and the place to confirm the loan really ends when the calculator says it does.
Key facts
- AmortisationEach payment covers the interest on the remaining balance first; the remainder reduces principal. Early payments are mostly interest, late payments are mostly principal.Source:Loan amortisation
- Biweekly26 biweekly payments per year equal 13 monthly equivalents, so biweekly scheduling adds one extra monthly payment annually with no extra effort.Source:Payment scheduling
- Interest totalTotal interest paid equals total of all payments minus the original principal borrowed. Extra principal lowers that total directly.Source:Loan arithmetic
- Prepayment effectAn extra principal payment immediately shrinks the balance, so all subsequent interest is computed on a smaller amount — the saving compounds for the rest of the loan.Source:This tool
- PrivacyAll schedules are computed locally in your browser; no loan figures are transmitted.Source:This tool
How this calculator works
The loan is amortised one month at a time. Each month, interest is charged on the outstanding balance at the annual rate divided by twelve; the scheduled payment covers that interest first, and whatever is left reduces the principal. Any extra payment is applied to principal in the month you choose, so every later month begins from a smaller balance and therefore accrues less interest. Payments are assumed to fall at the end of each month, and the loan ends in the month the balance reaches zero.
Two conventions move the answer and are worth stating plainly. The rate is treated as a nominal annual rate compounded monthly, which is how US mortgages are amortised — it is not an effective annual rate, and it is not continuously compounded. The result is also nominal dollars: no inflation adjustment is applied, and no mortgage-interest deduction is modelled, so a figure here is not the after-tax cost of the loan.
Sources and standards
Disclaimer
This is an educational calculator, not financial advice. The result is an arithmetic projection of the figures you enter: it does not know your full circumstances, the terms of a specific offer, or the tax and regulatory rules that apply where you live. Confirm any figure against the terms of the product itself, or with a licensed professional, before acting on it.
Last reviewed:
Frequently asked questions
How much earlier will I pay the loan off?
It depends on how much extra you pay and when. A common rule of thumb is that an extra monthly payment equal to one principal-and-interest instalment, paid from the start, can cut a 30-year loan by roughly six to eight years. The savings are front-loaded: the same extra dollar applied in year two removes more total interest than the same dollar applied in year twenty, because early in the loan the balance — and therefore the interest it accrues — is largest.
Does an extra payment really reduce total interest?
Yes, and the reason is mechanical rather than magical. Each scheduled payment first covers the interest charged on the remaining balance, and only what is left reduces principal. When you send extra money marked for principal, the balance falls immediately, so the very next month interest is computed on a smaller number. Less balance, less interest, every month thereafter — that is where the saving comes from.
Biweekly vs extra monthly — which saves more?
A biweekly schedule makes 26 half-payments a year, which equals 13 full monthly payments — one extra monthly payment per year with no extra effort. An explicit extra monthly amount gives you direct control over the size of the boost. Biweekly is effectively a fixed ~8% annual overpayment; if you can afford more, an explicit extra amount beats it, and if you want "set and forget", biweekly is the simpler habit.
Should I shorten the term or reduce the monthly payment?
When you overpay, the lender usually offers a choice: keep the term and lower the monthly payment, or keep the payment and shorten the term. Shortening the term saves dramatically more interest, because the balance is cleared sooner and stops accruing. Reducing the payment improves monthly cash flow but leaves the balance outstanding longer, so you pay for that flexibility in extra interest. Choose based on whether your priority is the total cost or the monthly burden.
Is my data uploaded to a server?
No. The schedule and every comparison are computed in your browser from the numbers you type; nothing is sent to or stored on any server. You can model real loan figures, including identifying amounts and balances, without any of it leaving the page — useful because the whole point is to experiment with figures you would not want sitting in someone else's logs.
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