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Mortgage overpayment calculator
What does paying a bit extra each month actually remove — in months, and in interest?
Your mortgage
Month and year only — the schedule shows months, not days.
What you would pay extra
on top of the required payment
What happens
Paying $200.00 extra a month removes 6 years 8 months and $110,890.56 of interest from this schedule.
How this was calculated
Each month, interest is charged on the balance you owed at the start of it, and the rest of your payment reduces the balance:
interest = balance × (annual rate ÷ 12)
principal = payment − interest
new balance = balance − principal
Anything you pay above the required amount has no interest left to cover, so every cent of it reduces the balance. That is why the effect grows: a smaller balance is charged less interest next month, and in every month after.
Month 1, in full:
interest = $320,000.00 × (6.706% ÷ 12) = $1,788.27
principal = $2,266.16 − $1,788.27 = $477.89
balance = $320,000.00 − $477.89 = $319,522.11
Figures are rounded to the cent every month, the way a statement does it, and the final payment is adjusted to clear the balance exactly.
Not modelled: prepayment penalties, escrow for tax and insurance, PMI, rate changes on an adjustable loan, or fees. Those move the real number and none of them is something a calculator can know about your loan.
| Month | Due | Payment | Interest | Principal | Balance |
|---|---|---|---|---|---|
| 1 | Sep 2026 | $2,266.16 | $1,788.27 | $477.89 | $319,522.11 |
| 2 | Oct 2026 | $2,266.16 | $1,785.60 | $480.56 | $319,041.55 |
| 3 | Nov 2026 | $2,266.16 | $1,782.91 | $483.25 | $318,558.30 |
| 4 | Dec 2026 | $2,266.16 | $1,780.21 | $485.95 | $318,072.35 |
| 5 | Jan 2027 | $2,266.16 | $1,777.49 | $488.67 | $317,583.68 |
| 6 | Feb 2027 | $2,266.16 | $1,774.76 | $491.40 | $317,092.28 |
| 7 | Mar 2027 | $2,266.16 | $1,772.02 | $494.14 | $316,598.14 |
| 8 | Apr 2027 | $2,266.16 | $1,769.26 | $496.90 | $316,101.24 |
| 9 | May 2027 | $2,266.16 | $1,766.48 | $499.68 | $315,601.56 |
| 10 | Jun 2027 | $2,266.16 | $1,763.69 | $502.47 | $315,099.09 |
| 11 | Jul 2027 | $2,266.16 | $1,760.88 | $505.28 | $314,593.81 |
| 12 | Aug 2027 | $2,266.16 | $1,758.06 | $508.10 | $314,085.71 |
Why an overpayment does so much
A mortgage payment is not one thing. It is interest on the balance you owe today, plus whatever is left over to reduce that balance. Early in a thirty-year loan the first part dominates: on $320,000 at 6.706%, the first month charges$1,788.27 of interest against a payment of$2,066.16. Only$277.89 of that first payment actually reduces what you owe.
An extra payment behaves completely differently, because the interest for that month has already been covered. Every cent of it goes to principal. In month one, an extra $200 does the work of about 72% of the entire contractual payment's principal reduction — for less than a tenth of the money.
And it compounds. A balance $200 smaller is charged roughly $1.12 less interest the next month, which frees another $1.12 to reduce principal, and so on for three hundred months. That is the whole mechanism, and it is why the totals look implausible until you follow the schedule.
A worked example you can check
Take the default scenario: $320,000 borrowed at 6.706% over 30 years. The required payment is$2,066.16 a month — the figure the standard amortisation formula gives, and the same one calculator.net publishes for these inputs.
Now add $200 a month:
| Paying | Term | Total interest |
|---|---|---|
| $2,066.16 (contractual) | 360 months | $423,821.51 |
| $2,266.16 (+$200) | 280 months | $312,930.95 |
| Difference | 80 months | $110,890.56 |
$200 a month for 280 months is $56,000 of extra payments. It removes $110,890.56 of interest — very nearly twice what was put in, and it ends the loan six years and eight months early. The ratio is that favourable because the money is applied at the point in the schedule where the balance, and therefore the interest, is largest.
Where our figure differs from other calculators
calculator.net reports $423,818.78 of total interest for the contractual schedule. We report$423,821.51 — $2.73 more, and the difference has a specific cause rather than being noise.
They compute from the unrounded payment, $2,066.163273, carried at full precision across 360 months. That is why their "total of 360 payments" is $743,818.78 rather than $2,066.16 × 360 = $743,817.60 — a payment nobody could actually make. We charge the rounded payment a lender collects, round interest to the cent every month, and collect the residue in the final payment.
Ours being higher is the direction that makes sense: a payment rounded down by a fraction of a cent retires the loan marginally more slowly. $2.73 over thirty years is under a cent a month. Neither figure is wrong; they answer slightly different questions, and our verification page covers how to check both.
Why the last payment is bigger
The exact amortising payment is $2,066.1633. Rounded to the cent it becomes $2,066.16 — very slightly too little to retire the principal to exactly zero over 360 months. The residue has to go somewhere, and lenders collect it in the final month rather than issuing a 361st bill for a few dollars.
On this scenario the last payment is $2,070.07. Our engine models it that way deliberately: a schedule that reported 361 months would be arithmetically defensible and wrong about the product.
What this does not include
The model covers principal and interest on a fixed rate. It does not include escrow for property tax and insurance, PMI, HOA dues, or any fee — which is why the payment here is smaller than the one your servicer collects. It does not model rate changes on an adjustable loan, and it does not know whether your note carries a prepayment penalty. Check the note before acting on any figure here.
Common questions
- Does paying extra on a mortgage actually save money?
- It removes future interest, because interest is charged on the balance and the extra reduces the balance sooner. On $320,000 at 6.706% over 30 years, $200 a month removes 80 months and $110,890.56 of interest. That is arithmetic, not advice — whether the money is better used elsewhere is a question this calculator cannot answer and does not try to.
- Where does the extra payment go?
- Straight to principal. The contractual payment covers that month’s interest first and reduces the balance with the remainder; anything above the contractual payment has no interest left to cover, so all of it reduces the balance. That is why the effect compounds — a smaller balance is charged less interest next month, and every month after.
- Why does your required payment differ by a cent from my lender’s?
- The exact amortising payment for $320,000 at 6.706% over 360 months is $2,066.1633. Rounded to the cent that is $2,066.16, which is what we and most published calculators show. A lender rounding up would charge $2,066.17. One cent a month over thirty years is $3.60, and it lands in the final payment either way.
- Why is my final payment larger than the others?
- Because the monthly payment is rounded. A payment rounded down by a fraction of a cent cannot retire the principal to exactly zero over 360 months, so a small residue is left. Lenders collect it in the final month rather than adding a 361st payment. On the default scenario the last payment is $2,070.07 against a normal $2,066.16.
- Does this account for a prepayment penalty?
- No. Some mortgages charge a fee for paying down principal early, and the rules vary by loan type and by state. Check your note before acting on any figure here. The CFPB source below explains what to look for.
- Is anything I type sent to your server?
- No, and there is no server to send it to. This is a static page and every figure is computed in your browser. The shareable link in your address bar carries the loan amount, rate, term and overpayment — nothing else, and no name.
Sources
- CFPB — What is an amortization schedule? — retrieved 2026-08-08
- CFPB — Understanding loan prepayment penalties — retrieved 2026-08-08
- The required payment is cross-checked against the standard amortisation formula, P·i(1+i)n ÷ ((1+i)n − 1), and against calculator.net's published output for the same inputs — $2,066.16, which our fixture asserts to the cent.
Written and maintained by Vikash Singh. Last verified 2026-08-08.