How do extra mortgage payments save interest?

Interest is charged on the remaining balance. Extra principal lowers that balance sooner, so every later month is charged on less. The required payment stays the same, which means a larger share of each remaining payment is principal, and the loan ends early.

What is a mortgage overpayment in the UK?

An overpayment is extra money you pay on top of the contracted monthly amount. It is the UK term for what US borrowers usually call an extra payment. This calculator models both the same way: extras go to principal and the required payment does not change.

Do UK mortgages really allow 10% overpayments a year?

Many do, and MoneyHelper describes 10% a year without a penalty as a common lender rule. It is not a legal limit and it is not applied here. Your mortgage offer sets the allowance, how the year is measured, and any early repayment charge on the excess.

How does a biweekly mortgage payment work?

You pay half the scheduled monthly amount every two weeks. That is 26 half-payments a year — one extra monthly payment. This page spreads that extra across 12 months. Confirm extras go to principal, and skip paid programs that only hold your money.

Should I recast instead of making extra payments?

Recast if you want a lower required payment and can live with the original remaining term. Pay extra if you want to finish sooner and save more interest. The recast calculator shows both a recast and a keep-paying-extra comparison.

Will I pay a prepayment penalty or early repayment charge?

Sometimes. US prepayment penalties are less common on recent mortgages, but you should still check. UK early repayment charges usually apply during a fixed period if you exceed the overpayment allowance. This calculator does not subtract either fee.

Does a lump sum lower my monthly payment?

Not on this page. The lump sum is extra principal, so you keep the original payment and finish earlier. To lower the payment without changing the rate, you need the lender to recast.

Can I share or download a calculation?

Yes. Use Share to copy a link that reopens the calculator with the same inputs, or Download to save the results and the yearly comparison as a text file.

Mortgage extra payment calculator

Enter the loan amount, the interest rate, the term, any extra you can pay each month, an optional lump sum, and whether you pay monthly or biweekly. The calculator keeps the original payment, sends extras to principal, and shows how many months you cut and how much interest you avoid.

Written by Kenneth Schrøder, Founder of WiseCalcs

Updated November 3, 2026·Sources verified June 10, 2026

Use it to see whether extra mortgage payments — a little more each month, a lump sum, or a biweekly schedule — are worth it. The result is time saved and interest saved compared with the original schedule, not a lower required payment.

Interest saved

$103,448.79

Time saved: 6 yr 11 mo

Total interest

Original

$382,633

With extras

$279,185

Original monthly payment: $1,896.20
Extra applied each month: $200.00
Original payoff time: 30 yr
Payoff with extras: 23 yr 1 mo
Original total interest: $382,633.47
Interest with extras: $279,184.67

Balance remaining

Remaining balance at the end of each year. The original loan is muted; extras are in colour.

Year-by-year comparison

Year-by-year comparison
Year	Original balance	Balance with extras	Original interest	Interest with extras
1	$296,647	$294,174	$19,401	$19,328
2	$293,069	$287,958	$19,177	$18,938
3	$289,252	$281,325	$18,937	$18,522
4	$285,179	$274,249	$18,681	$18,078
5	$280,833	$266,698	$18,409	$17,604
6	$276,196	$258,642	$18,118	$17,098
7	$271,249	$250,046	$17,807	$16,559
8	$265,970	$240,875	$17,476	$15,983
9	$260,338	$231,089	$17,122	$15,369
10	$254,328	$220,648	$16,745	$14,713
11	$247,916	$209,507	$16,343	$14,014
12	$241,075	$197,621	$15,913	$13,268
13	$233,776	$184,938	$15,455	$12,472
14	$225,987	$171,407	$14,966	$11,623
15	$217,677	$156,968	$14,445	$10,716
16	$208,811	$141,563	$13,888	$9,749
17	$199,351	$125,127	$13,294	$8,718
18	$189,257	$107,589	$12,661	$7,617
19	$178,487	$88,877	$11,985	$6,442
20	$166,996	$68,912	$11,263	$5,189
21	$154,735	$47,609	$10,494	$3,852
22	$141,653	$24,880	$9,673	$2,425
23	$127,695	$629	$8,797	$903
24	$112,803	$0	$7,862	$3
25	$96,912	$0	$6,864	$0
26	$79,958	$0	$5,800	$0
27	$61,868	$0	$4,665	$0
28	$42,567	$0	$3,453	$0
29	$21,973	$0	$2,161	$0
30	$0	$0	$781	$0

The result updates as you type. Interest saved is the headline. Time saved is how many years and months you knock off the original term. The bars compare total interest, the chart compares remaining balances, and the table is the year-by-year amortization comparison.

How does it work?

Each month, interest is the remaining balance times r. The rest of the scheduled payment plus any extra monthly amount, any biweekly extra, and any lump sum in that month, reduces the balance. The required payment stays the original M; extras never recast it. A biweekly schedule is modeled as one extra monthly payment a year (M ÷ 12 each month), because 26 half-payments equal 13 monthly payments. The last payment is trimmed so the balance lands on zero. You supply the rate; the calculator does not look one up, apply a UK overpayment cap, or charge an early repayment fee.

Extra payments go to principal

M = P\cdot\frac{r\,(1+r)^{n}}{(1+r)^{n}-1},\quad \text{extra}\to\text{principal}

M: Scheduled monthly principal-and-interest payment. Extra amounts do not change it.
P: Loan amount (the principal you still owe).
r: Monthly interest rate (annual rate ÷ 100 ÷ 12).
n: Scheduled number of monthly payments (years × 12).

A $300,000 loan at 6.5% for 30 years has a payment of about $1,896.20. Paying $200 extra each month, with no lump sum, finishes the loan in 277 months instead of 360 — 83 months (6 years 11 months) sooner — and cuts interest by about $103,449.

Method & sources

The calculator builds two amortizing schedules on the same loan: the original equal monthly payment, and the same payment plus extra monthly amounts, an optional lump sum, and an optional biweekly extra. Every extra dollar goes to principal. The comparison is months saved and interest saved versus the original schedule.

Sources

Where this method comes from — use these references to understand the formula, assumptions, and limits.

What is an amortization schedule? — U.S. Consumer Financial Protection Bureau, verified 2026-06-10
How does paying down a mortgage work? — U.S. Consumer Financial Protection Bureau, verified 2026-11-03
Mortgage key terms — bi-weekly payment — U.S. Consumer Financial Protection Bureau, verified 2026-11-03
Should you pay off your mortgage early? — MoneyHelper, verified 2026-11-03

How we calculate

The loan is fully amortizing with a fixed rate and equal monthly principal-and-interest payments. Tax, insurance, and mortgage insurance are not included.
The required monthly payment stays the original amount. Extra payments do not recast or refinance the loan.
Extra monthly amounts, the biweekly extra, and the lump sum are applied entirely to principal. Some lenders apply extra money to the next instalment instead, which does not shorten the loan.
A lump sum is applied at the start of the month you enter, before that month's interest, as extra principal. If the loan is already paid off by then, the lump sum is not counted and the calculator says so.
Biweekly means half the scheduled monthly payment every two weeks, which is 26 half-payments a year — one extra monthly payment. That extra is modeled as the original payment divided by 12, added each month, because most mortgages still accrue interest monthly.
You supply the rate. The calculator does not look up market rates, UK overpayment allowances, early repayment charges, or US prepayment penalties.

Limitations

This is a planning comparison, not financial, tax, or legal advice. Your lender's statement and mortgage offer are the figures that matter.
Early repayment charges, prepayment penalties, and annual overpayment caps are not subtracted from the savings.
Interest-only, adjustable-rate, offset, and tracker products are not modeled.
A paid biweekly program that holds funds and forwards one monthly payment, or that charges a fee, will not match this schedule.

Rounding

Money is rounded to two decimals for display. Time saved is shown in whole years and months. The schedule itself uses full precision, and the last payment is trimmed so the balance lands on zero.

What this extra payment calculator does

An extra payment is money on top of the scheduled principal-and-interest amount. In the UK that is usually called an overpayment. This page models both: it keeps the required payment, sends extras to principal, and compares the original schedule with the extra-payoff schedule. The amortization calculator shows equal payments with no extras. The debt payoff calculator is for a total monthly payment against a balance, such as a credit card.

How to use it

Enter the current loan amount, the annual interest rate, and the remaining term in years.
Enter the extra you can add every month. Leave it at 0 to test only a lump sum or only biweekly payments.
Enter a one-off lump sum if you have one, and the month it lands. Month 1 means it is applied at the start, before the first month's interest.
Leave the schedule on monthly, or switch to biweekly to model 26 half-payments a year.
Read interest saved, time saved, both payoff lengths, and the year-by-year comparison.

A worked example

A $300,000 loan at 6.5% for 30 years has a monthly principal-and-interest payment of about $1,896.20 and about $382,633 of interest over the term. Add $200 extra every month, with no lump sum, and the loan finishes in 277 months instead of 360 — 83 months, or 6 years and 11 months, sooner — and interest falls by about $103,449. Those are the default numbers. Your rate, remaining term, and extra amount will change the result.

Monthly extra, lump sum, and biweekly payments

A regular extra each month is the steady lever: every extra dollar reduces next month's interest. A lump sum does the same job in one hit — a bonus or savings — and saves more if it lands earlier. A biweekly schedule pays half the monthly P&I every two weeks. That is 26 half-payments a year, or 13 monthly payments, so you make one extra monthly payment a year. This page models that extra as the original payment divided by 12 each month, because most mortgages still accrue interest monthly. You can usually get the same effect by sending one extra monthly payment yourself and telling the servicer to apply it to principal.

Extra payments versus recast or refinance

Paying extra keeps your required payment and shortens the term. A recast uses a lump sum to lower the required payment and keeps the remaining term. Refinancing replaces the loan. If you have a lump sum and want a smaller bill each month, use the recast calculator. If you want the loan gone sooner, stay on this page. If a new rate would beat both, use the refinance calculator.

UK overpayments, the 10% allowance, and early repayment charges

On UK mortgages the usual word is overpayment. MoneyHelper notes that many lenders let you overpay up to 10% a year without a penalty. That 10% figure is a common product feature, not a legal cap, and it is not built into this calculator. The amount, the year it is measured against, and whether unused allowance carries over are in your mortgage offer. If you overpay more than the allowance during a fixed period, the lender may charge an early repayment charge on the excess. This page does not subtract that charge. Check the offer letter before you send a large lump sum.

What the result means

Interest saved is money you never pay if you stick to the extras until the loan is gone. Time saved is how much shorter that plan is than the original term. Neither figure is a cash refund. If you stop, the remaining balance amortizes at the original payment from there. The table shows both balances in the same year.

Common mistakes

Assuming extra money automatically shortens the loan. Some servicers apply it to the next payment due unless you say it is extra principal.
Treating a UK 10% allowance as a rule this calculator enforces. It does not. Your offer letter does.
Paying for a biweekly program that only forwards one monthly payment, or that charges a setup fee, and expecting this schedule.
Comparing extra payments with a recast as if they did the same thing. One shortens the term; the other lowers the bill.
Ignoring a prepayment penalty or early repayment charge on a large lump sum.

When this calculator is useful

Use it to test whether $100 or $200 extra a month is worth it, what a bonus does to the term, or how a biweekly habit compares with one extra payment a year. Pair it with the mortgage calculator for the full housing payment, and with the points calculator if the other option is buying a lower rate at closing.

A note on advice

This page applies a standard amortizing formula to the numbers you type. It is not financial, tax, or legal advice. Compare the result with your lender's payoff quote before you send extra money.

FAQ

How do extra mortgage payments save interest?: Interest is charged on the remaining balance. Extra principal lowers that balance sooner, so every later month is charged on less. The required payment stays the same, which means a larger share of each remaining payment is principal, and the loan ends early.
What is a mortgage overpayment in the UK?: An overpayment is extra money you pay on top of the contracted monthly amount. It is the UK term for what US borrowers usually call an extra payment. This calculator models both the same way: extras go to principal and the required payment does not change.
Do UK mortgages really allow 10% overpayments a year?: Many do, and MoneyHelper describes 10% a year without a penalty as a common lender rule. It is not a legal limit and it is not applied here. Your mortgage offer sets the allowance, how the year is measured, and any early repayment charge on the excess.
How does a biweekly mortgage payment work?: You pay half the scheduled monthly amount every two weeks. That is 26 half-payments a year — one extra monthly payment. This page spreads that extra across 12 months. Confirm extras go to principal, and skip paid programs that only hold your money.
Should I recast instead of making extra payments?: Recast if you want a lower required payment and can live with the original remaining term. Pay extra if you want to finish sooner and save more interest. The recast calculator shows both a recast and a keep-paying-extra comparison.
Will I pay a prepayment penalty or early repayment charge?: Sometimes. US prepayment penalties are less common on recent mortgages, but you should still check. UK early repayment charges usually apply during a fixed period if you exceed the overpayment allowance. This calculator does not subtract either fee.
Does a lump sum lower my monthly payment?: Not on this page. The lump sum is extra principal, so you keep the original payment and finish earlier. To lower the payment without changing the rate, you need the lender to recast.
Can I share or download a calculation?: Yes. Use Share to copy a link that reopens the calculator with the same inputs, or Download to save the results and the yearly comparison as a text file.