Mortgage Overpayment Calculator

A mortgage overpayment calculator shows how much interest you save and how many years you cut from your mortgage term by making regular or one-off extra payments. It is used by homeowners who want to reduce their total borrowing cost and pay off their mortgage earlier than the original schedule.

How to Use the Mortgage Overpayment Calculator

1. Enter your current outstanding mortgage balance.
2. Enter your current interest rate and remaining term in years.
3. Enter the amount you want to overpay, either as a regular monthly addition or a one-off lump sum.
4. The calculator computes your new payoff date and the total interest saved compared to making standard repayments only.
5. Review the before and after comparison to decide whether overpaying is the best use of your available funds.

The Formula

Regular payments reduce the outstanding balance faster than the scheduled amortisation. The interest saved is calculated by comparing the total interest paid under two scenarios:

Standard schedule: total interest = sum of all monthly payments minus the original loan balance.

Overpayment schedule: total interest = sum of all standard plus overpayments minus the original loan balance.

The difference is the interest saved. The reduction in term is the number of months removed from the original schedule.

Real-World Example

You have a £200,000 mortgage with 20 years remaining at 4.5% interest. Your standard monthly payment is £1,265.

Option A: You overpay by £200 per month, making your total monthly payment £1,465.

New payoff time: approximately 16 years and 3 months. Years saved: approximately 3 years and 9 months. Total interest without overpayment: approximately £103,600. Total interest with overpayment: approximately £80,200. Interest saved: approximately £23,400.

Option B: You make a single lump sum overpayment of £10,000 today.

Interest saved: approximately £13,000, and the term reduces by around 14 months.

Consistent monthly overpayment tends to save more interest than an equivalent lump sum, because the benefit compounds over a longer period.

Is Overpaying Always the Best Option?

Overpaying your mortgage is effectively earning a guaranteed return equal to your mortgage interest rate. If your rate is 4.5%, overpaying is equivalent to a 4.5% risk-free return on that money. Compare this to alternative uses such as paying down higher-rate debt, maximising your ISA allowance, or building an emergency fund. Most financial planners suggest prioritising high-interest debt and an adequate emergency fund before overpaying a mortgage. Also check your lender's overpayment limit. Most fixed rate mortgages allow overpayments of up to 10% of the outstanding balance per year without penalty. Exceeding this triggers early repayment charges.

Frequently Asked Questions

How much can I overpay without penalty? Most fixed rate mortgages allow overpayments of up to 10% of the outstanding balance per year. On a £200,000 mortgage, that is up to £20,000 per year in overpayments. Check your specific mortgage terms, as some lenders allow more or less. Variable and tracker mortgages often permit unlimited overpayments.

Does overpaying reduce my monthly payment or shorten my term? This depends on how your lender applies the overpayment. Some lenders recalculate the monthly payment to reflect the lower balance, reducing your monthly cost. Others keep the payment the same and shorten the term. Shortening the term saves more interest, so check with your lender which approach they use.

Should I save or overpay my mortgage? If your mortgage rate is higher than the after-tax return you can reliably achieve by saving, overpaying may be more beneficial. If your savings account or ISA earns more than your mortgage rate, saving may be preferable. The comparison should be made on an after-tax basis.

When is a lump sum overpayment most effective? Lump sum overpayments made early in the mortgage term save the most interest because the balance is largest and there are more years over which interest compounds. Even small lump sums early in the term can save disproportionately large amounts of interest.

What the page's example does over twenty years

The worked example above is a £200,000 mortgage at 4.5% over 20 years. The standard amortisation formula gives a monthly payment of £1,265.30. Left alone, that mortgage costs £103,671.70 in interest across its 240 months and £303,671.70 in total. Adding £200 to every monthly payment raises the payment to £1,465.30, clears the balance at month 192 and cuts the interest bill to £80,735.45. The saving is £22,936.25.

The table below runs both mortgages side by side, year by year. Every figure comes from the same monthly calculation: interest is charged on the balance at the start of the month, then the payment is applied.

Two things stand out. The gap between the two balances widens for most of the term, from £5,012.81 at the second year to £46,687.97 at the fourteenth, because the overpayment keeps arriving every month and every pound of it removes interest that would otherwise have been charged. Then the gap closes in a single step at month 192, when the smaller loan finishes and the larger one still owes £55,487.08. The standard borrower pays £2,800.82, £1,636.96 and £363.71 in the last three years of the schedule while the overpayer pays nothing at all.

Counted to the tenth year the overpayer has paid £67,684.06 in interest against £73,923.67 on the standard schedule, a difference of £6,239.61 at that point. The remaining £16,696.64 of the final saving arrives after year ten, which is the half of the term that the overpayment buys.

An overpayment ladder on the same mortgage

The next table holds the mortgage, the rate and the term fixed and moves only the size of the monthly overpayment. The term is what changes.

The ladder shows diminishing returns as the overpayment grows. The first £100 a month saves £12,948.80. The second £100 saves £9,987.45. The third saves £7,955.64. Moving from £300 to £500, which is £200 a month more, saves a further £11,904.09, or £5,952.05 for each extra £100. Each pound of overpayment only saves interest on the months it is actually outstanding, and a larger overpayment finishes the loan sooner, so the later pounds have fewer months left to earn anything. A borrower with £500 a month spare therefore gets more total benefit by applying it from the start than by adding it in steps later on.

A lump sum paid at three different points

A single £10,000 lump sum behaves the same way. Paid early it works on the whole balance for the whole term. Paid late it catches only the tail of the loan.

The Option B paragraph above quotes roughly £13,000 saved and a term reduction of around 14 months for a £10,000 lump paid today. The year-by-year calculation on the same balance, rate and payment gives £13,746.30 and 18 months. The difference is the assumption about how the lender applies the money. The figures in this table hold the monthly payment at £1,265.30 and shorten the term. A lender that instead recalculates the payment down to reflect the lower balance leaves the term close to 20 years and saves less, because the monthly outflow falls with the balance and less money stays at work.

The 10% annual allowance on this mortgage

Most fixed rate mortgages cap penalty-free overpayments at 10% of the outstanding balance each year. On the example above the cap moves as the balance falls.

The £200 a month in the worked example comes to £2,400 a year, comfortably inside the allowance for the whole term. The cap only becomes a real constraint for a borrower overpaying aggressively, and it binds hardest early, when the allowance is largest but so is the interest saving. A borrower who intends to overpay more than the allowance should check the charge for exceeding it before committing, because an early repayment charge can cancel the interest saved.

What the overpayment calculation assumes

Five assumptions sit behind every figure on this page.

1. Interest is calculated once a month on the balance at the start of the month, and the payment lands at the end of the same month.
2. The rate stays at 4.5% for the whole term. A variable or tracker mortgage changes this, and a rate rise makes overpaying more valuable.
3. The monthly payment is held at its original level and the overpayment shortens the term. This is the assumption that produces the largest saving.
4. There are no arrangement fees, no early repayment charge and no payment holidays.
5. The overpayment is made every month from the first month, with no gaps.

Change the third assumption and the answer changes materially. Where the lender keeps the term and reduces the payment instead, the saving is smaller and arrives as a lower monthly outflow rather than as an earlier payoff date. Both are legitimate outcomes, and the calculator on this page shows the term-shortening version.

Also try these free tools related to Mortgage Overpayment Calculator: - Mortgage Calculator

• Amortization Calculator