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Mortgage Overpayment Calculator

See how much interest you could save — and how many months or years sooner you could be mortgage-free — by overpaying your mortgage.

£

Your current outstanding mortgage balance.

%
years
£

Extra amount you'll pay on top of your normal monthly payment.

£

Optional one-off overpayment, e.g. from a bonus or inheritance. Leave at 0 to skip.

%

Most lenders allow penalty-free overpayments up to this % of your balance per year before Early Repayment Charges apply.

Interest you'd save£41,393.87
Time saved62 months≈ 5.2 years
New payoff time238 months≈ 19.8 years
Total interest (baseline)£177,588.02
Total interest (with overpayments)£136,194.15
Baseline monthly payment£1,425.29
Baseline term300 months≈ 25.0 years

How this calculator works

Every month, your mortgage interest is charged on your outstanding balance. An overpayment goes straight to reducing that balance, so every future month's interest is calculated on a smaller number — the mortgage pays itself off faster, and the total interest charged over its life falls. This calculator builds two full amortisation schedules side by side: a baseline schedule at your normal payment, and awith-overpayments schedule that applies your monthly overpayment (and optional lump sum) directly to principal each period, then compares the two:

Interest saved = Baseline total interest − New total interest

The calculator also checks your planned overpayments against a lender overpayment cap you can edit (most UK lenders default to around 10% of the balance per year, penalty-free) and warns you if your plan would exceed it in the first year, since going over the limit typically triggers an Early Repayment Charge (ERC).

Worked example

Take a £250,000 repayment mortgage at 4.75% over 25 years. Without overpaying, the baseline monthly payment is about £1,425, and total interest over the full 300-month term comes to roughly £177,590. Now add a £200 monthly overpayment: total interest falls to around £136,190 — a saving of about £41,390 — and the mortgage is paid off in roughly 238 months instead of 300, about 62 months (just over 5 years) sooner. That's the effect of a relatively modest overpayment compounding over two decades.

What affects your result

  • Size of the overpayment — larger regular overpayments save proportionally more interest, since more of the balance is cleared sooner.
  • Timing — overpaying earlier in the term saves more interest than the same amount paid later, because it stops accruing interest sooner.
  • Your interest rate — the higher your rate, the more each pound of balance costs you in interest, so overpaying is worth proportionally more on higher-rate mortgages.
  • Remaining term — a longer remaining term gives overpayments more time to compound, so the same monthly overpayment saves more interest on a 30-year term than a 10-year one.
  • Your lender's overpayment allowance — exceeding the penalty-free cap (commonly ~10% of the balance per year) can trigger an Early Repayment Charge that eats into any savings.

A note on accuracy

This calculator computes both schedules on full, unrounded figures and only rounds for display, so the interest and time-saved figures reflect your exact inputs. It assumes your lender keeps your contractual monthly payment fixed and simply shortens the term (the most common approach) — some lenders instead let you reduce the monthly payment while keeping the original term, so check which applies to your mortgage. For guidance on overpaying and Early Repayment Charges, seeMoneyHelperand the Financial Conduct Authority.

Frequently asked questions

How much will I save overpaying my mortgage?

It depends on your balance, rate, term, and how much you overpay — but even modest, regular overpayments can save tens of thousands of pounds in interest. For example, on a £250,000 mortgage at 4.75% over 25 years, overpaying by just £200 a month cuts total interest from around £177,600 to about £136,200 — a saving of roughly £41,400 — and pays the mortgage off around 5 years earlier. Enter your own numbers into the calculator above for an exact figure.

How much can I overpay my mortgage without a penalty?

Most UK lenders let you overpay a set percentage of your outstanding balance each calendar year — commonly 10%, though some products allow 20% or unlimited overpayments — before an Early Repayment Charge (ERC) applies to the excess. This limit is set by your specific mortgage product, not by law, so check your mortgage offer or annual statement for your exact allowance. The calculator above flags it if your planned overpayments would exceed the percentage you enter.

What are the benefits of overpaying my mortgage?

The two main benefits are less total interest paid (since interest is charged on your outstanding balance, so a smaller balance means less interest going forward) and a shorter remaining term. Overpaying can be especially effective early in the mortgage, when the balance — and therefore the interest charged on it — is at its highest. The trade-off is that the money is tied up in your home rather than available elsewhere, so it's worth weighing overpaying against other priorities like an emergency fund, higher-interest debt, or pension contributions.

Is it better to overpay monthly or with a lump sum?

Both reduce your balance and therefore future interest, and mathematically an earlier overpayment (of either kind) saves slightly more interest than the same amount paid later, because it stops accruing interest sooner. Regular monthly overpayments are easier to build into a budget and compound their effect every month; a lump sum (for example from a bonus or inheritance) delivers a one-off reduction in the balance. The calculator above lets you model either, or both together.

Do overpayments reduce my monthly payment or shorten my term?

This depends on your lender — most UK mortgages keep your contractual monthly payment the same and simply pay the mortgage off earlier (which is what this calculator assumes), but some lenders let you choose to instead reduce your monthly payment while keeping the original term. Ask your lender which option applies to your product, since it changes how the benefit shows up day to day even though the total interest saved is similar.

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