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Mortgage Overpayment Calculator

Use this mortgage overpayment calculator to see how extra payments could reduce your interest and shorten your mortgage. Enter what you still owe, your rate and term, then add a one-off or recurring overpayment plan.

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What kind of overpayments?
Choose one-off (a single extra payment), recurring (extra payments you make regularly), or both.
What type of mortgage is it?
Repayment pays off interest and the balance each month. Interest-only pays just the interest, and the balance is repaid separately.
One-off overpayment
When will you make it?
Without overpayments
Monthly payment£1,461.48
Total interest£188,442.53
Time remaining25 years
With overpayments
Monthly payment£1,461.48
Total interest£177,228.86
Time remaining24 years 1 months
Interest saved
£11,213.67
Time saved
11 months
Total overpaid
£5,000.00
Share this overpayment scenario
Copy a link that keeps your current numbers, so you can come back later or send it to someone.
How this mortgage overpayment calculator works

This calculator compares your mortgage with and without overpayments, to estimate how much time and interest you could save.

  • For a repayment mortgage, your normal monthly payment stays the same. Overpayments reduce the balance faster, so you may finish sooner and pay less interest.
  • For an interest-only mortgage, the monthly interest is based on what you still owe. Overpayments reduce what you owe, so the monthly interest can fall over time.

It’s a simplified model and does not include fees, rate changes, or lender rules (such as limits on overpayments).

Example model
This box is a fixed sanity-check using the default example numbers. It does not change when you edit the calculator.
Mortgage typeRepayment
Current balance£250,000.00
Interest rate5%
Remaining term25 years
One-off overpayment£5,000.00
One-off timing1 years
Example outputs
Interest saved£11,213.67
Time saved11 months
Total overpaid£5,000.00
Mortgage warning

This is a guide only. Some mortgages charge early repayment fees or limit how much you can overpay each year.

Before making overpayments, check your mortgage terms or speak to your lender or a qualified adviser.
How it works

This page compares two timelines side-by-side:

  • Baseline: what happens if you just keep paying as normal.
  • With overpayments: what happens if you add a one-off payment and/or recurring overpayments.

The calculator then reports the differences: months saved and interest saved.

Important: lender rules (ERCs, annual allowance caps, admin fees) are not included. Treat this as a planning model and check your mortgage offer for conditions.

Rates and thresholds

Last verified: 05/03/2026. Overpayment limits/charges vary by lender and product and aren’t modelled here.

There are no universal “overpayment thresholds” the calculator can apply automatically. Instead, you enter the overpayment amounts and timing you want to test, and the model shows the potential savings under a constant-rate assumption.

Worked examples

Worked examples (rounded):

Example 1 — £250,000 balance at 5.00%, 20 years remaining: £1,000 at month 12 + £50/month

  • Baseline term remaining: 240 months
  • With overpayments: 227 months
  • Months saved: 13
  • Interest saved: £9,546.43

Example 2 — £250,000 balance at 5.00%, 20 years remaining: £5,000 at month 6

  • With overpayment: 233 months (months saved: 7)
  • Interest saved: £8,039.62

Example 3 — £250,000 balance at 5.00%, 20 years remaining: £200/month recurring

  • With overpayments: 200 months (months saved: 40)
  • Interest saved: £27,578.29
Edge cases and exclusions
  • ERCs and allowances: some deals charge for early repayment or cap fee-free overpayments; this model ignores charges.
  • Interest-only vs repayment: interest-only overpayments behave differently; ensure you pick the correct mortgage type.
  • Changing rates: if your rate will change soon, test multiple rate scenarios.
  • Payment timing: an overpayment earlier in the schedule usually saves more interest than the same amount later.
  • Payment holidays: not modelled; these can increase interest and extend the term.
Assumptions
  • This is a simplified estimate and your lender’s rules may be different.
  • It assumes your interest rate stays the same for the remaining term.
  • Overpayments are applied after the normal monthly payment (simple model).
  • It does not include lender fees, early repayment charges, product fees, or taxes.
Methodology

Without overpayments: estimates interest and time remaining with your current balance, rate and term.

With overpayments: applies your one-off and/or recurring overpayments to reduce the balance faster.

Interest saved and time saved are the differences between the two scenarios.

What to do next
FAQ
What does the mortgage overpayment calculator compare?
It compares a baseline mortgage with a scenario that includes your overpayments, then estimates the difference in interest and time to repay.
Why does this overpayment calculator ask whether the mortgage is repayment or interest-only?
Overpayments behave differently: repayment overpayments usually shorten the term, while interest-only overpayments reduce the balance so the interest-only cost can fall.
If I choose “one-off” overpayment, what does the timing field mean?
It is when the extra payment happens, measured from now. You can enter it in years or months to match how you think about the timing.
How does the recurring overpayment frequency affect the results on this page?
A higher frequency means more overpayments over the year, which can reduce interest and term faster. The calculator applies your chosen schedule consistently.
Why does the calculator warn about early repayment charges (ERCs)?
Many mortgages limit annual overpayments or charge an ERC in certain periods. This tool does not apply those lender rules, so you should check your product terms.
Does the mortgage overpayment calculator model interest rate changes over time?
No. It assumes a constant rate for the period being modelled. Rate changes can materially change the savings.
What is counted as “total overpaid” in the results section?
It is the sum of your extra payments in the overpayment scenario. It is shown separately from the normal scheduled payments.
Why can the “monthly payment” look unchanged even after I add overpayments?
For repayment mortgages, lenders often keep the contractual payment the same and reduce the remaining term instead. This tool reflects that idea in a simplified way.
If I overpay, does it always make sense to shorten the term rather than reduce payments?
Not necessarily. Some lenders let you reduce payments, shorten the term, or both depending on your deal. This calculator focuses on time and interest impact, not lender-specific options.
How should I represent a lump sum I plan to pay after a bonus or inheritance?
Use the one-off overpayment amount and set the timing to when you expect the lump sum. If the date is uncertain, run a few timings to see sensitivity.
Why does changing the remaining term change the savings so much?
Savings depend on how long interest would have been charged on the balance. With a longer remaining term, there is usually more interest left to save.
What does the “interest saved” figure on this overpayment calculator include and exclude?
It is an estimated difference between the baseline and overpayment scenarios under a fixed-rate assumption. It excludes fees, ERCs, and any opportunity cost of using cash elsewhere.
Is this overpayment calculator accurate enough for a lender decision?
It is for guidance only. For an exact figure, check your lender’s overpayment terms and ask for a redemption or overpayment illustration.
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