Low return
Net return: 2.60%
Overpaying leads in this estimate
Estimated lead: £26,874.
Overpay vs invest calculator
Compare paying extra into your mortgage with investing instead.
Use one spare monthly amount and estimate whether mortgage interest saved or potential investment growth could leave you ahead over the same time horizon.
Current estimate
Investing leads in this estimate
£3,617
Difference between the two strategies by the original mortgage end date, using the assumptions below.
Low return: 2.6%Overpaying leads in this estimate by £26,874.
Base return: 4.6%Investing leads in this estimate by £3,617.
High return: 6.6%Investing leads in this estimate by £51,071.
Adviser report
Printable scenario summary
Build a print-friendly pack with the figures entered, main results, assumptions, caveats, and next discussion points. Nothing is saved and no PDF is generated on the server.
Result
Overpay vs invest snapshot
Investing leads in this estimate: £3,617
Compared at the end of the original 25 years mortgage term.
- Overpaying saves
- £51,515
- Mortgage cleared 6 years 11 months sooner.
- Investing grows to
- £168,359
- £300 invested each month.
- Overpay-first pot
- £164,743
- Freed payments invested after payoff.
- Break-even return
- 4.4%
- Annual return needed for investing to catch up.
Inputs
Your mortgage and spare cash
Planning context
These optional prompts help interpret the investment route. Fees affect the projection; wrapper and risk prompts are educational context only.
Low / base / high return assumptions
Current net return used in the calculation: 4.60%.
Result
Investing leads in this estimate
Estimated lead
£3,617
Compared at the end of the original 25 years mortgage term.
Overpaying saves
£51,515
Mortgage cleared 6 years 11 months sooner.
Investing grows to
£168,359
Investing £300 each month.
Overpay-first pot
£164,743
After mortgage payoff, the freed payment is invested.
Break-even return
4.4%
Estimated annual return needed for investing to catch up.
Result interpretation
What the overpay-vs-invest estimate may suggest
- What it means
- This may suggest which route has the higher modelled end value by the original mortgage end date, using the mortgage rate and return assumption entered.
- Number that matters most
- The estimated lead shows the gap between the strategies, while the break-even return helps show how sensitive the investment route is.
- Be careful about
- Investment returns are uncertain and can fall as well as rise. Taxes, platform fees, fund charges, ERCs, overpayment limits, and your risk tolerance are not personalised.
- Explore next
- You may want to compare a lower return assumption, check overpayment rules, and keep cash-buffer needs separate from long-term strategy choices.
Sensitivity range
See how investment return uncertainty changes the answer.
A single return assumption can make the result look too certain. These cases rerun the comparison with lower, base, and higher net investment returns.
Base return
Net return: 4.60%
Investing leads in this estimate
Estimated lead: £3,617.
High return
Net return: 6.60%
Investing leads in this estimate
Estimated lead: £51,071.
What would change this result?
- - A lower investment return can make overpaying look stronger because the mortgage saving is more certain.
- - A higher investment return can make investing lead, but it also implies accepting uncertainty and possible losses.
- - Tax wrapper, platform fees, fund charges, ERCs, and cash-buffer needs can move the real result.
Assumptions
Assumptions to review
Check these inputs before relying on the result.
- Mortgage balance
- £250,000
- The outstanding debt used for the overpayment strategy.
- Interest rate
- 4.50%
- Assumed to stay fixed for the mortgage term.
- Remaining term
- 25 years
- The comparison runs to the original mortgage end date.
- Overpayment amount
- £300
- The spare monthly amount is either paid into the mortgage or invested.
- Investment return assumption
- 4.60% after fees
- Projected annual growth after a 0.40% fee assumption, before tax.
- Tax and risk prompts
- unknown / unknown
- Used to frame the result, not to personalise tax, pension, ISA, or investment suitability.
- Product fees
- Not included
- Mortgage fees, ERCs, investment tax, and investment charges can change the comparison.
Last reviewed
12 June 2026
Interest method
Monthly interest is estimated from the annual rate unless a page states a different method. Lender figures can use daily interest, product-specific rules, and exact payment dates.
Educational scope
UK-focused calculator estimate. It explains trade-offs and does not make a personal recommendation.
How this is calculated
- The annual interest rate is converted into a monthly rate.
- Each month, interest is estimated on the current mortgage balance.
- The repayment first covers that month's interest. The rest reduces the balance.
- As the balance falls, less interest is charged and more of each payment goes towards the debt.
Main limitations
- The estimate uses the values entered on this page and does not check lender eligibility, affordability, credit history, product availability, or personal tax treatment.
- Fees, ERCs, insurance, valuation costs, legal costs, rate changes, and overpayment rules are included only where the calculator explicitly asks for them.
How investing is compared
- The overpayment route reduces the mortgage balance and estimates interest saved.
- The investing route puts the same monthly surplus into an investment projection.
- Both routes are compared at the original mortgage end date.
- Investment returns are assumed, not guaranteed, so test lower returns before relying on the lead.
Results are estimates based on the assumptions shown here. They are not financial advice and can differ from lender figures because real products, fees, rate changes, overpayment rules, and repayment timing vary.
Learn the maths
Understand the trade-off behind this comparison.
Follow the tutorial to see how mortgage interest, compound investment growth, opportunity cost, and uncertainty shape the estimate.
Timeline
How the strategies compare over time
Values are estimated strategy pots after mortgage debt is handled.
Year 0
- Overpay balance
- £250,000
- Invest balance
- £250,000
- Overpay pot
- £0
- Invest pot
- £0
Year 5
- Overpay balance
- £199,501
- Invest balance
- £219,645
- Overpay pot
- £0
- Invest pot
- £20,195
Year 10
- Overpay balance
- £136,287
- Invest balance
- £181,646
- Overpay pot
- £0
- Invest pot
- £45,601
Year 15
- Overpay balance
- £57,155
- Invest balance
- £134,080
- Overpay pot
- £0
- Invest pot
- £77,563
Year 20
- Overpay balance
- £0
- Invest balance
- £74,536
- Overpay pot
- £40,544
- Invest pot
- £117,773
Year 25
- Overpay balance
- £0
- Invest balance
- £0
- Overpay pot
- £164,743
- Invest pot
- £168,359
Recommended next steps
What to check next
A short follow-up can help put this estimate into context before you make a money decision.
Add offset savings to the comparison
Offsetting can preserve access to cash while reducing interest, so it is worth comparing alongside overpaying and investing.
Open next step
Return to the dashboard
Use the dashboard as a secondary checkpoint if you want to update your balance, LTV, monthly payment, or mortgage-free estimate before comparing offset and investment routes.
Open next step
Important disclaimer
This calculator provides estimates only and does not constitute financial, mortgage, tax, or investment advice. Investments can fall as well as rise, and mortgage overpayments may be limited by lender rules or early repayment charges.