Cautious case
- Mortgage rate
- 5.50%
- Offset savings
- £3,750
- Best growth route
- Invest
Strategy decision calculator
Compare offsetting, overpaying, and investing spare cash.
Estimate how a lump sum and monthly surplus could perform if used to overpay the mortgage, sit in an offset account, or be invested instead.
Quick read
Best for growth potential
Invest
Based on the net position at the end of the original mortgage term. This is an estimate, not a recommendation.
Adviser Mode
Review this calculation with a client or adviser.
Adviser Mode brings the charts, assumptions, and side-by-side comparison points into a print-friendly review pack. Nothing is saved and no account is created.
Comparison
Three-way strategy snapshot
Best for certainty: Offset
Standard payment estimate: £1,390 per month over 25 years.
- Best for flexibility
- Offset
- Useful if access to cash still matters.
- Best for growth
- Invest
- Depends on uncertain investment returns.
- Overpay interest saved
- £79,544
- November 2041
- Offset liquidity
- £120,000
- Cash modelled as accessible in the offset route.
Inputs
Mortgage, cash, and offset assumptions
Offset mortgage details
Add existing savings that would sit in the offset account and any monthly offset fee.
Planning context
These optional prompts help frame the offset route against cash access and the interest your savings might earn elsewhere.
Savings interest / opportunity cost explanation: offsetting can reduce mortgage interest while keeping cash accessible, but the same cash might otherwise earn savings interest. Tax, access rules, and offset fees can change the comparison.
Comparison
Strategy summary
Best for certainty
Offset
Best for flexibility
Offset
Best for growth
Invest
Standard mortgage payment estimate: £1,390 per month over 25 years.
Result interpretation
What the three-way strategy comparison may suggest
- What it means
- This may suggest how the same lump sum and monthly surplus compare across debt certainty, accessible cash, and growth potential.
- Number that matters most
- There is no single winner for every user. Interest saved, liquidity, and projected investment value each answer a different planning question.
- Be careful about
- Offset fees, savings interest you give up, overpayment limits, ERCs, tax, investment charges, investment volatility, and liquidity needs can change which route looks strongest.
- Explore next
- You may want to test conservative return assumptions, compare the narrower tools, and ask a mortgage adviser to confirm lender-specific rules.
Sensitivity range
Test how rate, offset, and return assumptions change the range.
These cases rerun the comparison with cautious, base, and optimistic assumptions so the result is not treated as a fixed prediction.
Base case
- Mortgage rate
- 4.50%
- Offset savings
- £5,000
- Best growth route
- Invest
Optimistic case
- Mortgage rate
- 3.50%
- Offset savings
- £6,250
- Best growth route
- Invest
What would change this result?
- - A higher mortgage rate can make overpaying or offsetting look stronger because mortgage interest avoided becomes more valuable.
- - A larger offset balance can improve flexibility, but offset fees and access rules still matter.
- - A higher investment return can improve the investing route, but it is uncertain and may not be achieved.
Assumptions
Strategy comparison assumptions
Check these inputs before relying on the result.
- Mortgage balance
- £250,000
- The outstanding debt used for the overpay and offset routes.
- Interest rate
- 4.50%
- Assumed to stay constant across the modelled mortgage term.
- Remaining term
- 25 years
- The time horizon used to compare all three strategies.
- Overpayment amount
- £25,000 lump sum + £300 monthly
- Used for the overpay route, or redirected to offset or invest in the alternative routes.
- Investment return assumption
- 5.00%
- Projected annual growth before tax, platform fees, and fund charges.
- Product fees
- £10 monthly offset fee
- Offset fees are included. ERCs, investment charges, tax, and other mortgage fees are not personalised.
- Savings opportunity cost
- 3.50% savings rate prompt
- Shown as planning context only; the core comparison still focuses on mortgage interest, offset fee, liquidity, and investment projection.
- Liquidity need
- unknown
- A prompt to consider how important access to the lump sum is before choosing between offsetting, overpaying, or investing.
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 the three strategies are compared
- The overpay route applies the lump sum and monthly surplus to the mortgage.
- The offset route keeps cash accessible while reducing the balance on which interest is charged.
- The invest route projects the lump sum and monthly surplus using the investment return assumption.
- The result compares interest saved, projected investment value, liquidity, and net position over the mortgage term.
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-offs behind the three-way comparison.
Follow the tutorial to see how offset savings, mortgage overpayments, and investing compare across interest, liquidity, growth potential, and risk.
Overpay
Best for debt certainty when overpayments are allowed and you do not need the cash back.
Interest saved
£79,544
Mortgage-free date
November 2041
15 years 5 months
Projected investment value
£0
Liquidity: Low
£0
Offset
Best for flexibility because savings can reduce mortgage interest while staying accessible.
Interest saved
£84,727
Mortgage-free date
June 2046
20 years
Projected investment value
£0
Liquidity: High
£120,000
Invest
Best for growth potential, but returns are uncertain and investments can fall as well as rise.
Interest saved
£0
Mortgage-free date
June 2051
25 years
Projected investment value
£265,685
Liquidity: Medium
£265,685
Visual comparison
Interest saved and accessible cash
Interest saved
Overpay
£79,544
Offset
£84,727
Invest
£0
Liquidity comparison
Overpay
£0
Offset
£120,000
Invest
£265,685
Timeline
Estimated net position over time
Year 0
- Overpay
- -£225,000
- Offset
- -£220,000
- Invest
- -£225,000
Year 5
- Overpay
- -£168,206
- Offset
- -£161,872
- Invest
- -£167,159
Year 10
- Overpay
- -£97,112
- Offset
- -£89,107
- Invest
- -£93,886
Year 15
- Overpay
- -£8,116
- Offset
- £1,978
- Invest
- -£1,050
Year 20
- Overpay
- £0
- Offset
- £102,000
- Invest
- £116,590
Year 25
- Overpay
- £0
- Offset
- £120,000
- Invest
- £265,685
Strategy cards
What each route tends to suit
Best for certainty
Overpaying or offsetting can reduce mortgage interest in a more predictable way than investment returns.
Best for flexibility
Offsetting keeps savings accessible while still reducing the balance interest is charged on.
Best for growth
Investing can lead over long periods, but the outcome is uncertain and depends on returns, tax, fees, and risk.
FAQ
Offset, overpay, and invest questions
Is it better to offset, overpay, or invest?
There is no single best choice. Overpaying can give more certainty, offsetting can preserve access to cash, and investing may offer higher growth with more risk. This calculator compares estimates, not advice.
Why does liquidity matter?
Liquidity is how accessible your money remains. A mortgage overpayment usually reduces debt but may be hard to access later, while offset savings and investments may be easier to use if your plans change.
Does an offset mortgage beat a savings account?
An offset can be attractive when the mortgage interest avoided is higher than the after-tax savings interest you would otherwise earn, but offset rates, fees, and access rules vary by lender.
What investment return should I assume?
Use a cautious long-term annual return assumption and remember that investments can fall as well as rise. The result is sensitive to the return you enter and does not include all taxes or platform costs.
Does this include tax or financial advice?
No. It is an educational estimate only. It does not replace mortgage, tax, or investment advice and does not include every fee, tax rule, early repayment charge, or product restriction.
Recommended next steps
What to check next
A short follow-up can help put this estimate into context before you make a money decision.
Review remortgage options
After comparing spare-cash strategies, check whether a new deal, fees, ERCs, and LTV bands could change the route to paying the mortgage off.
Open next step
Return to the dashboard
Use the dashboard as a secondary checkpoint to update equity, LTV, and mortgage-free timing before moving from strategy comparison into remortgage planning.
Open next step
Important disclaimer
This calculator is educational only and is not mortgage, investment, tax, or financial advice. It assumes constant mortgage rates, offset access, fees, surplus cash, and investment returns. Investments can fall as well as rise, and mortgage overpayments may be limited by lender rules or early repayment charges.