Mortgage overpayment calculator
Use this calculator to compare your current mortgage path with an overpayment plan. See how extra monthly payments and a one-off lump sum could change your payoff date and total interest.
- Interest rate stays fixed for the full comparison.
- Interest is modelled monthly using a simple APR ÷ 12 approach.
- Overpayments reduce the term rather than your contractual payment.
- No fees, penalties, rate changes, or payment holidays are included.
Mortgage details
Enter your remaining balance, rate, term, and any overpayments you want to test.
Results
Comparison breakdown
| Measure | Standard | With overpayments | Difference |
|---|
How this mortgage overpayment calculator works
Use it to estimate how much faster you could become mortgage-free.
This calculator compares two paths. The first keeps your mortgage on its current repayment track. The second adds your regular monthly overpayment and any optional lump sum from the start month you choose.
It can help you estimate how much term you could cut off your mortgage and how much interest you might avoid paying. It is most useful for fixed-rate planning or for checking whether a modest overpayment could make a meaningful difference over the years ahead.
Actual lender calculations can vary. Some use daily interest, overpayment allowances, and admin rules that may slightly change the outcome.
Assumptions and interpretation
This estimate is designed for repayment mortgages where you are considering regular monthly overpayments.
The calculator assumes your interest rate remains unchanged for the modelled period and that every overpayment is applied straight to reduce the mortgage balance. It then compares the standard repayment path with the overpayment version to estimate interest saved and time removed from the term.
It does not account for future remortgaging, changing rates after a fixed deal ends, offset features, or lender-specific overpayment limits.
Worked UK example
Example only. Real outcomes depend on lender rules and future rates.
Example: a £220,000 repayment mortgage with 23 years left at 4.8%, plus a £150 monthly overpayment. The tool compares the normal path with the overpayment path and estimates how much earlier the mortgage could finish and how much interest may be saved.
How the Calculation Works
The calculator compares a normal mortgage repayment schedule with one where extra money is paid into the mortgage.
The calculator starts with your current balance, rate and remaining term to estimate the normal repayment path. It then reruns the same mortgage with the overpayment applied, which reduces the balance faster and therefore reduces future interest.
The gap between those two paths shows the potential term reduction and interest saving from regular overpayments.
Limitations
Mortgage overpayments are highly affected by lender rules and future rate changes.
This tool does not model introductory deals ending, ERC rules, offset arrangements, product fees on a future remortgage, or the tax impact of keeping money elsewhere instead of overpaying.
Treat the result as a planning estimate, then confirm the detail with your lender or broker before making a large change.
What to Do Next
Use the result to decide whether regular overpayments fit your wider mortgage plan.
Read Mortgage Overpayments in the UK for the practical considerations around lender rules and penalties, then compare with the Remortgage Savings Calculator if a product switch may also reduce costs.
If your deposit, buying costs, or affordability assumptions are still changing, use the Mortgage Calculator and Stamp Duty Calculator alongside it.
FAQs
Common questions about mortgage overpayments
Do mortgage overpayments always reduce interest?
Usually yes. Reducing the balance earlier means less interest is charged later, assuming your lender applies overpayments straight to principal.
Should I enter my exact monthly payment?
You can. If you leave it blank, the tool estimates a standard repayment from your balance, rate, and term. Using your lender's actual payment can give a closer result.
What if my lender uses daily interest?
This calculator uses a monthly approximation, so daily-interest mortgages may produce slightly different real-world results.
Can I use this for an interest-only mortgage?
No. This version is intended for repayment mortgages where your regular payment reduces both interest and principal.
Does a lump sum help more than monthly overpayments?
A lump sum can have a strong impact because it reduces the balance immediately. Monthly overpayments can also be powerful when maintained consistently.
What happens if I start overpaying later?
Starting later usually reduces the benefit because more interest has already been charged before your balance starts falling faster.
Should I overpay the mortgage or save instead?
That depends on your rate, savings returns, liquidity needs, and tax position. Many people first keep an emergency fund, then compare the guaranteed mortgage saving with potential savings growth.