UK Mortgage Overpayment Calculator
Estimate how monthly and lump-sum overpayments change interest and mortgage payoff time.
What this calculator explains
Overpaying a mortgage reduces principal sooner, which can lower later interest and shorten the term. The saving depends on the current balance, rate, timing and whether the lender keeps the normal payment unchanged or recalculates it.
The calculator supports a lump sum paid now and recurring monthly extras while keeping the normal monthly payment unchanged. It shares the annual allowance across both, charges ERC only on the excess during the entered charge period, and deducts charges from interest savings. A visible warning reminds users to check product limits and charges before paying extra.
How the estimate is calculated
- Build the baseline amortisation schedule from balance, rate, payment and term.
- Apply the monthly overpayment after scheduled interest and payment.
- Apply the lump sum immediately, capped at the outstanding balance.
- Stop when the balance reaches zero and calculate interest paid.
- Compare baseline and overpayment term and interest.
- Reset the allowance at the entered month and annually thereafter. Apply the entered ERC rate only to excess overpayments within its remaining charge period, then subtract those charges from interest savings.
Formula
Interest saved = baseline total interest − overpayment-scenario interest; net saving also subtracts any entered early-repayment charge
Worked example
A £250,000 balance at 4.5% with 25 years remaining has a modeled payment of about £1,389.58. Adding £200 per month in a simplified constant-rate model repays the loan in roughly 239 months instead of 300, about 5 years and 1 month earlier, and reduces estimated interest by about £38,458 before any charge.
Rules and assumptions
- Overpayments reduce principal earlier and can lower later interest, but the saving depends on timing, rate and how the lender recalculates payments.
- The model must support both recurring and one-off overpayments and an optional early-repayment charge.
- Users must be told to check their mortgage contract for limits and charges.
Included in the estimate
- Monthly and lump-sum overpayments
- Baseline and revised amortisation
- Term reduction and interest saving
- Entered overpayment allowance
- Optional early-repayment charge
- Editable allowance percentage, balance basis, allowance already used and reset timing
Not included or not guaranteed
- A lender’s exact daily-interest calculation
- Automatic future rate changes
- Contract interpretation
- Offset-account treatment unless separately modeled
- Advice to overpay instead of saving or investing
Does mortgage overpayment reduce the term or payment?
That depends on lender treatment and borrower instruction. This calculator keeps the normal payment unchanged and estimates a shorter term; it does not recalculate a lower regular payment.
How much can I overpay without a charge?
Check the mortgage offer and current lender rules. A commonly seen allowance is not a universal entitlement.
Is a lump sum better than monthly overpayments?
Earlier principal reduction generally saves more interest, but liquidity, charges and personal goals matter.
Should I overpay or save the money?
Compare mortgage rate, savings return, emergency funds, taxes and access to cash. The calculator only models the mortgage effect.
Why does my lender quote differ?
Lenders can use daily interest, specific payment dates and contractual recalculation methods. Toolistify provides a planning schedule.
Can I model a future rate change?
No. This projection holds the entered mortgage interest rate constant throughout. A future rate change or changing ERC percentage requires a separate scenario.
Official sources
Applicable period: 2026 · Last reviewed: September 3, 2026
Related tools
Compare more than one overpayment amount
Try different monthly extras and a lump sum paid now, then compare net savings using your lender’s allowance and charge period.