Why remortgaging isn't just about the interest rate
It's tempting to remortgage the moment you spot a lower advertised rate, but the genuine financial benefit depends on the full picture: your monthly saving weighed against every cost of switching, including any Early Repayment Charge from your current deal, new product or arrangement fees, and valuation or legal costs. This calculator brings all of these together to show a realistic break-even point — the moment your switching costs are fully recovered through monthly savings.
The theory behind the break-even calculation
This simple but powerful formula tells you how many months it takes for your accumulated monthly savings to equal (and then exceed) what you spent to switch. If your break-even point is well within the period you intend to keep the new mortgage, switching is very likely a sound financial decision. If it's close to, or longer than, the length of your new deal, the switch may not be worthwhile — you could end up paying to switch again before you've even recouped the cost of this one.
How your monthly payment is calculated here
This calculator uses a standard mortgage repayment (capital and interest) formula, applying your outstanding balance, interest rate and remaining term to calculate a monthly payment — the same underlying mathematics used across the mortgage industry for standard repayment mortgages. It calculates this twice: once at your current rate, and once at the new rate on offer, to derive the monthly saving.
Why Early Repayment Charges deserve careful attention
If you're still within a fixed or discounted deal period, your current lender may charge an Early Repayment Charge (ERC) for leaving before the agreed term ends — often calculated as a percentage of your outstanding balance, sometimes tapering down each year of the deal. An ERC can be a genuinely significant cost, and it's essential to include it accurately in this calculation, since it directly extends your break-even point. See our full guide on what happens when your fixed deal ends for more on understanding and timing around ERCs.
Fees that are sometimes free
Many remortgage deals include free valuation and free basic legal work as an incentive to switch — always check exactly what's included in any specific deal before assuming these costs apply, since including unnecessary costs in this calculator will make switching look less attractive than it may actually be. Conversely, some deals with an apparently attractive headline rate carry higher fees than others, which is exactly why comparing total cost (not just the rate) matters.
Product transfer vs full remortgage: does this calculator apply to both?
Yes — this calculator works equally well for comparing a product transfer with your existing lender (where you'd typically have no ERC, and often lower or no new fees) against your current rate, or a full remortgage to a new lender (which more commonly involves the full range of switching costs modelled here). See our detailed comparison of product transfers vs remortgaging to decide which route is likely to suit your situation best before running your numbers.
What this calculator doesn't capture
This tool focuses purely on the financial break-even calculation. It doesn't account for non-financial considerations — such as wanting to change your mortgage structure (for example, moving to an offset mortgage), needing to borrow additional funds as part of the switch, or a lender's specific eligibility criteria, which could affect whether the "new rate on offer" you've entered is actually one you'd qualify for. Always confirm actual eligibility and rates with a lender or broker before making a final decision.
The bottom line
A lower headline rate doesn't automatically mean remortgaging is worthwhile once fees and any Early Repayment Charge are factored in. This calculator's break-even figure gives you a clear, honest way to judge whether a specific switch makes financial sense given how long you're likely to keep the new deal — the single most useful number in any remortgaging decision.