Compare two mortgages on a like-for-like basis over the fixed period.
Most mortgage calculators — and lender quotes — show you the headline monthly payment, which adds any upfront fee to the loan and spreads it over the full mortgage term (often 25–30 years). That flatters deals with large fees, because the fee barely nudges the monthly figure when smeared across decades, even though you only benefit from the low rate during the fix (typically 2–5 years).
This calculator shows the effective monthly payment, which is closer to how much a mortgage actually costs. It's the monthly payment that makes both mortgages end the fix at the same balance — so whichever costs less over the fix is unambiguously cheaper, because you walk away in the identical financial position either way. In effect, the fee is absorbed over the fix period rather than smeared over 25+ years, so a £2,000 fee on a 2-year fix shows up as roughly £83/month of extra cost, which can easily change the cheapest mortgage deal! Other mortgage calculators can make high-fee deals otherwise look deceptively attractive.
Here's the catch: the lender bills the headline payment, not the effective one. So if the effective payment is higher, and you just pay what they ask, the mortgage will end up with a higher balance, meaning the saving above isn't quite in your pocket yet. There are two ways you can approach this:
1. Overpay by the difference. Set a standing order for the effective payment instead of the headline one; the extra goes straight to capital. Most lenders allow 10% overpayments per year penalty-free, far more than you'd need. Even when overpaying, you're paying less per month than you would with the other mortgage.
2. Enjoy the extra cashflow. Paying only the headline means less going out each month, at the cost of a higher balance at the end of the fix. To be clear: you're still in a better financial position overall, but when the fix expires you can put that saved cash toward the mortgage, or simply remortgage the higher balance — though your payments at renewal will be a touch higher as a result. Comparing the "Balance at end of fix" can help you weigh this up.
It won't show you the full amortisation schedule, year-by-year interest and capital breakdowns, or projections past the fix period. This calculator is solely to help you compare two mortgages. For whole-term modelling, such as what happens when you roll onto the SVR or remortgage, use a dedicated amortisation tool or speak to a broker.
It also doesn't account for overpayments, early repayment charges, cashback, offset features, or product transfer quirks. These can swing the answer on edge cases, but for a standard fee-vs-rate tradeoff the effective payment gives you the right answer.
Headline monthly payment: the standard payment where the fee is added to the loan and amortised over the full mortgage term. This is typically the figure the lender quotes you.
Effective monthly payment: the payment that makes both mortgages end the fix at exactly the same balance, so cost over the fix is a direct like-for-like comparison.
Cost over fix: the effective monthly payment × number of months in the fix — the like-for-like cost of each deal during the fixed period.