Mortgage Comparison Calculator

What this calculator does

This mortgage comparison calculator sets two deals against each other on the same property price and deposit. Vary the rate, the term, or both, and it returns the monthly repayment, the total interest, and the total cost of each, so a choice between products becomes three concrete numbers rather than a hunch about which rate looks better.

It also produces a year-by-year table for each option showing annual payments, equity built, and the balance outstanding. The balance figure is the one to watch when a fixed deal ends, because it determines your LTV at remortgage and therefore which rate band you fall into next time around.

When to use it

The commonest use is comparing two products at remortgage — typically a lower rate with a product fee against a slightly higher rate with none. Because UK fees are charged as flat amounts rather than percentages, the answer flips depending on your balance, and only arithmetic settles it.

It is equally useful for the term question when a broker suggests stretching from 25 to 30 or 35 years to make the sums work, and for comparing a product transfer from your existing lender against a full remortgage elsewhere. Ahead of a deal expiry, model your current rate against your lender's standard variable rate to see the size of the jump you are trying to avoid.

Understanding the inputs

Property price and deposit set a shared borrowing amount, so only the rate and term differ between the two scenarios. Enter the product rate as advertised — the calculator handles the monthly conversion — and remember it applies only for the deal period of typically two, three, or five years.

Term in the UK commonly runs 25 to 35 years, and lenders increasingly approve terms that end well into your sixties. Product fees are not an input, so handle them separately: take the monthly difference the calculator gives you, multiply by the number of months in the fix, and compare that total against the fee. That single step is what most deal comparisons get wrong.

How is this calculated?

Calculate monthly payment and total cost for both mortgages using the standard amortization formula.

A worked example

Take a £300,000 property with a ten percent deposit, so £270,000 borrowed over 25 years. Deal A is a two-year fix at 4.35 percent with a £1,499 product fee, giving a repayment of about £1,478 a month. Deal B is a two-year fix at 4.75 percent with no fee, at about £1,539 a month.

Deal A saves roughly £61 a month, which across the 24-month fix is about £1,475 — just short of the £1,499 fee. On this balance the fee-free deal is marginally better, and the gap is small enough that the free valuation on one or the other would decide it. On a £400,000 balance the same rate gap would save around £2,190 over two years and the fee-paying deal would clearly win.

Limitations and assumptions

This compares repayments only. Product and arrangement fees, valuation fees, conveyancing on a remortgage, and any cashback incentive are excluded, and on UK deals those items frequently decide the outcome. Early repayment charges are also outside the model, which matters if you might sell or overpay heavily during the deal period.

The calculator assumes a single rate for the full term, which no standard UK fixed product actually does — figures beyond your deal period assume you keep the same rate rather than reverting to a standard variable rate or remortgaging. It models repayment mortgages, not interest-only or offset arrangements. Use it to rank products over the fix, then check the full illustration each lender provides for the fees it leaves out.