Mortgage rate change calculator: How much will my mortgage go up?

Use our calculator to work out how much your mortgage payments will go up if the interest rate changes.

A home owner moving from a fixed mortgage deal to the standard variable rate could pay significantly more in monthly mortgage payments if the rate is higher.

Research by Finder found that almost a third (31%) of home owners have let their mortgage slip into a higher rate for at least 1 month after their fixed-rate deal has ended.

Use our mortgage rate change calculator to see how any interest rate increase would impact you and work out whether you need to fix a new mortgage deal sooner rather than later!

Mortgage calculator
Use the fields above to estimate mortgage costs.

How does our mortgage rate increase calculator work?

1. Enter the total amount you owe on your mortgage into the “Drawdown” box.

2. Enter any mortgage fees into the “fees” box and then select whether you pay them upfront or with your mortgage repayments.

3. Enter your fixed rate into the “Intro rate” box and the length of your fix in months into the “Intro term” box. For example, if you have a 3-year fix, this will be 36 months.

4. Enter the “Revert rate” you will move into once your fixed-term mortgage ends.

5. Enter the full term of your mortgage into years to the “Full term” box and press calculate.

What do my results mean?

The results give you a breakdown of mortgage costs at your fixed rate and at the new “revert rate” as well as any fees you need to pay.

The “intro instalment” shows your monthly payments during the fix, while your “revert instalment” shows the new monthly payment when your mortgage rate changes. You might notice a significant jump in costs if you move from a lower fixed rate to a higher variable rate.

The results also show the total amount you would pay during your fixed mortgage period and the amount the mortgage would cost over the full term if you stayed at a higher rate.

How much does forgetting to renew a mortgage deal cost the average home owner?

Finder research shows that almost a third (31%) of home owners have let their mortgage slip into a higher rate for at least 1 month after their fixed-rate deal has ended. The total time during which people had let their mortgage “revert” to a higher rate was an average of 10 months over their mortgage, according to the survey.

This means that the average home owner could be paying a total of £3,000 in unnecessary mortgage repayments by forgetting to renew their mortgage deals.

Someone paying off the cost of the UK’s average house, worth £281,913 as of January 2024, on a competitive fixed 3-year rate of 5.5% would pay £1,361 per month during those 3 years.

But if they didn’t remortgage immediately at the end of the initial fixed term, the interest rate would revert to the lender’s standard variable rate, which was typically around 7.5% when the research was conducted. This would cost them £1,661 per month, which is an extra £300. The average person paying 10 months of this would, therefore, part with an extra £3,000 to pay the extra interest.

While the average time that home owners in the survey had left their revert rate going was 10 months, over 1 in 10 (11%) had paid a higher “revert rate” for more than 1 year. Worryingly, 3% said they’d paid a revert rate for over 5 years. This would cost over £30,000 in extra interest.


Finder commissioned Censuswide to carry out a nationally representative survey of adults aged 18 and over. The survey ran between 18-22 April 2024, and a total of 2,000 people were questioned throughout Great Britain, with representative quotas for gender, age and region.

A typical 3-year fixed-term mortgage rate of 5.5% was used, assuming a downpayment of 15% and, therefore, an LTV of 85%. A typical rate for this type of mortgage then reverts to (7.5%) was then used. No product fees or additional fees were included in the calculations.

Click here for more research. For all media enquiries, please contact:

Matt Mckenna
UK communications manager
M: +44 747 921 7816
T: +44 20 8191 8806

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