How much will my mortgage go up when my fixed rate ends

What changes when the fix ends

A fixed rate freezes your payment for the length of the deal. MoneyHelper puts it simply: if you have a fixed-rate mortgage, your payments won't change until the fixed period ends and you move to your lender's standard variable rate (SVR). That is the moment the payment is recalculated, and it is the number most people want to know in advance.

The SVR is set by each lender. MoneyHelper says it usually follows the Bank of England's base rate, and that when it changes you could see a different bill as soon as your next payment. It sits well above the base rate. The Bank of England calls it the revert-to rate and publishes a monthly average: 6.58% in August 2026, when Bank Rate itself was 3.75%.

To estimate the jump you need four things: your balance today, the number of years left on the mortgage, your current fixed rate and the rate you are moving to. That last one is either your lender's SVR, which your lender can confirm, or the rate on a new deal you are considering.

The formula behind every repayment calculator

For a repayment mortgage, the monthly payment is M = P × r × (1 + r)^n ÷ ((1 + r)^n − 1). P is the balance, r is the annual interest rate divided by 12 (so 6% becomes 0.005), and n is the number of monthly payments left. This is the standard annuity formula, and it assumes interest is charged monthly and the rate stays the same for the rest of the term.

You can check it against MoneyHelper's own worked example. On a £200,000 mortgage over 20 years at 5%, MoneyHelper shows a monthly payment of £1,320. Putting P = 200,000, r = 0.05 ÷ 12 and n = 240 into the formula gives £1,319.91.

Interest-only mortgages are simpler. The payment is the balance times the annual rate, divided by 12, because none of the capital is being repaid. On £186,400 that is about £278 a month at 1.79% and about £1,022 at 6.58%, which is why a rate change hits interest-only borrowers proportionally harder.

A worked example

Take a household that fixed for five years in February 2022, when the Bank of England's average quoted five-year fixed rate at 75% loan to value was 1.79%. Their fix ends in February 2027. They now owe £186,400 with 21 years (252 payments) left.

At 1.79% the formula gives a monthly payment of £887.94. At 6.58%, the August 2026 average revert-to rate, it gives £1,366.57. That is £478.63 more a month, or £5,743.56 over a year if the rate did not move. Spread across the year it comes to about £15.73 a day.

A new fixed rate changes the picture. The Bank of England's average quoted five-year fixed rate at 75% loan to value was 4.78% in August 2026. On the same balance and term that gives £1,173.37 a month: £285.43 more than the old fix, but £193.20 less than the average SVR. Averages are only a guide. The rate you are offered depends on your lender, how much equity you have and your circumstances.

What the Bank of England expects in 2026

The Bank of England's Financial Stability Report of July 2026 estimates that a typical owner-occupier rolling off a fixed rate in the next two years will see monthly repayments rise by £45, far less than the median rise of about £120 between the end of 2022 and the end of 2024. Most two-year fixes ending by the end of 2028 are expected to refinance close to their existing rate.

Older, cheaper fixes are a different story. The same report says nearly 750,000 households paying less than 3% interest will roll off their fixes in 2026 and see an average increase of £170 a month. A little over 5 million households are projected to see their repayments increase by the end of 2028.

Those projections assume borrowers move onto a typical new fixed rate when their deal ends, not onto the SVR. Staying on the revert-to rate, as the worked example shows, can cost considerably more than the figures in the report. UK Finance expected around 1.8 million fixed-rate mortgages to come to an end in 2026.

Why your lender's figure may not match

Your calculation and your lender's letter can differ by a few pounds, and sometimes by more. MoneyHelper's own example assumes, for simplicity, that interest is charged monthly and the rate never changes. Your lender's method and dates may not match those assumptions exactly, and on a repayment mortgage your balance on the day the fix ends will be a little lower than today's.

Fees change the result too. In MoneyHelper's example, a 4.4% deal with a £2,000 arrangement fee added to the loan ended up with higher monthly payments and a higher total cost than a 4.5% deal with no fee. If you add a fee to the balance, add it to P before you calculate.

Finally, the SVR is not fixed. MoneyHelper notes that lenders are likely to pass base rate changes on to SVR customers, so a figure worked out today is a snapshot.

Common calculation mistakes

Using the original loan. The payment is based on what you owe now and the years left, not the amount you borrowed or the term you started with. Your lender can confirm both.

Comparing the SVR with Bank Rate. In August 2026 the average revert-to rate was 6.58% while Bank Rate was 3.75%. Use your lender's actual SVR, not the base rate.

Multiplying the yearly rate by the balance. That gives interest-only interest for a year, not a repayment mortgage payment. Use the formula, or a calculator built on it.

Treating the result as final. Rates change, and the Bank of England averages are updated every month. RateCliff runs this same calculation on your own numbers, your fixed rate against your lender's SVR, and shows the SVR and the date you entered it next to the result so you can see when it needs checking.

This guide is general information, not financial advice. For your own figures and options, ask your lender, an FCA-registered mortgage broker or adviser, or MoneyHelper, which gives free guidance.

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