Mortgage Rate Calculator

Rather than answering a single “what would my payment be” question, this calculator is built for comparing rates against each other — stress-testing what your payment would look like if rates rose before your current fix ends, or comparing what a specific remortgage rate would actually cost against your current deal. The same loan amount and term, run through different rates, shows exactly how sensitive your payment genuinely is to rate movements.

Stress-Testing What Happens When Your Fix Ends

Anyone on a fixed-rate mortgage will, at some point, come to the end of that fix and need a new deal at whatever rate is available then — and that future rate is genuinely unknown today. Running your current balance and term through this calculator at a rate a percentage point or two above your current one gives a realistic sense of what your payment could look like if rates have risen by the time your fix ends, rather than being caught off guard by a payment increase that could have been reasonably anticipated in advance.

Doing this well before your fix actually ends, rather than only at renewal time, leaves genuine room to adjust — whether that means building a larger buffer into monthly savings, reconsidering other planned spending, or simply going into the renewal conversation with realistic expectations rather than an unwelcome surprise.

Why a 1% Rate Change Moves the Payment More Than It Sounds Like It Should

A single percentage point sounds modest, but its effect on a monthly mortgage payment is considerably larger than the number itself suggests, particularly on a sizeable balance over a long term — because the extra interest compounds against the balance every single month across the whole remaining term, not just once. This is genuinely worth seeing in your own numbers rather than assuming a small rate difference produces an equally small payment difference; the two are related, but the relationship is not a simple one-to-one scaling.

This is exactly why lenders themselves apply meaningful stress-test margins when assessing affordability at the application stage — they are working from the same underlying reality that a modest-looking rate movement can genuinely strain a household budget once translated into actual monthly pounds rather than a headline percentage.

The Same Balance, Three Rate Scenarios

A £240,000 balance with 22 years remaining costs roughly £1,394 a month at 4.2%. The same balance and term at 5.2% — just one percentage point higher — rises to roughly £1,528 a month, an extra £134 monthly, or over £1,600 a year. At 6.2%, two points above the starting rate, the payment climbs to around £1,668 — an extra £274 a month, or close to £3,300 a year, compared with the original rate. The same balance, the same term, a dramatically different monthly commitment depending purely on where the rate lands.

Using This Before You Remortgage, Not After

When comparing a remortgage offer against your current deal, running both rates through this calculator with the identical balance and term gives a genuinely fair, apples-to-apples comparison — rather than trying to compare two headline rates in the abstract without seeing what each actually translates to in pounds per month. This is worth doing before committing to a specific new deal, not after, since it is precisely the comparison that reveals whether a marginally better-looking rate genuinely produces a meaningful saving once run through your actual numbers.

Any product fee attached to a new deal is worth factoring into this comparison too, rather than looking at the rate and monthly payment alone — a lower monthly payment with a large upfront fee can occasionally work out more expensive overall than a slightly higher payment on a fee-free deal, depending on how long you actually intend to keep the mortgage on that specific rate.

The Assumption Every Rate Stress Test Should Include

A realistic stress test should use a rate meaningfully above current levels, not simply a fraction of a percent, since rates have moved by two percentage points or more within a relatively short period in recent UK mortgage history — treating a rise of that magnitude as implausible would have been a mistake for anyone whose fix ended during a period of rising rates. Checking whether your household budget could genuinely absorb a payment at a meaningfully higher rate, not just the rate you currently enjoy, is the whole point of running this exercise before it becomes an urgent, real decision rather than a hypothetical one. Our Mortgage Calculator is useful for a single, straightforward calculation including your LTV band, and our Ditch Your Fix Calculator is worth checking if you are specifically weighing up whether to break your current fix early for a better rate now, rather than waiting for it to end naturally.

Frequently Asked Questions

Important Information

This calculator provides an estimate based on the figures you enter and does not constitute financial advice or a prediction of future interest rates. For advice specific to your circumstances, consult a mortgage broker or FCA-regulated adviser. See our Disclaimer for further information.