UK mortgage & home-finance calculators

What makes a UK mortgage different

If you have borrowed in another country, four things about the UK will surprise you, and each one changes the arithmetic rather than the paperwork.

There is no mortgage insurance. A US borrower below 20% down pays PMI, a Canadian pays a CMHC premium, an Australian pays LMI. A UK borrower at 90% or 95% loan-to-value pays none of these. What you pay instead is a higher interest rate, priced into the deal by LTV band. That makes the UK unusually cheap at high LTV in monthly terms and unusually expensive on the tax side, which is the second surprise.

Stamp Duty Land Tax is large and paid upfront. It is charged in bands on the purchase price, with a separate and lower set of bands for first-time buyers, and a surcharge on additional property that applies from the first pound rather than above a threshold. On a second home it can be several times the bill on the same property bought as a main residence. It is due within 14 days of completion and cannot be added to the mortgage, so it has to be found in cash alongside the deposit.

Fixed rates are short. A US borrower fixes for 30 years. A UK borrower fixes for two or five, occasionally ten, and then faces the deal ending. What happens at that point is the single most expensive moment in a UK mortgage, and it is covered below.

Borrowing is capped by an income multiple, not only by affordability. Most lenders work to around 4.5 times income, and the Financial Policy Committee limits how much of a lender's book can sit above that multiple. So two borrowers with identical outgoings and different salaries can be offered very different amounts, and clearing an existing monthly commitment often raises your figure more than a pay rise would.

The moment a UK mortgage gets expensive

When a fixed deal ends, you do not stay on that rate and you are not automatically moved to a new one. You revert to the lender's standard variable rate, which is set by the lender rather than tracking the Bank of England base rate, and which averages a little over 7%.

The gap is the point. A competitive five-year fix at low LTV has recently been available near 4.25%. Reverting to an SVR near 7.13% on a £250,000 balance costs roughly £450 a month more, and six months of drift while you get round to remortgaging costs about £2,700. Nothing warns you, and the lender has no obligation to move you.

This is why the UK has two words for the same act. A product transfer is taking a new deal from your existing lender, which usually needs no affordability check and completes quickly. Aremortgage is moving to a new lender, which needs a full application and often gets a better rate. Most people take the transfer because it is easy, and the difference between the two is worth checking rather than assuming.

Overpaying, and the limit nobody reads

Most UK fixed deals allow you to overpay 10% of the outstanding balance each year without penalty. Above that, an early repayment charge applies, typically a percentage of the amount repaid and usually stepping down each year of the fix.

Two things about overpaying are worth knowing before you start. Ask your lender to apply the overpayment toreduce the term rather than to reduce the monthly payment, because the two produce very different savings from the same money. And check whether your deal reprices the overpayment allowance annually or on the anniversary of the deal, since the two dates are rarely the same.

What the rate actually follows

The Bank of England base rate was held at 3.75% at the Monetary Policy Committee meeting on 30 July 2026, on a 6-3 vote, with the three dissenters preferring a rise.

That rate governs trackers directly and standard variable rates loosely, at the lender's discretion. It does not set fixed rates. Fixed deals are priced off swap rates, which move on expectations rather than on decisions, which is why a fix can get cheaper in the weeks before a cut and why the headline after a base rate announcement often has no effect on the deal you are offered.

Where the calculators fit

Each of the calculators below exists because one of the above needs arithmetic rather than explanation. The affordability calculator applies the income multiple and the outgoings test. The Stamp Duty calculator holds the current bands, including the first-time buyer relief and the additional property surcharge, each checked against HMRC and dated. The overpayment calculator shows the term-reduction difference. And the LTV calculator tells you which rate band your deposit puts you in, which on a UK mortgage is the closest thing there is to mortgage insurance.

Every figure is computed from the underlying formula rather than repeated from another site, and every statutory value carries the date it was last checked against the primary source.