UK lenders calculate mortgage interest by taking your outstanding balance, multiplying it by your annual interest rate, and dividing by 365 to produce a daily charge. Most lenders then recalculate that charge every day, so each repayment shrinks the balance the formula runs against almost immediately. Understanding how mortgage interest is calculated in the UK means looking at four things together: the daily formula, how often your balance is refreshed inside it, the rate that goes into it, and the actions that change the balance itself.
The Daily Interest Formula
The calculation runs in three steps. Your annual interest rate is expressed as a decimal, so 4% becomes 0.04. That decimal is multiplied by your current outstanding balance. The result is divided by 365 to give the interest charged that day. In a leap year, some lenders divide by 366 instead.
Take a £200,000 balance at 4%. The yearly interest is £200,000 × 0.04 = £8,000. Divide by 365 and the daily charge is roughly £21.92. A 30-day month brings about £657.53 in interest; a 31-day month, £679.45. Nothing else has changed, but the interest portion of your bill moves slightly with the calendar.
A small number of older contracts divide annual interest by 12 to produce a flat monthly figure regardless of how many days a month contains. That method is simpler and less precise, and daily calculation has largely replaced it across the industry.
Rest Periods: How Often Your Balance Is Refreshed
The “rest period” is the interval at which your lender updates the balance the formula runs against. It has a bigger effect on your total cost than most borrowers notice.
On a daily rest, the lender recalculates interest against whatever your balance is at the end of each day. Pay on the 5th and the remaining 25 or 26 days of that month reflect the smaller balance straight away. This is now the standard approach for the vast majority of UK residential mortgages.
On a monthly rest, interest is calculated on the balance at the start of the month. A payment made on the 5th does nothing to your interest charge until the next month begins, so you lose a few weeks of benefit from every payment. An annual rest updates the balance only once a year, so repayments made during the year keep incurring interest at the higher figure until the anniversary. Annual rest terms are rare now but still appear in some legacy products.
Daily rest is significantly better for the borrower because every pound repaid starts saving interest the same day. If you are on an older product with monthly or annual rest, switching to a daily-rest mortgage at your next remortgage can produce a meaningful saving across the remaining term.
What Determines the Rate in the Formula
The rate that goes into the daily calculation depends on the type of deal you took.
A tracker mortgage is tied directly to the Bank of England base rate, which sits at 3.75% as of February 2026. A typical tracker set at “base rate plus 0.75%” gives you 4.5%, and any base rate move changes your mortgage rate by the same amount.1Bank of England. Interest Rates and Bank Rate
A fixed-rate mortgage locks your rate for a set period, commonly two, five, or ten years. Base rate movements during that window do not affect your monthly payment. When the fixed period ends, you usually roll onto the lender’s standard variable rate unless you remortgage.
The standard variable rate (SVR) is set by the lender itself rather than pegged to the base rate. Lenders can change it at their discretion, though in practice SVRs tend to follow base rate trends with a lag. SVRs are almost always higher than the deals available to new borrowers, which is why sitting on one for long is expensive.
Between late 2024 and early 2026, the Bank of England cut the base rate from 5% to 3.75% across several decisions.1Bank of England. Interest Rates and Bank Rate Each cut lowered tracker payments directly and fed gradually into the fixed-rate deals lenders offered new borrowers.
Repayment and Interest-Only Mortgages
On a standard repayment mortgage, each monthly payment covers that month’s interest plus a slice of the debt. In the early years, interest takes most of the payment because the balance is still large. As the capital falls, the interest share shrinks and the repayment share grows. On a £200,000 mortgage at 4% over 25 years, roughly two-thirds of the first payment goes to interest and one-third to capital; by year 20, those proportions have flipped. Total interest across the full term can easily pass £100,000, which is why even small rate differences matter most at the start.
On an interest-only mortgage, the monthly payment covers only the interest. The outstanding balance stays the same throughout the term, so the interest charge does not fall unless the rate itself moves. A £200,000 balance at 4% costs about £667 a month for the whole term, and the full £200,000 is still owed at the end. Because the capital is untouched, the Financial Conduct Authority treats repayment of the capital at the end of the term as a contractual requirement and expects lenders to engage borrowers early to confirm they are on track.2Financial Conduct Authority. Guidance Consultation – Dealing Fairly With Interest-Only Mortgage Customers
APRC: The Comparison Figure Lenders Must Show
The headline rate does not capture the full cost of borrowing. The Annual Percentage Rate of Charge (APRC) expresses the total annual cost across the whole mortgage as a single percentage, rolling in arrangement fees, valuation charges, and the reversion to a higher rate after any initial deal period ends.
Lenders are required to disclose the APRC when offering a mortgage. Two products with the same headline rate can carry very different APRCs: a low starter rate paired with a large arrangement fee and a steep SVR reversion will produce a higher APRC than a slightly higher headline rate with no fees.3Financial Conduct Authority. Annual Percentage Rate of Charge (APRC) Calculations The figure is most useful when comparing deals of the same term length, and less useful for short-horizon decisions, because it assumes you stay with the same lender for the full term.
How Overpayments Cut the Interest You Pay
Because the daily formula multiplies your rate by your outstanding balance, every extra pound you pay in reduces tomorrow’s interest charge. On a daily-rest mortgage, that reduction kicks in the same day.
Most fixed-rate mortgages allow overpayments of up to 10% of the outstanding balance each year with no penalty. On a £200,000 balance, that means an extra £20,000 across the year before any charge applies; if the balance has dropped to £150,000, the allowance drops to £15,000. The allowance usually resets on the anniversary of the mortgage or the fixed-rate start date. Going beyond the 10% limit triggers an early repayment charge on the excess. Tracker and SVR products often have no overpayment cap at all, which suits borrowers with irregular income or occasional lump sums.
One boundary worth flagging: early repayment charges also apply if you clear the mortgage or remortgage away during the initial deal period, and they typically run from 1% to 5% of the outstanding balance. The charge schedule sits in your mortgage offer, and it is worth checking before making any decision that pays down more than the allowance.
How Your Term Length Changes Total Interest
Stretching a mortgage over a longer term reduces the monthly payment but pushes total interest up sharply. On a £200,000 mortgage at 4%, moving from a 25-year term to a 35-year term can add more than £70,000 to the total interest bill, because the balance falls more slowly and the daily formula runs against a higher figure for longer.
Longer terms have become more common, with many first-time buyers now taking 30- or 35-year mortgages to pass affordability checks. If your finances improve later, shortening the term at your next remortgage, or using the annual overpayment allowance, recovers a meaningful share of that extra interest.
Notice You Must Receive Before a Rate Change
The FCA regulates UK mortgage lending through its Mortgage Conduct of Business (MCOB) rules. MCOB 7A.2 requires your lender to notify you before any change to your interest rate or monthly payment takes effect, including the new payment amount and details of any change to payment frequency.4FCA Handbook. MCOB 7A.2 Notification of Interest-Rate Changes
The point of that notice is time to act. If a tracker rate is moving because the base rate changed, or a fixed period is ending and the loan is about to revert to the SVR, you should hear from the lender with enough lead time to remortgage elsewhere if you decide to. The APRC disclosure at offer stage gives you the standardised figure to compare those alternatives against your current deal.3Financial Conduct Authority. Annual Percentage Rate of Charge (APRC) Calculations