| TL;DR: Most UK mortgages charge interest daily, calculated as the outstanding balance multiplied by the annual rate and divided by the number of days the lender uses in a year, which is not always 365, and this convention difference explains most small discrepancies borrowers find when checking their own numbers. Last reviewed July 2026 |
| MORTGAGES : DAILY INTEREST CALCULATIONS |
Most residential mortgages in the UK calculate interest on a daily basis: the outstanding balance is multiplied by the annual interest rate, then divided by the number of days the lender treats as being in a year, before being multiplied by the number of days in the period. Some lenders use 365 consistently, others use 365.25 to average in leap years, and this difference alone can explain why a borrower's own spreadsheet does not exactly match the lender's official figure.
KEY FACTS
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The basic daily interest formula
The standard approach most UK mortgage lenders use is to calculate a daily interest charge as the outstanding mortgage balance multiplied by the annual interest rate, divided by the number of days the lender uses to represent a year, giving a daily interest amount, which is then accumulated over the number of days in the period being calculated, commonly a calendar month.
As a simplified illustration, on a balance of £250,000 at an annual rate of 4%, using a 365-day year, the daily interest would be £250,000 multiplied by 0.04, divided by 365, giving approximately £27.40 a day. Over a 30-day month with no change in balance, that comes to roughly £821.92 for the month.
Why the day-count convention creates small discrepancies
The detail that trips up many borrowers checking their own figures is which number of days the lender actually uses to represent a year. Some lenders use exactly 365 days every year, ignoring leap years for this specific calculation. Others use 365.25, which averages in the extra day that occurs roughly every four years, producing a very slightly lower daily rate and therefore a slightly lower monthly interest figure than a 365-day calculation on the same balance and rate.
The table below illustrates this using a representative £250,000 balance at 4% for a 30-day period, to show how much difference the convention alone makes.
| Day-count convention | Daily interest rate factor | Interest over 30 days |
| 365 days used | Annual rate divided by 365 | Approximately £821.92 |
| 365.25 days used | Annual rate divided by 365.25 | Approximately £821.30 |
The difference in this example is only around 62p over a month, which is genuinely small, but it is enough to explain why an independent spreadsheet built assuming one convention will not exactly match a lender's statement built on the other, without anything having actually gone wrong.
Why interest is calculated daily but charged monthly
Even though the interest calculation itself happens daily, most mortgage accounts only apply, or debit, the accumulated interest to the balance once a month, typically on the anniversary of the mortgage or a fixed statement date. This means the balance used for the daily calculation stays the same throughout the month unless a payment or overpayment changes it, and then updates in one step when the monthly interest is applied.
This distinction, between daily calculation and monthly application, is a common source of confusion, since it can look at first glance as though interest is simply a flat monthly charge, when it is actually the sum of many small daily calculations that happened to be totalled and applied together.
Why the timing of an overpayment changes how much you save
Because interest is calculated daily on the current outstanding balance, an overpayment made early in a month reduces the balance that interest is calculated against for every remaining day of that month, saving more in interest than an identical overpayment made a few weeks later, even though both reduce the balance by the same amount overall. This is why some mortgage guidance specifically suggests making a planned overpayment as early in the month, or as early in an interest period, as possible.
The effect of timing on any single overpayment is generally modest, but for someone making regular monthly overpayments over many years, consistently paying early in the cycle rather than late can meaningfully add up over the life of the mortgage.
Why your own maths might not match your lender's exactly
Beyond the day-count convention, small discrepancies between a borrower's own calculation and a lender's official statement can also arise from exactly when in the day a payment is treated as received, how the lender rounds intermediate figures, and whether any fees or adjustments were applied during the period being checked. None of these are usually large enough to represent a genuine error, but together they can produce a gap between an independent spreadsheet and an official statement that is worth understanding rather than assuming is a mistake.
If a discrepancy is larger than a few pounds over a typical month, or persists and grows over several months, that is a reasonable point at which to contact the lender directly and ask them to explain, in writing, the exact convention and dates used for the specific period in question, since this removes any ambiguity about which numbers are correct.
Where to find your lender's exact method
The specific day-count convention a lender uses, along with the exact date interest is applied each month, is set out in the mortgage's terms and conditions document, though it is not always prominently highlighted in customer-facing summaries or online account dashboards. Requesting this detail directly from the lender, or checking the original mortgage offer documentation, is the most reliable way to confirm exactly how your specific mortgage calculates interest.
Once you know the exact convention your lender uses, building your own tracking spreadsheet to match it becomes straightforward, and is a reasonable way to independently verify your statements over time, particularly if you are making irregular overpayments and want to track their effect precisely.
Does this work differently on a fixed or variable rate mortgage
The daily calculation method itself, balance multiplied by rate divided by the day-count convention, applies in the same way whether a mortgage is on a fixed rate, a tracker, or a standard variable rate. The only thing that changes between these product types is how often, and under what circumstances, the annual rate itself can move.
On a fixed-rate deal, the rate stays constant for the fixed period, so month-to-month changes in the interest charged come only from changes in the outstanding balance. On a tracker or variable rate mortgage, the rate itself can change between statement periods, meaning both the rate and the balance can shift the monthly figure, which is worth remembering if comparing interest charged across several months on a mortgage that is not fixed.
| Note: Day-count conventions and interest application dates vary between lenders and mortgage products. Confirm the exact method used on your specific mortgage directly with your lender rather than assuming a single standard applies universally. |
| RELATED GUIDES |
| Disclaimer: Kael Tripton Ltd is an independent editorial publisher, ICO-registered (ZC135439). This guide is general information, not financial, legal or debt advice, and carries no commission or referral arrangement. Your circumstances may differ; consider speaking to a regulated adviser or a free debt charity before acting. Figures and thresholds change; verify current numbers with the primary sources listed below. |
Frequently asked questions
Why does my own interest calculation not exactly match my mortgage statement?
Most commonly because of the day-count convention: some lenders use 365 days in a year for this calculation, others use 365.25, which produces small but genuine differences.
Is mortgage interest charged daily or monthly?
Interest is typically calculated daily based on the outstanding balance, but only applied, or debited, to the account once a month on a fixed date.
Does it matter when in the month I make an overpayment?
Yes, to a modest degree. An overpayment made earlier in the month reduces the balance that interest is calculated on for more days, saving slightly more interest than the same overpayment made later.
How can I find out exactly how my lender calculates interest?
Check your mortgage's terms and conditions document, or contact your lender directly and ask them to confirm the day-count convention and interest application date used on your specific account.
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