Daily interest and UK mortgage overpayments

Daily interest in plain English

A daily-interest mortgage tracks interest for each day using that day’s relevant balance and an annual rate converted according to the lender’s convention. The daily amounts are then charged or reported under the account process. When capital falls, later daily interest is calculated on a smaller amount, assuming the rate and other factors are unchanged.

“Calculated daily” does not tell you every detail. Lenders can differ in day-count basis, rounding, transaction cut-off, effective date and when the monthly payment is recalculated. A planning calculator can approximate the calendar spacing but cannot reproduce an undisclosed servicing system.

Posting date matters more than transfer date

The day money leaves a current account may not be the day it reduces the mortgage balance. A bank transfer made late on Friday or with an incorrect reference may post later. Ask the lender which payment route and reference to use, whether there is a cut-off time, and when the account will show the credit.

Keep confirmation and check the mortgage account. For a large transfer, consider sending a small verified payment first if the lender recommends it and if doing so does not create extra fees or allowance complications.

Why a lump sum usually acts sooner

If £1,200 is available now and no charge applies, crediting it today lowers the relevant balance before twelve monthly £100 payments would. That timing difference is why lump-sum and monthly scenarios should not be treated as identical. However, the advantage must be weighed against the need for accessible cash and any income the cash could earn while held.

A calculator should place the lump sum at its assumed date and recurring extras on each payment date. A tool that simply subtracts £1,200 at year end can understate the effect of monthly overpayments; a tool that subtracts all future payments today can overstate it.

Payment recalculation can change the path

After an overpayment, a lender may reduce the required monthly payment. If the borrower then pays only the new lower amount, some of the planned acceleration is given back through smaller future capital payments. Alternatively, keeping the total outgoing unchanged can continue the faster path. Neither is universally right: payment relief and earlier payoff are different goals.

Nationwide’s published guidance, for example, says it calculates interest daily and describes different recalculation treatment by payment size, while allowing eligible borrowers to choose an overpayment preference. That is a lender example, not a rule for all mortgages.

A transparent daily-interest estimate

1. Start with today’s balance and the next actual payment date.
2. Accrue interest for the calendar days to that payment.
3. Apply the regular payment, then the extra amount, according to the stated model.
4. Repeat using actual month lengths until the balance reaches zero.
5. Run a baseline without the extra payment using the same assumptions.
6. Compare total modelled interest and payoff dates, then subtract any known ERC or fee.

The calculator method guide explains further limits, including rate changes and negative amortisation checks.

Example: month end and a bank holiday

A borrower wants to pay a lump sum on the final day of an allowance period, which is also a bank holiday. Their current-account transfer date does not prove the lender will post it that day. They ask the lender for the cut-off and allowance treatment, then schedule with enough margin. Avoiding uncertainty over a contractual charge is more important than squeezing out one extra day of modelled interest reduction.

Checks after the payment

• Did the amount reach the correct mortgage account and sub-account?
• What effective date is shown?
• Did the balance fall by the expected amount?
• Was any ERC or fee added?
• Did the required monthly payment or term change?
• How much allowance remains?

Daily calculation supports the general logic of earlier balance reduction; it does not override the contract. Keep emergency money accessible and use the lender’s records as the source of truth.