How savings interest is calculated daily

Your savings statement says you earned £12.41 this month. Where that number comes from is not magic or an estimate — it is a one-line formula run every night, and once you can run it yourself, no bank can confuse you again.

The per-day formula

Each day, the bank computes: today's interest = balance × annual rate ÷ days in the year. A £10,000 balance at 4% in a 365-day year earns 10,000 × 0.04 ÷ 365 ≈ £1.10 per day. The bank does this calculation once a day, records the accrual, and adds it to your balance monthly or annually depending on the product. Interest itself is the price of lending your money to the bank; the accrual mechanics are the same machinery described generally at Wikipedia's interest entry. The formula never changes — only the timing of the additions does.

When your money actually starts earning

The timing rules are where real money leaks. Deposits typically start earning on the day they clear (which is why the deposit date and the value date can differ). Withdrawals stop earning the day they leave. Bonuses often run on a separate clock entirely. And the daily calculation is exactly why: because the formula runs on the balance each night, every pound's journey through the month is priced separately. Two accounts with the same headline rate and different timing rules can pay noticeably different totals — the statement is the only honest comparison.

Daily versus monthly compounding

When interest is added to the balance before the next day's calculation, it starts earning interest itself — compounding. Daily compounding (accrual calculated daily and added to the balance regularly) beats monthly, which beats annual, at the same headline rate. This is why the AER (annual equivalent rate) exists: it is the rate that would produce the same final total if interest were paid once a year, so products with different compounding schedules can be compared on one number. If you remember one thing: compare AER, not the headline rate. The arithmetic of compounding itself is in compound interest explained, and the rate-comparison habit is in APR versus APY.

The two-line check

Once a month, run it yourself: take your average balance for the month (the statement usually shows it), multiply by the rate, divide by 365, multiply by the days in the month. If the answer is within pennies of the statement's interest line, the account is doing what it says. If it is consistently far off, three suspects arrive in order: a rate change you missed (the bank must notify you), a bonus clock that has expired, or a timing rule you misread. That check takes ninety seconds and is the entire maintenance burden of savings interest — the rest is choosing a rate worth checking.

Leap years and changing balances

Some products use 365 as the denominator throughout the year; others use 366 during a leap year or follow a contractual convention described in the account terms. The difference on an ordinary balance is small, but it matters when you are reconciling a statement to the penny. A changing balance also means the month’s interest is not simply the closing balance multiplied by the rate: each day has its own starting balance, and deposits or withdrawals affect only the days after their value date. This is why a rough average-balance check is useful but not an exact audit.

Rate changes and bonus periods

Variable-rate accounts can change the rate during a statement period. The bank then calculates each day using the rate that applied that day, so a simple month-end calculation will be off. Introductory bonuses may expire on a set date or after a fixed number of months, and the underlying rate can be much lower. Keep the opening confirmation and rate-change notices, then compare the statement’s credited interest with the daily formula across each rate period. If the terms are unclear, ask the provider to explain the value date, compounding frequency and rate history for the account.

Sources and further reading

Links were reviewed 2026-09-29. Regulatory permissions, firm status and product terms can change; use the current official register before acting.

Wikipedia — Interest (accrual and compounding)