Key takeaways
What this article covers, in order:
- Before you start the clock
- Minutes 0 to 5: Is the opening balance still right?
- Minutes 5 to 10: Search for the exact amount
- Minutes 10 to 15: Halve it
- Minutes 15 to 20: Divide it by 9
- Minutes 20 to 25: Dates, period and duplicates
If your bank reconciliation won't balance, don't start again from the top. Check the opening balance, search for the exact difference, then halve it, then divide it by 9, then look at dates and duplicates, in that order. Those five checks find the large majority of differences, and you can run all of them in half an hour. Here's the routine for finding a bank reconciliation difference, minute by minute.
Before you start the clock
Dev runs a print shop in Leicester. It's Friday afternoon, 2 Oct 2026, and his September 2026 reconciliation is out by £418.50. The adjusted bank balance is £418.50 higher than the adjusted book balance.
He's already been staring at it for an hour, scrolling up and down. That's the least effective way to find a difference, because your eyes skim right past the problem line every time.
So: put the kettle on, write the difference on a sticky note, and set a timer for 30 minutes. You're going to run tests, not hunt.
One thing to confirm first. Have you actually finished matching? If there are still unticked lines on the statement, you don't have a difference yet. You have unfinished work.
Minutes 0 to 5: Is the opening balance still right?
Compare this period's opening balance in your books with the closing balance on last period's reconciliation report.
If they don't agree, stop here. Someone (possibly you) has changed, deleted or back-dated a transaction in a period you'd already reconciled. Your difference lives in that earlier month, and searching Sep 2026 won't find it.
Dev checks: last period closed at £6,204.33, and the books open Sep 2026 at £6,204.33. Fine. The problem is in Sep 2026.
This test takes two minutes and saves hours. It's also the best argument for locking periods once they're reconciled.
Minutes 5 to 10: Search for the exact amount
Search both the books and the statement for £418.50. A single missing or extra transaction often equals the difference precisely.
Things to try:
- Search the books for 418.50
- Search the statement or bank CSV for 418.50
- Search for 418 and 419, in case of rounding or a typo in the pennies
Dev finds nothing. No transaction for £418.50 anywhere. Next test.
Minutes 10 to 15: Halve it
Divide the difference by two. £418.50 ÷ 2 = £209.25.
Why halve? Because if a transaction is posted in the wrong direction (money in recorded as money out, or the other way round) your books are wrong by twice the amount. A £209.25 receipt recorded as a £209.25 payment swings the balance by £418.50.
Dev searches for £209.25. There it is: a refund from a paper supplier for a damaged delivery, £209.25 received on 17 Sep 2026. In the books it's been recorded as a payment to that supplier, because the description had the supplier's name on it and the categorisation defaulted to "paid out".
He flips it to money in, against the supplier. The reconciliation balances to the penny. Fourteen minutes.
This is the single most useful test in the routine. Refunds, reversals and chargebacks are the usual suspects, because they look like their opposite.
Minutes 15 to 20: Divide it by 9
If halving doesn't work, divide the difference by 9. If the answer is a whole number (or whole pennies with nothing left over), you've probably got a transposition error, where two digits swapped places.
Examples:
| Should be | Typed as | Difference | Difference ÷ 9 |
|---|---|---|---|
| £1,450.00 | £1,540.00 | £90.00 | 10.00 |
| £63.00 | £36.00 | £27.00 | 3.00 |
| £812.40 | £821.40 | £9.00 | 1.00 |
The test works because swapping any two adjacent digits always produces a difference divisible by 9. It won't tell you which transaction, but it tells you what kind of mistake to look for: a typed entry, probably manual, where the digits look familiar but slightly wrong.
A related trick: if the difference is a round number like £100.00 or £1,000.00, look for a slip in one digit, such as £356.20 entered as £456.20.
Minutes 20 to 25: Dates, period and duplicates
Still out? Now look at things that don't change the amount but put it in the wrong place.
Wrong period. A payment dated 1 Oct 2026 in your books but 30 Sep 2026 on the statement will sit outside the period you're reconciling. Sort your Sep 2026 transactions by date and look at the first and last few days.
Wrong account. If you have more than one bank account, a transaction might be recorded against the savings account instead of the current account. Check the other account for an entry of the difference amount.
Duplicates. If you've imported a CSV and you also have a live bank feed, or you entered something by hand and then the feed brought it in again, you'll have two of the same transaction. Sort by amount and look for pairs.
Batched amounts. One deposit on the statement might be three customer payments in your books. Make sure the three add up to the deposit, and that none of them is also matched elsewhere.
Minutes 25 to 30: Park it properly
If the timer goes off and you're still out, stop. Seriously.
Post the difference to a clearly named suspense account with a note: "Sep 2026 bank rec difference, £xx.xx, under investigation". Finish the rest of your month-end, and come back on Monday with fresh eyes.
What you must not do is post it to a random expense to make it disappear. A suspense entry is honest. It says "this is unexplained". An entry to "sundry expenses" pretends it's been explained.
If it's still a mystery after a second look, ask your bookkeeper or accountant. Someone who didn't create the error spots it far faster than someone who did.
The cheat sheet
Stick this inside the cupboard door, or pin it in your notes.
| The difference is... | Most likely cause | Where to look |
|---|---|---|
| Exactly one transaction amount | Missing or extra entry | Search books and statement for the amount |
| Twice a transaction amount | Posted in the wrong direction | Refunds, reversals, chargebacks |
| Divisible by 9 | Transposed digits | Manually typed entries |
| A round number | One digit wrong | Larger manual entries |
| Equal to last period's difference | Opening balance changed | Last period's reconciled transactions |
| Several hundred pounds, no pattern | Duplicate or batch mismatch | Sort by amount, check imported CSVs |
| A few pennies | Rounding on foreign currency or a typo | Non-sterling transactions, pence fields |
Why this works better than scrolling
Scrolling relies on spotting an error you don't know the shape of. The tests turn the difference into a clue. Most reconciliation differences in a small business come from a short list of mistakes, and each test points at one of them.
It also stops the panic. Having a method means you're making progress even when the first test comes up empty.
How HelloBooks helps
A lot of these errors start at entry, so the review step matters. In HelloBooks, transactions from your Open Banking feed (most UK banks and cards work) or a CSV import land in a review list, where you confirm or change the category before they hit your books.
On the reconcile screen, each statement line has an AI match suggestion with a confidence score and the reason it chose that match. A low-confidence suggestion is a hint to look properly before accepting. If you accept a wrong match, you can unmatch it in one click.
Once a period is reconciled and signed off, you can lock it. Any reopening is logged, which makes the "has the opening balance changed?" test much less likely to fail in the first place. The reconciliation report (opening balance, cleared items, outstanding items, closing balance) exports as PDF or CSV. More on the bank reconciliation page and the AI bookkeeping page.
FAQs
Why does the divide-by-9 trick work?
When two digits swap places, the difference between the right and wrong number is always a multiple of 9. It's a quirk of how our number system works, and bookkeepers have relied on it for generations.
My difference is 1p. Should I bother?
Yes, quickly. Check foreign-currency transactions and anything with fractions of a penny. If it's genuinely rounding on a currency conversion, record it to bank charges with a note.
What if two errors cancel each other out?
They can, and that's one reason a balancing reconciliation isn't a guarantee everything is right. Matching line by line, rather than just comparing totals, catches most of these.
How long should I leave something in suspense?
Clear it by the next month-end at the latest. A suspense account that keeps growing is a warning sign.
Can a bank feed cause a difference?
A feed can occasionally duplicate or miss a transaction, especially if a connection drops and reconnects. Comparing against the actual statement, not just the feed, is what catches it.
Set the timer, run the tests in order, and stop at 30 minutes. You'll be surprised how often you're done in fifteen.
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