Bank rec off by a number divisible by 9? You probably swapped two digits
Short answer: if the difference in your bank reconciliation divides evenly by 9, the most likely cause is a transposition error: two digits typed in the wrong order, such as $45.30 entered instead of $54.30. Divide the difference by 9 to narrow down which entry it is.
Why the 9 trick works
Swapping two neighbouring digits always changes a number by a multiple of 9. For example, 54 − 45 = 9 and 83 − 38 = 45 = 9 × 5. It works wherever the digits sit, including after the decimal point: $54.30 − $45.30 = $9.00. A "slide" error, where the decimal point moves (for example $45.00 entered as $450.00), also leaves a difference divisible by 9 ($405.00 = 9 × $45).
How to find the entry, step by step
- Make sure the difference is real. Check that the statement's opening balance plus all its transactions equals its ending balance, and that your book balance is for the same date.
- Divide the difference by 9. If the answer is a whole number of cents (no remainder), suspect a transposition or slide.
- Read the result. For a swap of two neighbouring digits, the result shows how far apart the two digits are. $9.00 ÷ 9 = 1, so look for two digits one apart (4 and 5, 2 and 3…). $27.00 ÷ 9 = 3 points to digits three apart, such as 1 and 4.
- Look at the size. A difference in whole dollars points to the dollar digits; a difference in cents points to the cents.
- Scan your unmatched lines for an amount on one side whose digits are the same as an amount on the other side, just in a different order.
- Fix it at the source: correct the amount in your books, keeping the bank's figure. The bank's amount is what actually cleared.
A worked example
| Bank statement | Amount | Your books | Amount |
|---|---|---|---|
| STAPLES 0412 | −$54.30 | Staples: printer ink | −$45.30 |
The reconciliation is off by $9.00. $9.00 ÷ 9 = 1, so look for two neighbouring digits one apart: 5 and 4. The Staples line is the culprit. Change the book entry to −$54.30 and the difference goes to $0.00.
When the 9 trick doesn't apply
- The difference equals one transaction: an entry is missing or entered twice.
- The difference is exactly double a transaction: it was probably entered with the wrong sign (money in recorded as money out, or the reverse). Halve the difference and look for that amount.
- The difference is a fee-sized round number ($15, $25, $35): look for a bank fee you haven't recorded yet.
- Several errors at once can cancel each other out or hide the pattern. Fix the ones you can find, then check again.
Common mistakes
- Changing the bank's figure instead of your books. The bank amount is what actually happened.
- Assuming every multiple of 9 is a swap. Some genuine differences are multiples of 9 by coincidence.
- Comparing against a book balance from the wrong date.
General guidance, not accounting or tax advice.