Check 1: does the difference divide by 9?
If it does, two digits were probably swapped somewhere: 540 entered as 450, or 1,263 as 1,236. Look for amounts on the statement and in the books that use the same digits in a different order.
Check 2: search for the exact difference
One transaction of exactly that amount is missing from one side, or is in the books twice. Search the register and the statement for the number.
Check 3: search for half the difference
An amount entered on the wrong side is off by twice its value. A 75.00 deposit entered as a payment puts the reconciliation off by 150.00. So if you are off by 150.00, look for 75.00.
Check 4: did the opening balance move?
The opening balance of this reconciliation must equal the closing balance of the last one. If it does not, someone changed, deleted or un-reconciled a transaction in a period that was already reconciled. That is not this month’s problem, and nothing you do this month will fix it. See why a beginning balance is wrong.
Check 5: bank fees and interest
They are small, they are on the statement, and they are often not in the books. A monthly service fee, a wire fee, a few cents of interest.
Check 6: timing and split amounts
A check that cleared for a different amount than it was written for. A deposit the bank split into two lines. A card payment that the bank dated a day later than you did.
Still off? Reconcile in halves
Tick off the first half of the month against the statement’s running balance. If the books agree with the bank on the 15th, the problem is in the second half. Halve again. Four or five rounds find a single wrong transaction in any month.
What should I not do?
Do not post a “reconciliation adjustment” to make the difference go away. The entry is easy and the cost comes later: an unexplained balance at year end, and no record of what was actually wrong.
What does a clean reconciliation prove?
That every bank and card account agrees with its statement. In a close this is the first check, before clearing accounts and schedules. See when a month is really closed.