Check Number Gap Finder
Paste or type a list of check numbers and this tool reports the range, the count, any missing numbers in the sequence, and any duplicates. It is built for reconciling a check run and for confirming that a positive pay issue file lists every serial number the bank should expect.
Gaps and duplicates in your serial numbers are a quiet source of exceptions. Finding them before you send the file is faster than clearing them at the bank afterward.
Why check number sequence matters for positive pay
Positive pay works by matching each check the bank pays against the serial number and amount you listed in your issue file. When a paid check has a serial number that is not in your file, the bank cannot match it, and it becomes an exception you have to approve or return. A missing number in your file, a check you issued but forgot to list, is exactly the kind of gap that turns into an exception when that check is presented.
Duplicates cause the opposite problem. The same serial number listed twice, or reused across two real checks, makes matching ambiguous and can trip your bank's validation or your reconciliation. Catching both gaps and duplicates up front keeps the file clean and the exception queue short. For more on how exceptions arise and what to do with them, see positive pay exceptions explained.
Using the results for reconciliation
The gap finder gives you a fast reconciliation check against your check register. Compare the count and range with the number of checks you believe you issued. If a check number is missing from your sequence, it is either still unissued, was voided, or was left out of your file by mistake. Voided checks deserve special care, because some banks want voids reported in the issue file rather than simply skipped. See how to handle void checks in positive pay before you decide to leave a number out.
If your bank also reconciles on the MICR line, it helps to confirm the serial number printed on the check matches the one in your records. Our MICR line decoder and the note on the MICR line and positive pay explain how the serial number on the check ties back to your file.
From a clean list to a finished file
Once your serial numbers are confirmed continuous and free of duplicates, you are ready to build the issue file. Send your verified list into the positive pay file generator to produce the layout your bank expects, then run the file validator to confirm the structure before upload. If you are unsure of the layout your bank uses, the positive pay file format reference covers the common field sets. Browse the rest of the free tools for related checks.
Frequently asked questions
What does the gap finder report?
It reports the lowest and highest check number, the total count of numbers you entered, every missing number in the sequence between the lowest and highest, and any number that appears more than once. That gives you a quick reconciliation snapshot before you build a positive pay file.
Why do missing check numbers matter for positive pay?
Positive pay matches paid checks against the serial numbers in your issue file. If you issued a check but its number is missing from your file, the bank cannot match it when it is presented, and it becomes an exception you have to approve or return. Finding the gap first lets you add the check or confirm it was voided.
What should I do about a gap in my sequence?
A missing number usually means the check is unissued, was voided, or was accidentally left out of your file. Check your register. If it was voided, review how your bank wants voids reported, since some require voids in the issue file rather than a simple skip in the sequence.
Why are duplicate check numbers a problem?
A serial number listed twice, or reused on two real checks, makes positive pay matching ambiguous and can fail your bank's validation or your own reconciliation. The tool flags duplicates so you can correct the list before sending the file.
Can I paste numbers separated by commas or new lines?
Yes. The tool accepts check numbers separated by new lines, commas, or spaces, so you can paste a column from a spreadsheet or a register export directly. It sorts them and reports the range, count, gaps, and duplicates.
Does this tool send my check numbers anywhere?
No. The gap finder runs entirely in your browser, so your check numbers are never uploaded. It is part of the free PositivePayMaker toolset alongside the file generator and validator.