How to use the invoice and check number gap finder
- Paste your numbers, one per line or separated by commas. They may have a prefix or a suffix such as INV- or -A.
- Leave the box ticked to treat each prefix as its own series, or clear it to treat all the numbers as one series.
- Read the missing numbers, grouped as ranges, for each series.
- Check the repeats and the entries that could not be read as numbers.
Formula
each entry = prefix + last run of digits + suffix; a series shares the prefix and suffix
distinct numbers are sorted, and a jump of more than 1 is a gap
missing = (last - first + 1) - distinct numbers present
repeats = entries whose number appears more than once
Digits are handled as exact whole numbers however long they are, and numbers with leading zeros keep their width when shown.
Invoices and checks
The default list has invoice numbers INV-1001 to INV-1010 with INV-1004, 1005 and 1009 absent, INV-1003 entered twice, check numbers CHK-0201, 0202 and 0204, and the word void. The invoice series runs 1001 to 1010 with 7 distinct numbers, so 3 are missing, in two ranges: 1004 to 1005 and 1009. The check series misses 0203. In total, 4 numbers are missing, one entry is repeated and one entry has no digits.
Without separate series, the plain list 1, 2, 3, 7, 8, 12 and 12 again runs from 1 to 12 with 6 distinct present numbers, so 6 are missing in two ranges: 4 to 6 and 9 to 11.
Why gaps matter
Invoice and check numbers are normally issued in order, and a gap can mean a voided document, a lost one, one used out of order or a missing entry in your records. Auditors and bookkeepers look for gaps and repeats as a first step. Finding them takes a minute with a tool and much longer by eye.
To create the next numbers in a series, use the Document Number Sequence Generator. To match what cleared the bank, see the Bank Reconciliation Template, and to check that the books balance, the Trial Balance Checker. The Date Gap and Streak Finder does the same for dates.
Separate series
Two books of checks or two kinds of invoice can run in parallel with different prefixes, and mixing them would hide real gaps. With separate series, each prefix and suffix pair is checked on its own. Without it, the tool uses only the digits, and different prefixes share one run of numbers.
Assumptions and limits
- The number in each entry is the last run of digits, so a date inside a number such as 2026-0005 is read by its last part.
- Up to 20,000 entries are accepted.
- Entries are trimmed of spaces, and blank lines are skipped.
- A gap is any number between the first and last that is not present, whatever the reason.
- The tool cannot tell a void from a loss.
Frequently asked questions
Does it keep leading zeros?
Yes. If any number in a series has leading zeros, the missing numbers are shown with the same width.
What if my numbers have letters in the middle?
Only the last run of digits is treated as the number, and the text before it is the prefix. Entries that differ in the letters before the digits are separate series.
How many missing numbers can it count?
Any amount, since it uses exact big numbers. It lists the ranges, not each missing number.
What does Not read as a number mean?
The entry has no digit in it, like the word void in the example, so it cannot be placed in a series.