Invoice number

Mandatory requirements for invoice numbering

Anyone starting a business has to decide how to number invoices. Belgian law requires invoice numbering to be unique, chronological and continuous. All three criteria at the same time.

  • Unique: an invoice number is unique if it occurs only once in the series of invoices. So never duplicates. This criterion guarantees that, thanks to its number, an invoice in the accounts cannot be confused with another invoice (it also offers a means of searching systematically and therefore quickly).
  • Chronological: means that invoices are numbered according to the time at which they are issued: a newer invoice therefore has a higher number than an older invoice. An invoice from March cannot have a lower invoice number than an invoice from January of the same accounting period.
  • Continuous: the numbers must form an uninterrupted consecutive series, without gaps (and, as said above, without duplicates). It follows that an incorrectly drawn up invoice may not disappear from the series. The law requires all issued invoices, including incorrect ones, to be left in the series and kept. Cancel the invoice with a credit note and draw up a new, correct invoice with its own unique, higher sequence number.

The requirements of uniqueness and chronology also apply to credit notes. But every entrepreneur and company may choose either to give credit notes their own number series, for example CN with a sequence number, or to number them along with the invoice series (e.g. F220001, F220003, CN220004, F220005, …).

A practical approach to numbering invoices

With this basic knowledge of numbering, the next step is to choose the format, i.e. the content, composition én length of the invoice numbers. Will it be a plain running sequence number, will the number consist of different elements such as F + sequence number, and how many positions will each number have? Be careful: invoice zero (0) is not possible; the first invoice gets sequence number 1.

A company that issues few invoices, for example a few dozen per year, can manage with a running number. Anyone producing many invoices then runs into the inconvenience that long strings of digits are hard for many people (difficult to read, risk of errors when copying). Estimate in advance how many invoices you will make, so that the sequence number can consist of enough characters. If you expect 100+ invoices, the sequence number can consist of e.g. three digits from the start (001, 002, …. 101, 102, …).

The composition of an invoice number can be chosen freely: digits only, a combination of letters and digits (always including digits), and other connecting characters such as / or -, but no spaces. So F/22/101 is allowed, F 22 101 is not. Do think carefully about the format, though. A smart choice can help and speed up processing and searching, or actually slow it down. Everyone in the company then knows that F-2019-82 is the 82nd invoice in the financial year 2019. A sequence number in the flat format 96482 says nothing, except that many invoices have already been issued. Anyone incorporating a date into the number format should (again) preferably use a quickly recognisable series such as F-2021-008. Note that a number such as F2022030001, where 2022 indicates the year and 03 the month, is also possible provided the rest of the number is the sequence number within the month. In April this becomes F2022040001 etc.

The possible combinations are endless. Some companies even incorporate the customer number into their invoice numbers. Tip: keep the numbers and formats clear. Informative numbers are an advantage, but numbering with too many information parts quickly becomes awkwardly complicated.

In onFact you can work with various tags to compose the invoice number. For example, the (fill) tag %Y changes into the relevant year (calendar year), so in 2022 ‘2022’ appears in the invoice number, and in 2023 this automatically changes to ‘2023’, etc.

Restarting the sequence number

Part of the invoice number, and therefore the whole number itself, can change based on the calendar year, provided it remains unique and ascending. But the series may not simply stop and restart. Restarting the numbering is only allowed at the start of a new taxable period, i.e. the financial year, where the first invoice then gets number 1, the second number 2 etc. Because an invoice number must be unique, with a new start another element besides the sequence number itself will have to identify the new series. Letting a year change along with it is a frequently used option.

Numbering per journal

Anyone with several activities may have a separate number series per activity. For example F-AD-22001 for the first invoice as an adviser and F-W-22088 for the 88th invoice issued by his or her supplies shop. In accounting terms, this means a separate journal is kept per activity. Per journal, the numbering again follows the basic rules, so the number is chronological, unique and continuous. Anyone choosing an invoicing program should check carefully whether and to what extent the software can handle the choices described above. Invoicing software that leaves no choices can have annoying drawbacks, such as the impossibility of giving an invoice number ‘meaning’ according to the activity or the date. In that respect onFact is definitely one of the better choices: you can easily create multiple journals with their own number structure.