Invoicing comes with a lot of rules to follow. And invoicing itself often takes a lot of time, too, and now and then a mistake can happen that attracts a financial audit.

One of the rules, which may seem like a small thing, includes the correct numbering of invoices. If you are still using excel spreadsheets for invoicing, such a mistake can happen right away due to inattention.

What are the rules for numbering invoices?

The numbering series is not clearly defined by a rule, but it is a good idea to follow the following:

  • Chronological numbering to build a sequential series over time,
  • The final form is up to each individual, but it is recommended to use a system where the period of invoice is clearly identifiable: undefined
  • Pairing of invoice number and variable symbol - to make it easier to identify whether the invoice has already been paid,
  • Characters in the numeric string such as dashes, slashes, etc. are allowed, they just cannot be used as a variable symbol for payment.

Invoice numbering and its rules also give entrepreneurs and companies a lot of freedom to set up their own number series for each month, year or client so that the system suits the company and its needs.

Invoice numbering errors - what to do about them?

If a mistake is made, it is possible to edit the invoice before it is sent to the client and they register it (for example, if one of the online tools such as has been used), but it is no longer a good idea to replace a missing number series with an invoice that was issued later, or delete it, for the sake of order in the accounts, for example.

In order to save time when issuing invoices and to minimize errors (not only in numbering), it is good to use online tools that have number series already set up - for example, with you choose your number series and it is automatically increased by one for the next invoice.

For example, you issue your first invoice with the number 2020 (year) 06 (month) and the sequence 01 - 20200601, the next one will be automatically 20200602.

Invoicing doesn't have to be lengthy and you can eliminate the risk of error to a minimum with invoicing tools.