Invoice number - What is an invoice number?

An invoice number is a unique, sequential code that is systematically assigned to invoices.

Invoice numbers are one of the most important aspects of invoicing as they ensure that income is properly documented for tax and accounting purposes. They also make it easier to track payments and manage overdue invoices.

Each invoice issued should be assigned a unique invoice number. The invoice number should always be clearly stated on the invoice – without an invoice number, an invoice is not considered a legal document.

As proforma invoices are not ‘true’ invoices, they shouldn’t be assigned invoice numbers.

How to assign invoice numbers

Invoice numbers are a mandatory invoice field. They should be assigned sequentially, meaning that the number of each new invoice increases. Legally, an invoice number sequence should never contain repeats or gaps.

It’s not recommended to simply number invoices '1', '2', '3', etc. as this can be confusing when invoice numbers reach double or triple figures. New businesses may not have high numbers of invoices to begin with, but it’s important to consider future growth when choosing an effective invoice numbering system.

A common way to create a longer, more specific invoice number is to include the month or year the invoice was issued.

For example, within a sequence that refers to the year, the first invoice in 2021 could be numbered '210001'. The next invoices would be '210002', '210003' and so on. At the start of a new year, the numbers would be reset. The first invoice number of 2022 would be '220001'.

This system allows you to quickly see which year an invoice is issued and still follows the sequential numbering requirement.

Alphanumeric invoice numbers

An invoice number doesn’t always contain just numbers – it’s also possible to create a unique code that includes numbers, letters, and special characters such as slashes (/) or dashes (-).

Adding letters and characters to an invoice number series can help make it easier to organise records and identify invoices.

An alphanumeric invoice number series must follow the same rules as regular invoice numbers series – they should be sequential and should never contain gaps.

Letters can be used to identify different types of documents, like quotes and delivery notes. For example, an invoice number sequence could contain 'INV' and a quotation number sequence could contain 'QUT'. If you choose to include letters in your invoice number series, the letters should not be changed – only numbers should increase.

Examples of alphanumeric invoice sequences include:

2018/INV/0001
18-QUT-0001
2018/0001
2018-0001
18/0001

Learn more about formatting your invoice numbers in our article: “How to format invoice numbers”.

When to update invoice numbers

If you use an invoice number series that follows the date, it should be updated accordingly. For example, the invoice number 'INV-21-12-009' includes the month and year, referring to the ninth invoice issued in December 2021. The first invoice of the next month, January 2022, would be numbered 'INV-22-01-001'.

If you want to change your invoice number series entirely, it’s best to wait until the next accounting period as this helps avoid confusion and prevents missing or duplicate invoice numbers.

However, if you change your invoice number series, you should be sure that you don’t repeat or exclude any numbers. This means that if your last invoice number was ‘023’, your next invoice number must include '024' – for example '21/INV/024' or '2021-024'.

Invoice numbers and deleted invoices

As a standard rule, invoices should not be deleted. Because invoices are numbered sequentially, an invoice that has been completed in your records and is then deleted could show up as a gap in your financial records.

If you want to remove a transaction, it’s usually better to issue a credit note for the same amount. This will effectively cancel out the invoice while ensuring that your number sequence is complete.

Invoice numbers with SumUp Invoices

With SumUp Invoices, it’s easy to keep track of your invoice numbers.

Use our default numbering system, or choose your own custom invoice number series by typing your preferred number into the ‘Invoice No.’ box. The number will automatically be updated to the following sequential number whenever you create a new invoice.