What Are Invoice Numbers, and How to Make Them

Invoice numbers allow you to locate an invoice quickly and with ease, which is key for anyone who needs to be organised.  Despite being called numbers, they can contain both numbers and letters. Each invoice number should be unique and follow a sequence.

invoice numbers catalogued ring binders

Why do you need to use invoice numbers?

It may be a legal requirement to use invoice numbers, depending on your jurisdiction. They make it much easier to track who has and hasn’t paid, as well as keep a record of customers’ previous payments. For more information about what invoices should contain, see our blog.

Ways of making invoice numbers

Your invoice numbers can be any string of letters and numbers that can increase sequentially, but there are different ways you can make them.


Many people use letter or number codes in each invoice number to allow for easier cataloging.

Popular methods are:

  • Date strings – Using an abbreviation for the month the job happened in. Ie for January, you may put JAN at the start of your invoice number.
  • Initials/Abbreviations- Using the customer’s initials to make a short letter code. Ie for Alan Baker, you may put AB at the start of your invoice number.
  • Customer numbers- You may assign each customer a number. Ie for your sixth customer, you might assign them 06, and put 06 at the start of your invoice number.

Number sets

After your letter-based identifier, it’s ideal to add a string of numbers. These should increase consecutively with each invoice. You should use several digits, to make each number easy to set apart. Instead of 1, putting 0001 makes it easier to compare invoice numbers.

Combining identifiers and number sets make for a robust method of cataloging invoices.

Example invoice numbers

The seventeenth invoice made in October: OCT0017.
The third invoice for Paul Anderson: PA0003.
The tenth invoice made in July: 07-0010.

Automated invoice numbers

Invoicing solutions such as i24 can generate invoice numbers automatically for you, meaning that you never repeat one. You can still customise them, allowing you to fit the invoices created with the solution in with all of the invoices made previously.

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