What is an Invoice Number – Invoice Basics Made Clear

Roll numbers for students to employee numbers for payroll purposes, every number uniquely and unmistakably stands out as it works as an identifier. Invoicing is no different. A unique number is assigned to each invoice.

The basic purpose of the invoice is to request timely payment from customers. You might have multiple clients, and tracking payments made, partial payments, and overdue payments, can be a real headache. Invoice number simplifies the payment tracking process and avoids all the confusion.

Quick insight:

What is an invoice number?

Just like any identification number, an invoice number is a unique number generated for easy payment tracking. It acts as a reference to track the status of the payment of a particular client. The number should be visible on the invoice.

The invoice number could be sequential, chronological, numeric, a combination of numbers and letters, project name and number, date of issue, or any combination. But most importantly, each number should be unique. Examples of invoice numbers: 001, CN01, 20210430-403-01

Why is an invoice number important?

The invoice number is the most crucial aspect of every invoice. It’s a legal requirement. Not just for payment tracking, a properly assigned invoice number is important because:

  • It helps to check on the income for tax and accounting
  • It avoids duplication of invoices which would create a lot of confusion
  • It’s essential for professional accounting services and financial records
  • Easy to track outstanding invoices
  • It avoids duplicate payments
  • Invoice number makes your business look legit
  • Effortlessly tracked down invoices from several months ago

What information should be included on an invoice?

  • Business Information including company name, logo, and contact details 
  • Recipient information including name and contact details.
  • Invoice Number: A unique invoice number for tracking and reference.
  • Invoice issue Date: The date when the invoice is issued.
  • Invoice Due Date:  date by which the payment should be made.
  • Description of Goods/Services including quantity, unit price, and total price for each item.
  • Subtotal of all items/services including Applicable taxes, Any discounts applied, Shipping or handling fees, if applicable, and Total amount due.
  • Terms of payment (e.g., Net 30, payment due upon receipt).
  • Payment methods (e.g., bank transfer, credit card, PayPal).
  • Bank account information if payment is to be made via bank transfer.
  • Notes or Terms and Conditions related to the sale or service.

What should I put for the invoice number?

Before you decide on your invoice number, it is recommended to go through your country’s or state’s professional and legal requirements.

There is no hard and fast rule of following a particular format of invoice numbering. You can structure your invoice number the way you want. The assigned invoice number could be just numbers or a combination of alphabets and numbers. Eventually, the ideal criteria are to keep the number unique and sequential.

Invoice numbering system best practices

A complicated style of assigning invoice numbers should be avoided as it will consume a lot of time and are much more prone to errors. The best practice is to keep the invoice number in order like 1, 2,3. This way there will be no duplicate numbers, especially, if you are assigning numbers manually. It’s a good practice for new businesses as initially they wouldn’t have much number of invoices.

Additionally, the other practices to number invoices are:

A combination of letters and numbers (alphanumeric) – INV/01, INV/02, INV/03…..

An invoice number including letters, special characters, and numbers will help to identify the invoices easily. Let the letters be the same, you just need to increase the number with each invoice. It should be sequential and not contain any gaps.

Customer name + number – CN01, CN02, CN03…

Another option is to give the client invoice a letter code along with a unique number code. You can use the initials of the client name and prefix in the invoice number.

Project name + number – PN001, PN002, PN003…

If you are doing project-based work then you can format invoice numbers with project code prefixes. This invoice number structure will include the project name and sequence number. You can even use any special character in between, like for example PN-001.

Chronological order with date and customer ID – 20210430-403-01, 20210430-403-02, 20210430-403-03 (yyyy/mm/dd-customer id-unique sequence number).

Chronological order with date and customer name – 20210430-CN-01, 20210430-CN-02, 20210430-CN-03 (yyyy/mm/dd-customer name-unique sequence number).

It’s a detailed technique of invoice numbering in which you can include the date of the invoice issued. The date is used as a prefix, followed by customer ID or customer name short form, and in between special characters are included for more clarity. However, this format makes the invoice numbers quite long.

Whichever invoice structure you follow, the sequential number should always come at the end as it will be easy to differentiate each invoice and track.

What if you want to change your invoice number series?

Never change invoice numbers mid-transaction.

In case you are using the date or year in your invoice number then it has to be updated timely. If you are planning to completely change your invoice number series then it is better to wait until the next financial period to avoid confusion. Make sure you do not leave or repeat numbers while switching to another invoice number format.

How to avoid invoice numbering errors?

Incorrect invoice numbers due to typos, spelling errors, or sequence disorder can be a nightmare, especially while filing taxes. To prevent such errors it is better to switch to some online invoicing software that automatically generates the invoice number and sequentially assigns the next invoice number as soon as you create the invoice. Online invoicing eliminates duplicate and missing numbers.

Alternatively, you can run a tool to detect duplicate invoice numbers and trace the errors.

Is the order number the same as the invoice number?

An order number is assigned to the goods ordered by the customers while the invoice number is the seller’s number for billing the goods ordered. In a few cases, one order number can have multiple invoice numbers.

To know more, head over to our blog post on the differences between purchase orders and invoice numbers.

Is the invoice number the same as the bill number?

The invoice number and the bill number are not the same. An invoice number reflects a particular order while the bill number reflects a particular bill. A bill can be generated for multiple invoices.

The bill number is the total amount to your company and the total amount on the bill comes from the total of all the invoices.


What is an invoice ID?

The invoice ID is similar to the invoice number.

Does an invoice number need to be in the invoice?

Yes! The invoice number is a unique identifier. Hence, it needs to be added in an invoice.

How to number the first invoice?

You can number your first invoice by starting with 000001. The number of zeros preceding depends on your preference so you can adjust that as per your requirement. You can also make it an alphanumeric digit format: A00001

How to find the invoice number?

Every invoice must have an invoice number. It is found in the top-left or top-right corner of the invoice.

Why Use Online Accounting And Invoicing Software for Small Business?

Saves Time: Automates invoicing and bookkeeping tasks.
Increases Accuracy: Reduces human errors with automatic calculations.
Offers Accessibility: Access financial data anytime, anywhere.
Cost-Effective: Lowers paper and storage expenses.
Improves Cash Flow: Tracks payments efficiently to avoid late payments.


Do you want to be a thriving business owner, not one struggling with cash flow issues? Automating your invoicing process is key to maintaining smooth and efficient financial operations. Implement an invoicing system that automates and streamlines tasks, including invoice numbering, to eliminate confusion and errors.

Try using the user-friendly Billbooks cloud-based invoicing software and experience how effortless invoicing can be. With Billbooks, managing invoices and invoice numbers becomes a breeze.

Divya Mudliar
Divya Mudliar
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