What is an Invoice Number? Invoice Numbering Guide

January 15, 2024

This blog post offers a comprehensive guide on what an invoice number is, why it's essential, and how to generate unique ones efficiently. It emphasizes the importance of keeping billing organized and provides practical tips for generating invoice numbers.

James Fyfe
Co-founder - CEO

As a business owner, an invoice is one of the crucial documents you must prepare when providing goods or services to a client. 

An invoice number is a unique identifier assigned to an invoice that helps businesses keep track of their financial transactions. Invoice numbers have a combination of letters or digits that distinguish each invoice from one another. 

This article will discuss why an invoice numbering system is important and how to generate it.

Why is it Important to Assign Invoice Numbers?

An invoicing system is important for several reasons. Here are five reasons why:


When you number invoices, you'll keep track of their financial transactions and organize their financial records. Companies can easily retrieve and reference specific invoices by assigning a unique number.


An invoice number helps ensure accuracy in financial records, especially when sending multiple invoices. It allows businesses to avoid duplicate payments or missing invoices, which can cause discrepancies in financial statements.


Invoice numbers provide a means of communication between the buyer and the seller. They help buyers and sellers identify specific transactions and ensure they agree on the details.

Legal compliance

In some countries, businesses are legally required to assign invoice numbers. This ensures that the invoices are legal documents that can be used for tax purposes.

Payment processing

Invoice numbers help businesses process payments more efficiently. They allow companies to easily track which invoices have been paid and which are still outstanding. 

An invoice number also helps businesses manage their cash flow and ensures they are paid for the goods or services they have provided.

How to Generate An Invoice Number [With Practical Examples]

The process of generating an invoice number can be done in several ways. You can use a simple numbering system, a combination of letters and numbers, or a more complex system that includes the date or the customer's name. 

Here are some practical examples of how to generate an invoice number:

Simple numbering system 

You can start with a basic sequential numbering system that assigns a unique number to each invoice you issue. For example, you can start with 001, 002, 003, etc.

Combination of letters and numbers

You can combine letters and numbers to make your invoice number more unique. For example, you can use a combination of the client's initials and a sequential number, such as "AC-001," "AC-002," "AC-003," and so on.

Complex system 

You can use a more complex system that includes the date or the customer's name. For example, you can use the year, month, and day of the invoice date, followed by a sequential number, such as "20220317-001," "20220317-002," "20220317-003," and so on.

Invoice Number Errors: How to Correct Them?

Invoice number errors can confuse and lead to discrepancies in financial records. Common invoice number errors include duplicates, missing numbers, incorrect formats, and incorrect sequencing. 

Here are some steps to correct invoice number errors:

Identify the error

Review the invoice number to identify the error. Determine if it's a duplicate, missing number, incorrect format, or sequencing error.

Notify the customer

If the error was made on an invoice sent to a customer, notify them and provide them with a corrected invoice.

Update financial records

Update financial records to reflect the corrected invoice number. This includes updating any accounting software or spreadsheets used to track financial transactions.

Maintain consistency

Ensure the corrected invoice number is consistent across all financial records to avoid further errors.

Prevent future errors

Put measures in place to prevent future invoice number errors. This can include implementing a standardized numbering system, double-checking invoice numbers before sending them out, and regularly reviewing financial records for mistakes.

Correcting invoice number errors requires identifying the error, notifying the customer, updating financial records, maintaining consistency, and preventing future errors. 

Businesses can ensure accurate financial records and maintain good customer relationships by taking the steps mentioned above.

The Difference Between an Invoice and a Receipt

While both the invoice and receipt are part of a payment process, they are different in their outcome and each plays an important role throughout the sales of goods and services.

An invoice is a document that lists the details of a transaction, including the products or services provided, the quantity, the price, and the payment terms. It is issued by the seller to the buyer and serves as a request for payment. 

A receipt, conversely, is a document confirming that the payment has been made. It is issued by the seller to the buyer and serves as proof of payment.

Generate Invoice Numbers with the Help of Portant

Portant is a powerful tool that can help you generate invoice numbers automatically, making your invoicing process much more efficient and streamlined. 

With Portant, you no longer have to worry about manually generating invoice numbers or keeping track of them in a spreadsheet.

Start making use of Portant’s free invoice template to generate invoice numbers in bulk and streamline your whole invoicing process. 

Final Word

A business's invoicing process relies heavily on invoice numbers because they are unique identifiers. It allows for efficient tracking of payment status and helps to reference specific invoices in case of future inquiries or disputes. 

A well-organized and consistent system for assigning invoice numbers is essential for accurate record-keeping and effective communication with customers and clients. Businesses can streamline their invoicing process and maintain accurate financial records by following the tips outlined in this guide.


Here are some frequently asked questions about invoice numbers:

Q. How do I generate my invoice?

You can generate your invoice manually or by using invoicing software. You can use a template or create one from scratch if you develop it manually. If you choose invoicing software, you can select from various templates and customize them according to your needs.

Q. How do I automatically create an invoice number in Word?

You can automatically create an invoice number in Word using a macro or a formula. You can find various tutorials and templates online to guide you.

Q. Can invoice numbers be random?

Yes, invoice numbers can be random. However, it's important to ensure they are unique and not duplicated.

Q. How many digits are in an invoice number?

The number of digits in an invoice can vary depending on the business's preference and the number of transactions. However, typically, invoice numbers contain three to five digits. 

Some businesses may also include letters or special characters in their invoice numbers. Ensuring that each invoice number is unique allows easy tracking and organization of financial records.

Businesses can streamline their invoicing process and maintain accurate financial records by following the tips outlined in this guide.

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