What Is An Invoice Number? [Methods And Examples]

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As a business owner, what are your major responsibilities?

  • To handle multiple clients.
  • To bookkeep payment processing.
  • To deal with partial payments or overdue payments.
  • To manage tax and accounting purposes.

Right? If we talk about payment processing, invoice numbers simplify the payment tracking process and help you get paid faster. In short, your business needs a well-organized invoice management system.

You might be wondering how. From this guide, you will learn the concept of what is an invoice number and the best methods with examples for assigning the first invoice number in your invoice.

Invoice Number Explained

An invoice number is a unique identifier, or we can say a sequential code, that is assigned to invoices. The invoice number is primarily used to keep track of payments. In simple words, it helps to track the payment status of your client.

Moreover, an invoice number (also known as an invoice reference number) is assigned sequentially to each invoice to track and identify the transactions easily. It is also assigned to your invoice to help you avoid duplicate payments.

Generally, a unique invoice ID appears on the top of the invoice, making it easy for both the recipient of the invoice and the business providing it.

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What Makes Assigning Invoice Numbers Important?

It is a unique invoice number assigned to each first invoice created for your clients on behalf of services provided. Such a unique invoice number helps you to:

Importance of an invoice number

  • Easily track payments

    Invoice numbers help you keep track of paid and unpaid jobs for your new and old customers.

    Using a logical invoice number system, you can easily deal with the customer’s issues related to several months-old invoices.

    Once your customer pays the total amount due, you must mention your alphanumeric invoice numbers as paid. So, it becomes easy for you to track payments.

    Apart from the payment terms, you can even note payments with the help of job details or a customer’s contact details.

    Otherwise, tracking it down can become a real headache and time-consuming for both the buyer and seller.

  • Avoid duplicate payments

    An invoice number keeps track of your first invoices and helps you avoid duplication.

    If you bill your clients twice or let’s say incorrectly, there are chances of facing big losses for your business in terms of reputation, time, and money.

  • Provide transparency and professionalism

    An invoice with an invoice ID shows professionalism, and your clients are more likely to take your business seriously. Not only an invoice ID but a business logo, contact details, pricing, and job details make your client ready to take your service.

  • Properly document income for taxes

    Assigning invoice numbers helps tax officers review invoices against the business expenses and sales easily.

    One of the major tasks for any business is to track multiple jobs, clients, and income received every financial year.

    Invoice numbers follow the sequential number methodology, which helps you show the total number of invoices you have sent to your customers at a specific time.

    This is especially useful, to your accountant or when you are using accounting software, for taxes and accounting tracking purposes.

    You will know exactly how many jobs you did and how much they were for. Thus, it is essential for payment tracking purposes.

  • Manage your invoices smoothly

    Without invoice numbers, it’s hard to find an invoice from multiple invoices when you need them. So, you need to organize invoices correctly.

    For example, when you need to confirm a payment, answer a question from a client, or send them to your accountant, invoice numbers are a simple way to search through and find a particular invoice, regardless of the job, the customer’s name, or the cost of the job.

    By including a unique bill number for each of your digital or paper invoices, it’s easy to find the information you’re looking for in your records.

Ways to Assign Invoice Numbers

Before we start understanding the steps to generate an invoice number, note that it’s not mandatory to just stick to the methods described here. Instead, it entirely depends on you and how you want to keep your invoice numbers.

Best practices to generate an invoice number

1. Numbering invoices in sequence

This method allows you to assign invoice numbers like 1, 2, or 3—or even 00001, 00002—to every new invoice you create.

The most common sequential invoice numbering approach is nine zeros followed by a 1.

It means that you can start with your first invoice number as 0000000001.

If you do this sequential numbering manually, it’s going to be a challenging and time-consuming activity. You will have to note down the customer’s name and the amount owed on a separate sheet. The reason to note it down in the sheet is to get rid of any typos or duplication errors if they occur.

Moreover, you can also make a combination of letters and numbers (alphanumeric) like INV/01, INV/02, INV/03, and so on.

In fact, it is the easiest way to identify the invoice. You just need to keep the letters the same and increase the number with each invoice. It should be sequential and not contain any gaps.

An alternative and the best option is to use the best invoicing software that automatically assigns invoice numbers alphanumerically to every invoice you create. If you try to enter duplicate invoice numbers, you can’t assign one invoice number to another invoice.

2. Numbering invoices in chronological order

One of the best ways to create a longer and more detailed invoice number is by mentioning the invoice date. This method is known as the chronological method.

Let’s say your first invoice number for the invoice created on January 20th, 2021, would look like this:

  • Invoice number example: 20210120-001
  • Invoice number format: YYYYMMDD-001

There are two main sections:

  • The first one is the date on which the invoice is created.
  • Second is the order amount which increases with every new purchase.

Apart from this invoice number format, there are other formats that you can use in your invoice number, like:


You can even customize the invoice by including the customer’s name or client number and the invoice number.

For example, if the client’s number for the previous sale is 407 (CCC), the invoice ID would look like this:

  • Invoice number: 20201030-407-001
  • Format: YYYYMMDD-CCC-001

Similarly, the following invoice ID for the same date and same client will be 20201030-407-002.

3. Numbering of invoices based on customer number

When you assign invoice numbers based on the customer number, you have the option to select whether you want to enter the date or not.

The custom invoice number series begins with the customer name, and the invoice date (an optional one), and finally ends with a sequence number.

Note: The invoice number sequence always comes at the end of every invoice numbering system.

Using the customer invoice sequencing method, you can easily keep track of numbering. This effective invoice numbering system further helps you get rid of duplicate payments.

You have an invoice for client number 4567 with an invoice date of August 15th, 2021. The invoice number would like: Invoice no: 4567-001


Invoice no: 4567-20200815-001

4. Project-based numbering of invoices

The most widely used invoice sequencing method is project-based. As the name suggests, you can easily reference your invoices using the project number.

This method is an ideal option for new businesses that regularly take on multiple projects. Moreover, many construction companies use the project number to identify their clients, and it is the best option for them to use this method.

There are two main organizational ways to use this method:

  • project number + customer number + sequence number
  • customer number + project number + sequence number

For project number SOTS104 and client number 5278, the assigned invoice number would be:

Invoice number: SOTS104-5278-01
Invoice number: 5278-SOTS104-01

These are some of the standard methods by which the online invoice creator can number your invoices.

Still have any questions related to invoice numbers?
Fair enough. We’ve answered a few frequently asked questions in the next section to resolve your queries.

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Frequently Asked Questions
  1. Do invoice numbers have to be sequential?

    Yes, it is! Businesses are legally bound to number invoices in sequential order. This is majorly due to:

    • Taxation audits
    • Organized bookkeeping

  2. What is the invoice id number?

    The invoice id number is the unique number assigned to the invoice for the identification of every invoice.

  3. What is the invoice number on a receipt?

    The invoice number on a receipt is to link the receipt to the invoice it is related to.

  4. Why do invoices generally include an invoice number?

    Invoices generally included an invoice number because:

    • It is legally binding
    • It helps to audit taxations
    • It helps to organize invoices


The primary purpose of invoice numbers is to help differentiate and track payments.

For small business owners, generating numbered invoices is very simple, as you can opt for any of the methods like sequential, chronological, customer number, and project-based.

If you plan to update your invoice numbering system, please do it before the accounting period. Moreover, you also need to overcome many invoice sequencing errors, as they may lead to payment duplication.

The best option to avoid such complications is to adopting online invoicing software like InvoiceOwl and use reliable accounting software.. It allows you to create invoices using a free invoice template and send invoices to your clients easily. You just have to mention your client’s company details and let our software handle it. The software itself will automatically create an invoice number. and know how it helps your business in different ways.

Moreover, to find invoicing basics explained in detail, visit our invoicing guide.

Author Bio
Jeel Patel
Jeel Patel

Jeel Patel is the Founder of InvoiceOwl and is the main curator & writer of the content found on this site. With ideals of quality, commitment, and perseverance, he believes in creating lasting business relationships with the clients.

United States