Key Takeaways
An invoice number is a unique reference that helps you, your clients, accountants, and banks identify the same invoice without confusion.
There is no fixed format, but invoice numbers must follow a clear, logical system and should never be reused.
If an invoice contains an error, keep the invoice number on record and void or correct the invoice; do not delete it.
Many businesses handle dozens or even hundreds of invoices each month, so having a consistent system to manage and organise them is essential to stay on track and avoid drowning in paperwork. The invoice number may seem minor, but it’s a simple yet crucial tool for keeping your business running smoothly.
Throughout this article, you’ll discover essential information about invoice numbers, including what an invoice number is, why it is important, and how to set up a reliable numbering system that makes tracking your finances efficient, along with practical considerations.
What Is an Invoice Number?
An invoice number is a unique identifier assigned to an invoice, so it can be tracked, referenced, and verified.
Every final invoice you issue should have its own invoice number. The number should be unique, logical, and never reused.
The invoice number is usually placed near the top of the invoice, close to the invoice date and issue details. This makes it easy for clients, banks, and accountants to reference the correct document when processing payments or reviewing financial records.
Note: Invoice number is just one part of an invoice. Learn more about invoices and the 13 different types you may need in our guide.
Why Are Invoice Numbers Important?
Here are 4 reasons why invoice numbers are essential for business owners.
- Keep invoices organised
Invoice numbers make it easy to locate a specific invoice when responding to client questions, reviewing records, or preparing for audits. - Track payments accurately
Invoice numbers serve as references to quickly determine which invoices are paid, unpaid, or overdue (also known as past due). This makes it easier to manage outstanding invoices and follow up on payments. - Support accounting and tax reporting
A clear numbering sequence shows that all income is recorded, making bookkeeping, reconciliation, and tax preparation easier for you or anyone managing your accounts. - Stay in control as your business grows
A well-planned numbering system grows with your business, preventing duplication and errors.
1. Do you always include an invoice number on your invoices? Why?
2. How does an invoice number help in your invoicing management?
How to Generate Invoice Numbers & Examples
There is no single legal requirement you must follow when assigning invoice numbers. What matters is having a clear, consistent system that uniquely identifies each invoice you issue.
A good invoice numbering system should be easy to apply, easy to understand, and scalable as your business grows. While it is technically possible to use more than one format, small businesses are usually better off sticking to a single invoice system. This reduces errors and makes bookkeeping easier.
Below are 4 practical ways businesses assign invoice numbers.
|
Sequential Numbering
Examples:
✅ 001, 002, 003
✅ 0001, 0002, 0003 - The simplest option. Leading zeros keep numbers consistent. - Suitable for small businesses with low invoice volume. |
Chronological Numbering
Examples:
✅ 20250812-001, 20250812-002
✅ 20260109-001, 20260109-002 - Easy to see when each invoice was issued. - Suitable for frequent daily or monthly invoices. |
|
Client-based Numbering
Examples:
✅ STAT-001, STAT-002
✅ ACME-001, ACME-002 - Invoices are grouped by customer, using customer name abbreviation - Useful when invoicing the same clients regularly. |
Project-based Numbering
Examples:
✅ BL-51-001, BL-51-002
✅ PRJ-104-001, PRJ-104-002 - Invoices are grouped by project, using project name abbreviation - Useful for project or milestone billing. |
At Statrys, invoice numbers are part of how we keep things clear and compliant. They make it easier to track transactions and meet Hong Kong’s regulatory expectations. For any business, having a simple and consistent numbering system is simply good housekeeping.
Key Considerations When Numbering Your Invoices
Once you have chosen an invoice numbering format, a few practical considerations can help you avoid issues later, especially as invoice volume increases.
Handling Cancelled or Voided Invoices
Cancelled invoices should not be deleted. Keep the invoice number on record and clearly mark the invoice as void. This preserves a complete transaction history and avoids gaps that are difficult to explain later.
In case of errors in invoice numbers, such as duplicate invoice numbers, deleting the invoice can break the numbering order and create issues for payment tracking, accounting, and audits.
Here’s what you can do when you detect mistakes in the invoice numbers:
| Scenario | What to do | Why |
|---|---|---|
| Invoice has not been sent | Correct the invoice number before sending it and save the updated number in your system | Nothing has been shared yet, so it is safe to correct |
| Invoice has been sent, but not paid | Send a new invoice with a new number and mark the original invoice as void | Avoids confusion when the client pays |
| Invoice has been paid under the wrong number | Record the mistake, issue a credit note for the incorrect invoice, then issue a new invoice with the correct invoice number | Keeps your records clear and accurate |
Proforma Invoices and Drafts
Proforma invoices and drafts should use a separate reference or quotation number. They should not use invoice numbers that are reserved for final invoices.
What is a proforma invoice?
Resetting Invoice Numbers
Some businesses reset invoice numbers each year, while others run a continuous sequence. Both approaches are acceptable, but the key is to decide upfront and apply it consistently.
If you reset annually, do so only at the start of the new financial year.
Use Invoicing Software
Invoicing software helps automate numbering so each invoice is issued with a unique invoice number in the correct sequence.
Using an invoicing platform, such as Statrys allows businesses to issue numbered invoices, track payments, and keep records aligned with their accounting workflow in one place.
How Statrys Can Help
Managing invoice numbers is much simpler when the invoicing process is automated. Statrys Invoicing helps you create, send, and track invoices without worrying about duplicates, gaps, or manual mistakes.
With Statrys, you can:
- Issue professional invoices in multiple currencies
- Duplicate and edit past invoices for recurring billing
- Share them easily by link, email, or messaging apps
- Add payment options, such as PayPal and QR codes.
You can also see when invoices are viewed, paid or overdue, and keep both customer and supplier invoices organised in one place.
And it’s free to get started!
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FAQs
How do I find an invoice number?
An invoice number is typically located near the top of the invoice, on the right or left side of the page. It is often accompanied by the date the invoice was issued, as well as the company name and contact information of the issuer.
