An invoice number is the unique reference used to identify an invoice. A clear numbering system helps you find payments, answer customer questions and keep reliable tax records.
For VAT invoices, section 153 of the Nigeria Tax Act 2025 requires sequential invoice numbering.
What does sequential numbering mean?
Sequential means that your invoice numbers follow an organised order.
For example:
- INV-2026-0001
- INV-2026-0002
- INV-2026-0003
The number does not need to look exactly like this. It simply needs to be unique, consistent and easy to trace.
Good invoice number formats
| Format | Example | Best for |
|---|---|---|
| Simple sequence | INV-000145 | Small businesses |
| Year and sequence | INV-2026-0145 | Businesses that group invoices by year |
| Branch and sequence | LAG-INV-0145 | Businesses with several branches |
| Client reference and sequence | ACME-0145 | Businesses that need a client code |
Do not make the format so complicated that staff cannot use it consistently.
Can you restart invoice numbers each year?
You can use a new annual sequence if the year is part of the number and your records make the change clear.
Example:
- final invoice in 2026: INV-2026-0382
- first invoice in 2027: INV-2027-0001
Keep a written note of how the numbering system works, especially if several people create invoices.
What should you do with a cancelled invoice?
Do not give its number to another invoice. Mark the original invoice as void or cancelled and keep it in your records with the reason.
Example:
- INV-2026-0088: issued
- INV-2026-0089: cancelled because the customer order was withdrawn
- INV-2026-0090: next invoice
This keeps the sequence understandable.
Can quotations, receipts and credit notes use the same sequence?
It is clearer to give each document type its own prefix.
| Document | Example |
|---|---|
| Invoice | INV-2026-0041 |
| Quotation | QT-2026-0018 |
| Receipt | REC-2026-0035 |
| Credit note | CN-2026-0004 |
| Debit note | DN-2026-0002 |
This prevents a quotation from being mistaken for a tax invoice.
Common invoice numbering mistakes
Using the same number twice
Duplicate numbers make payment matching and tax records unreliable.
Using only the customer's name
"ADEOLA INVOICE" is not a useful unique sequence. Use a proper number and add the customer name separately.
Deleting numbers without a record
Keep cancelled documents and record why they were cancelled.
Allowing each team member to create a different format
Agree one company-wide system. If teams need separate sequences, define the branch or department code clearly.
Changing the format too often
Choose a simple format that can last. Document any necessary change.
Invoice number checklist
Your system should be:
- unique
- sequential
- easy to read
- separate from quotation and receipt numbers
- consistent across the business
- supported by a record of cancelled invoices
Create an invoice with a clear reference number and keep it with your customer and payment records. For the other fields you may need, read the VAT invoice format guide and sample invoice guide.