What Is a PO Number on an Invoice?

What is a PO number on an invoice?

PO stands for purchase order. A PO number is an identifier assigned to a purchase order and can be included on an invoice to connect the invoice with the customer's original order.

Purchase orders are generally created by buyers before an invoice is issued. When the seller later invoices the customer for the relevant goods or services, the purchase order number can be added to the invoice as a reference.

For example, if a customer issues purchase order PO-2026-104 for a piece of work, the seller can include PO-2026-104 on the invoice created for that work.

What does PO stand for on an invoice?

PO stands for purchase order. If an invoice contains a field labelled PO Number, Purchase Order Number, or PO Reference, it generally refers to the identifier of the purchase order associated with the transaction.

The PO number does not normally replace the invoice number. Instead, the two identifiers can appear together because they refer to different business documents.

The invoice number identifies the seller's invoice, while the PO number identifies the buyer's related purchase order.

PO number example

An invoice containing both an invoice number and purchase order number could show:

• Invoice number: INV-2026-052
• PO number: PO-2026-104
• Invoice date: 29 August 2026
• Due date: 28 September 2026
• Customer: Example Business Ltd

Who provides the PO number on an invoice?

The PO number is normally provided by the buyer because the buyer creates the purchase order as part of its purchasing process. The seller then uses the supplied number when creating the corresponding invoice.

A supplier should not normally invent a customer's PO number. If the customer says that a PO number is required but has not provided one, the supplier can ask the relevant customer contact or purchasing department for the correct reference.

Using the number supplied by the customer helps ensure that the invoice can be connected with the correct purchase order.

Why do businesses use PO numbers on invoices?

PO numbers help connect different documents relating to the same transaction. A business receiving many invoices can use the purchase order number to identify the order connected with a particular supplier invoice.

This can be particularly useful for larger organisations with purchasing and accounts processes involving several employees or departments. The person processing an invoice may not be the same person who originally requested the goods or services.

A PO number gives the business another reference it can use to identify what the invoice relates to.

How does a PO number work?

A typical transaction may begin when the buyer creates a purchase order for goods or services. That purchase order is given a PO number that allows the buyer to identify it within its records.

The seller provides the agreed goods or services and later creates an invoice requesting payment. If the buyer supplied a PO number, the seller can include it on the invoice.

The buyer can then use the PO number to connect the received invoice with the corresponding purchase order.

• Buyer creates a purchase order
• Purchase order receives a PO number
• PO number is supplied to the seller
• Seller provides the goods or services
• Seller creates an invoice
• PO number is included on the invoice
• Buyer can connect the invoice with the original order

Is a PO number the same as an invoice number?

No. A PO number and invoice number identify different documents. The invoice number identifies the invoice, while the PO number identifies the related purchase order.

They are also commonly created by different parties. The seller normally assigns its invoice number, while the buyer normally supplies the purchase order number.

An invoice can therefore contain both an invoice number and PO number without either one replacing the other.

PO number vs invoice number example

For example, a transaction could contain:

• Purchase order number: PO-1452
• Invoice number: INV-328
• PO-1452 identifies the buyer's purchase order
• INV-328 identifies the seller's invoice
• Both numbers can refer to the same transaction while identifying different documents

Is a PO number the same as an invoice reference number?

A PO number can act as a reference on an invoice, but an invoice reference does not always refer to a purchase order.

Businesses can use references for projects, jobs, contracts, customers, orders, or other records. A PO number has a more specific purpose because it identifies a purchase order.

If a customer specifically requests a PO number, an unrelated project or customer reference should not be treated as a replacement for the requested purchase order number.

Does every invoice have a PO number?

No. An invoice does not automatically have a PO number because not every purchase begins with a purchase order.

A freelancer providing a small service directly to a customer, for example, may issue an invoice without ever receiving a purchase order. Other organisations may regularly create purchase orders and expect suppliers to include the corresponding numbers on their invoices.

Whether a PO number is needed therefore depends on the transaction and the customer's purchasing process.

What if a customer does not provide a PO number?

If the customer does not use purchase orders for the transaction, there may simply be no PO number to include. The invoice can still have its own unique invoice number and any other useful reference information.

If the customer requires a PO number but has forgotten to provide it, asking for the correct number before submitting the invoice can help avoid using an incorrect reference.

It is particularly useful to check this when working with organisations that have formal purchasing or accounts procedures.

Where should the PO number be shown?

The PO number should be clearly labelled and easy for the customer to find. It is commonly included with the main invoice details near the invoice number, invoice date, and due date.

A clear PO Number or Purchase Order Number label helps distinguish it from other references that might appear on the invoice.

Avoid hiding an important PO number inside a long description or notes section if the customer needs it to identify the transaction.

Can the same PO number appear on more than one invoice?

The same PO number can sometimes appear on multiple invoices when one purchase order covers goods or services that are billed separately or in stages.

For example, work covered by one purchase order might be invoiced at different points during a project. Each invoice can reference the same relevant PO number while retaining its own unique invoice number.

This demonstrates why PO numbers and invoice numbers should remain separate. One purchase order can potentially relate to several individual invoices.

Common PO number mistakes

PO numbers are simple references, but several mistakes can make an invoice harder for the customer to identify or process:

• Confusing the PO number with the invoice number
• Creating a PO number instead of using the customer's number
• Entering the wrong purchase order number
• Leaving out a PO number requested by the customer
• Using an unrelated reference instead of the PO number
• Failing to label the PO number clearly
• Hiding the PO number inside invoice notes
• Removing the invoice number because a PO number is present

How to check a PO number before sending an invoice

Before sending an invoice, compare the PO number entered on the invoice with the reference originally supplied by the customer. Small errors in the number can make it harder for the customer to connect the invoice with the correct order.

It is also useful to make sure the PO number has been placed in the correct field and has not accidentally been entered as the invoice number.

The finished invoice should clearly distinguish the seller's invoice number from the customer's purchase order number.

Create an invoice with a PO number

When a customer has supplied a purchase order number, including it clearly on the corresponding invoice can help connect the payment request with the correct order.

InvoiceAtlas allows you to create professional invoices with invoice details, customer information, itemised goods or services, taxes, discounts, payment terms, and relevant reference information.

Once the invoice is complete, it can be reviewed and downloaded as a professional PDF for free.