Invoice References and Purchase Order Numbers Guide

What are invoice references and purchase order numbers?

Invoices can contain several identifiers that help businesses organise and track transactions. The invoice number identifies the invoice itself, while other references can connect the invoice with related business records.

One of the most common examples is a purchase order number, often shortened to PO number. A customer may provide a PO number when placing or approving an order and ask the supplier to include that number on the corresponding invoice.

An invoice can also contain a broader reference number for a customer, project, job, contract, order, or another record that both parties use to identify the transaction.

What is an invoice number?

An invoice number is a unique identifier assigned to an invoice. It allows the seller and customer to identify a particular invoice without relying only on the customer's name, date, or invoice value.

Businesses may use simple sequential invoice numbers such as 001, 002, and 003, or create a consistent numbering format that includes additional information.

The important distinction is that the invoice number identifies the invoice itself. A PO number or reference number may instead identify another document, order, customer, or project connected with that invoice.

What is a PO number on an invoice?

A PO number on an invoice is a purchase order number associated with the customer's purchase order. The purchase order is normally created by the buyer as part of their purchasing process before the corresponding invoice is issued by the seller.

When the seller later creates an invoice, the buyer may require the relevant PO number to be included. This helps the buyer connect the invoice with the purchase that was previously requested or approved.

For example, a customer might issue purchase order PO-2026-105 for a particular job. The supplier can include PO-2026-105 as the PO number when invoicing for the goods or services connected with that order.

PO number example on an invoice

A simple invoice could contain several different identifiers:

• Invoice number: INV-2026-042
• PO number: PO-2026-105
• Customer: Example Construction Ltd
• Project reference: SITE-24
• Invoice date: 29 August 2026

Who provides the PO number?

A purchase order number is generally provided by the buyer because it relates to a purchase order created through the buyer's purchasing process.

The supplier should therefore use the PO number supplied by the customer rather than creating a replacement PO number for the invoice.

If a customer requires a PO number but has not provided one, the supplier can ask the relevant customer contact for the correct reference before sending the invoice.

Why put a PO number on an invoice?

Including the correct PO number can make it easier for the customer to identify what an invoice relates to. This can be particularly useful for businesses that receive invoices from many different suppliers.

The customer's accounts or purchasing team can compare the PO reference on the invoice with its own records and identify the related order more quickly.

Some organisations may also require suppliers to quote a valid PO number as part of their internal invoice processing procedure, making it important to check the customer's requirements before submitting an invoice.

Where does a PO number go on an invoice?

A PO number should normally appear somewhere clearly visible near the main invoice details. It is commonly placed close to information such as the invoice number, invoice date, and payment due date.

The field should be clearly labelled PO Number or Purchase Order Number so the customer can identify it without searching through the invoice description, notes, or payment terms.

Keeping important references together near the top of an invoice can also make the document easier for customers and accounts teams to review.

What is an invoice reference number?

An invoice reference number is a reference used to connect an invoice with another useful piece of information. Depending on the business, a reference might relate to a customer account, project, job, contract, order, or another internal record.

Unlike the invoice number, an invoice reference does not necessarily identify the invoice itself. Instead, it provides additional context that can help the seller or customer understand what the invoice relates to.

The exact format can vary because different businesses organise customers, projects, and orders in different ways.

Invoice reference number examples

Different types of invoice references could include:

• Customer reference: CLIENT-125
• Project reference: PROJECT-042
• Job reference: JOB-2026-18
• Contract reference: CONTRACT-54
• Order reference: ORDER-291
• Purchase order number: PO-2026-105

Invoice number vs PO number

The main difference between an invoice number and a PO number is what each number identifies and who normally provides it.

An invoice number identifies the seller's invoice and is normally assigned when the invoice is created. A PO number identifies the buyer's purchase order and is normally supplied by the customer when an order is placed or approved.

An invoice can therefore contain both numbers. They are not duplicates because each one connects the transaction with a different document or record.

• Invoice number: identifies the invoice
• PO number: identifies the purchase order
• Invoice number: normally assigned by the seller
• PO number: normally supplied by the buyer
• Both numbers can appear on the same invoice

Invoice number vs invoice reference number

An invoice number and invoice reference number can also have different purposes. The invoice number is the identifier for the invoice itself, while an additional reference can link the invoice with a customer, project, job, contract, or order.

For example, a freelance designer could issue invoice INV-045 while using PROJECT-12 as the project reference. INV-045 identifies the invoice, while PROJECT-12 identifies the project the billed work relates to.

Keeping these identifiers separate makes it easier to search for either the individual invoice or the wider project connected with it.

Is a PO number the same as an invoice reference?

A PO number can be used as a reference on an invoice, but not every invoice reference is a PO number. Purchase order numbers specifically relate to purchase orders.

A general invoice reference can instead represent many different things, such as a customer account, project, job, contract, or internal order.

If a customer specifically asks for a PO number, the correct purchase order number should therefore be used rather than replacing it with an unrelated internal reference.

Does every invoice need a PO number?

Not every invoice will have a purchase order number because not every transaction begins with a purchase order. Whether one is needed depends on how the customer purchases goods or services and the process agreed between the parties.

A freelancer working directly with an individual client may never receive a PO number, while a supplier working with a larger organisation may regularly be asked to quote one.

If the customer has provided a PO number for a transaction, including it on the corresponding invoice can help the customer identify and process the invoice.

What if there is no PO number?

If no purchase order was created and the customer does not require one, there may be no PO number to include on the invoice.

The invoice can still use its own unique invoice number and any other useful reference information associated with the work.

If you are unsure whether a PO number is required, it can be useful to check with the customer before sending the invoice, particularly when dealing with an organisation that has its own purchasing or accounts process.

Can one PO number appear on multiple invoices?

A purchase order may sometimes relate to work or goods that are invoiced in more than one stage. In those situations, multiple invoices can potentially refer to the same purchase order.

Each invoice should still have its own unique invoice number even when the same PO number is referenced.

This is another reason invoice numbers and purchase order numbers should be treated as separate identifiers. One identifies each individual invoice while the other connects it with the relevant purchase order.

Can an invoice have more than one reference?

An invoice can contain several useful references when they help identify the transaction. For example, a business might include an invoice number, PO number, project reference, and customer account reference.

However, adding unnecessary identifiers can make an invoice harder to read. References should have a clear purpose and use labels that allow the customer to understand what each value represents.

Consistent labels are especially useful when the same business regularly invoices the same customer or works across several projects.

Common PO number and invoice reference mistakes

Incorrect or unclear references can make an invoice more difficult for a customer to identify. Common mistakes include:

• Confusing the invoice number with the PO number
• Creating a PO number instead of using the one supplied by the customer
• Entering the wrong purchase order number
• Leaving out a PO number requested by the customer
• Using an unclear reference without a label
• Putting important references somewhere difficult to find
• Using the same field for several unrelated identifiers
• Removing the unique invoice number because a PO number is present

How to display invoice references clearly

Important invoice identifiers should be easy to find and clearly labelled. Keeping them near the main invoice details allows the customer to identify the invoice and related records without searching through the rest of the document.

A simple layout could show the invoice number first, followed by the PO number or another relevant reference, invoice date, and due date.

The rest of the invoice can then focus on the seller and customer details, goods or services supplied, pricing, totals, payment information, and other necessary information.

• Invoice Number: INV-2026-042
• PO Number: PO-2026-105
• Reference: PROJECT-24
• Invoice Date: 29 August 2026
• Due Date: 28 September 2026

Create an invoice with a PO number and reference

When creating an invoice, references should support the information already on the document rather than replace the invoice's own identifier. A clear invoice number identifies the invoice while PO numbers and other references provide additional information about the transaction.

InvoiceAtlas can be used to create a professional invoice containing invoice details, customer information, itemised charges, 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.