What Is an Invoice Reference Number?

What is an invoice reference number?

An invoice reference number is a reference added to an invoice to help identify what the invoice relates to. Depending on the business and transaction, the reference could represent a customer, project, job, contract, order, or another useful record.

For example, a contractor working on several jobs for the same customer might use a different job reference for each project. Adding that reference to the invoice makes it easier to identify which job the charges relate to.

An invoice reference does not normally replace the invoice's own unique invoice number. Instead, it provides additional information that helps connect the invoice with another record.

Invoice reference number example

An invoice could contain several different identifiers while keeping each one clearly labelled:

• Invoice Number: INV-2026-142
• Reference Number: PROJECT-28
• Customer: Example Business Ltd
• Invoice Date: 29 August 2026
• Due Date: 28 September 2026

What is an invoice reference used for?

An invoice reference can make it easier to connect an invoice with the work, customer, or transaction it relates to. This becomes particularly useful when the same customer receives several invoices or when a business manages multiple projects at the same time.

Instead of identifying an invoice only by the customer's name or invoice date, a reference provides another searchable piece of information.

The seller and customer can also use an agreed reference when discussing the transaction, making it easier to identify the relevant project, job, or order.

What can an invoice reference number refer to?

There is no single type of information that every invoice reference must represent. Businesses can choose references that match how they organise their work and records.

The important thing is that the reference has a clear purpose and can be understood by the people who need to use it.

• Customer account
• Project
• Job
• Contract
• Order
• Booking
• Work reference
• Purchase order

Invoice reference number vs invoice number

An invoice reference number and invoice number should not automatically be treated as the same thing. The invoice number is the unique identifier assigned to the invoice itself.

An additional invoice reference can instead identify something connected with the invoice. For example, invoice number INV-105 could contain project reference PROJECT-18.

In that example, INV-105 identifies the specific invoice while PROJECT-18 tells the business which project the invoice relates to.

Invoice number and reference number example

The difference can be shown with a simple example:

• Invoice Number: INV-105
• Invoice Reference: PROJECT-18
• INV-105 identifies the invoice
• PROJECT-18 identifies the related project

Invoice reference number vs PO number

A PO number is a specific type of reference associated with a purchase order. It normally comes from the buyer and identifies the purchase order connected with the transaction.

An invoice reference number is broader. It could refer to a project, customer, contract, job, order, or another useful record rather than specifically identifying a purchase order.

This means a PO number can be used as a reference on an invoice, but not every invoice reference is a purchase order number.

Can an invoice have both a reference number and PO number?

Yes. An invoice can contain both when the references serve different purposes. A PO number could identify the customer's purchase order while another reference identifies the project or job being invoiced.

For example, a construction supplier could receive PO-4205 from a customer while using SITE-18 as its own project reference.

Both references can appear on the invoice alongside its unique invoice number as long as each field is clearly labelled.

• Invoice Number: INV-260
• PO Number: PO-4205
• Project Reference: SITE-18

Who creates an invoice reference number?

Who creates the reference depends on what the reference represents. A business may create its own project, job, or customer reference as part of its internal organisation.

In other situations, the customer may provide a reference and ask for it to be included on the invoice. Purchase order numbers are a common example because they are normally supplied by the buyer.

If a customer has specifically requested a particular reference, using the exact reference they supplied can make the invoice easier for them to identify.

Does an invoice need a reference number?

Not every invoice needs an additional reference number. A straightforward invoice may only need its unique invoice number along with the normal customer, date, item, total, and payment information.

Additional references become useful when there is another record worth connecting to the invoice, such as a project, job, contract, customer account, or purchase order.

Businesses should therefore use invoice references when they provide useful information rather than creating unnecessary numbers simply to fill a reference field.

Where does the reference number go on an invoice?

An invoice reference number should normally be placed somewhere easy to identify within the invoice details. It can appear near the invoice number, invoice date, due date, and other important identifiers.

The reference should also have a descriptive label. If it represents a project, Project Reference may be clearer than simply writing Reference. The same principle can be applied to customer, job, contract, and order references.

Clear labelling prevents an additional reference from being mistaken for the invoice number or purchase order number.

How should an invoice reference be formatted?

There is no single reference format that works for every business. A useful format should be consistent, readable, and appropriate for the type of information being identified.

Some businesses use simple numerical references, while others combine letters and numbers to make the purpose of the reference easier to recognise.

For example, PROJECT-024 immediately suggests a project reference, while CLIENT-105 could identify a customer account.

• PROJECT-024
• JOB-105
• CLIENT-052
• ORDER-2048
• CONTRACT-18

Can an invoice reference contain letters and numbers?

Yes. Invoice references can contain letters, numbers, or a combination of both when the format suits the business.

Using letters can help communicate what a reference represents. For example, JOB-104 is easier to recognise as a job reference than an unexplained number such as 104.

Whatever format is chosen, using it consistently can make references easier to recognise across future invoices and business records.

Can the same reference number appear on multiple invoices?

The same reference can potentially appear on multiple invoices if each invoice relates to the same underlying project, job, customer, contract, or order.

For example, a six month project could generate several invoices while each invoice continues to use the same project reference. The individual invoices would still have their own unique invoice numbers.

This is another important difference between an invoice reference and the invoice number. A broader reference may connect several documents, while the invoice number identifies one specific invoice.

Using project references on invoices

Project references can be particularly useful for businesses that complete several pieces of work for the same customer. Without a project reference, invoices may only contain the customer's details and descriptions of the work.

Adding a project reference gives both parties another way to connect the charges with a particular project.

For example, a designer could use PROJECT-WEBSITE-04 for work relating to a website project while using a different reference for branding work completed for the same customer.

Using job references on invoices

A job reference works in a similar way by connecting an invoice with a specific piece of work. This can be useful for contractors, tradespeople, repair businesses, and other services where customers may request several separate jobs.

For example, a contractor could use JOB-204 for one repair and JOB-205 for another. The corresponding invoices can include those references while still using separate invoice numbers.

A clear job reference can make it easier to determine which work an invoice relates to without relying only on the item descriptions.

Common invoice reference mistakes

Invoice references should make a document easier to identify rather than creating additional confusion. Common mistakes include:

• Using an invoice reference as a replacement for the invoice number
• Leaving a reference without a clear label
• Using an incorrect customer supplied reference
• Confusing a project reference with a PO number
• Changing reference formats without a clear reason
• Adding unnecessary references that serve no purpose
• Putting important references somewhere difficult to find
• Using the same label for several different types of reference

How to make invoice references clear

Every reference should have a clear purpose. If the invoice contains an invoice number, PO number, and project reference, each identifier should have its own label so the customer does not need to guess what the numbers represent.

Keeping the references together within the main invoice details can also make them easier to find.

Before sending the invoice, check any customer supplied references carefully to make sure the correct project, order, or purchase order has been connected with the invoice.

Create an invoice with a reference number

Adding a useful reference can make an invoice easier to connect with a customer, project, job, order, or other business record while the unique invoice number continues to identify the invoice itself.

InvoiceAtlas can be used to create professional invoices with organised invoice details, customer information, itemised charges, taxes, discounts, payment terms, and relevant reference information.

Once your invoice has been reviewed, it can be downloaded as a professional PDF for free and sent to the customer.