PO Number on an Invoice: What It Is and Where to Put It

A PO number (purchase order number) is the reference a buyer's company gives to an approved purchase. When you invoice them, including that number lets their accounts payable team match your invoice to the approval, and pay it.

If a client gave you a PO number and your invoice doesn't show it, expect the invoice to be sent back or quietly parked.

Where it goes

Put it near the top, with the invoice number and dates, where AP staff look:

Invoice no.: INV-0162 Invoice date: 28 August 2026 Due date: 27 September 2026 PO number: 4500123987

Also include it:

In the email subject: "Invoice INV-0162 for PO 4500123987"
In the PDF filename, if the client's system is strict: INV-0162_PO4500123987.pdf

The generator has a "PO / reference" field that prints alongside your invoice number on every template.

Copy it exactly

PO numbers are often long and systems match them character for character. Copy and paste it; don't retype. Include any prefix or suffix ("PO-", "/001") exactly as given.

If you don't have one

Before you start work with any company bigger than a handful of people, ask: "Do you need a PO number on invoices?" If yes, get it before starting. After the work is done, getting a PO raised can take weeks.

If the work is already done and you're missing one:

Ask your contact to raise the PO now, referencing your quote.
Ask accounts payable whether they can process the invoice with a different reference (some can, with the budget holder's approval).
Send the invoice anyway with "PO: to follow" only if the client agrees; otherwise it may be rejected.

Other references clients may ask for

Vendor or supplier number (your ID in their system)
Cost centre or project code
Contact name of the person who ordered the work
Contract number

Add them in the notes or reference field. The more their paperwork matches yours, the faster you're paid.

Add the PO number in the generator's reference field and it prints in the invoice header.

Open the invoice generator

Matching amounts

A PO usually has an approved amount. If your invoice is higher, it will fail the match. Options:

Invoice up to the PO amount and ask for a new or increased PO for the rest
Get the change approved and the PO amended before invoicing
Never quietly exceed a PO

One PO, several invoices

For a blanket or multi-stage PO, quote the same PO number on each invoice, and consider adding a line like "Invoice 2 of 4 against PO 4500123987 (remaining after this invoice: $6,000)". It helps both sides keep track.

More on how POs fit into the buying process: invoice vs purchase order.

Getting set up as a supplier

Many larger organisations can't raise a PO for you until you exist in their system as a supplier (or "vendor"). Onboarding commonly asks for:

Your legal business name and address
Tax details (a W-9 in the US, a VAT number in the UK or EU)
Bank details, often verified by a call-back or a bank letter
Proof of insurance for some kinds of work
Agreement to their supplier terms, including payment terms

Start this the moment a new client says yes. It can take a week or more, and no invoice can be paid until it's done.

Terms hidden in POs

A PO often comes with the buyer's standard terms attached or linked. Read them at least once per client. Things to look for:

Payment terms longer than you quoted (Net 60 or Net 90 isn't unusual)
Invoice requirements, such as a specific email address, portal or file format
Rights and ownership clauses that differ from your proposal
Cancellation rights for the buyer

If something conflicts with what you agreed, raise it before you deliver, not after you've invoiced.

Frequently asked

Is a PO number the same as an invoice number? No. The PO number is the buyer's reference; the invoice number is yours. Both go on the invoice.

Where do I find the PO number? On the purchase order document the client sends, or in the email approving the work. If you only have a verbal go-ahead, ask for it in writing.

Do small businesses use PO numbers? Some do, especially for suppliers on account. If a small client doesn't mention one, you don't need one.

Can I invoice without a PO if the client says it's fine? Yes, but get that in writing, and ask who will approve the invoice instead.