If you invoice businesses — especially larger ones — you will often be asked to include a purchase order (PO) number on your invoice. Without it, the accounts payable department may refuse to process payment.
What is a purchase order?
A purchase order is an internal document a client company creates to authorise a purchase. It is issued by their procurement or finance team and contains a unique number that ties the invoice to the approved spend. For them, it is part of their financial controls. For you, it is a bureaucratic step that you need to manage proactively.
Ask before you start work
The best time to ask for a PO number is when the client confirms they want to proceed. Include it in your new client onboarding questions: "Will you need to raise a purchase order before I start? If so, could you send me the PO number as soon as it is raised?"
What if they forget?
If you get to invoice time and still do not have a PO number, follow up with the person you worked with directly. They will usually know who in their organisation to ask and can chase internally faster than you can from the outside.
Where to put the PO number on your invoice
Include the PO number in a prominent place on your invoice, usually near the top — "PO Reference: [number]". This ensures the client's accounts payable team can immediately match your invoice to their internal records.
Some clients will not pay without a PO number regardless of the circumstances. Make it your standard practice to confirm this at the outset of any new client relationship, particularly with corporates.
Frequently asked questions
What is a purchase order number?
A purchase order (PO) number is an internal reference a client company creates to authorise a purchase, issued by their procurement or finance team, and it ties your invoice to their approved spend.
When should I ask a client for a PO number?
Ask for it when the client confirms they want to proceed, as part of your new client onboarding questions, for example asking whether they will need to raise a purchase order before you start.
Where should the PO number go on an invoice?
Include it in a prominent place near the top of the invoice, such as "PO Reference: [number]", so the accounts payable team can immediately match the invoice to their internal records.
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