A PO number is the identifier a business puts on a purchase order so that everything downstream can point back to it: the supplier's paperwork, the delivery note, and above all the invoice. It is the smallest part of a purchase order system and the part that does the most work, because without it an invoice is a claim and with it an invoice is a claim you can check. This page is about what the number is, how to structure one when you are starting from nothing, and what it means when you see one on an invoice.
What the number is for
One thing: making an invoice checkable. When a supplier quotes your PO number back on their invoice, anyone in your office can find the order it belongs to, see what was approved, see what was received, and decide whether the invoice is right, without asking the person who ordered it. That is the whole value, and it is why the number has to reach the supplier on the order rather than living only in your own records. An internal-only number solves nothing, because the invoice will arrive without it.
How to structure one
Sequential, unique, and boring. A simple running number with an optional year prefix is enough for almost every small business: it is short enough to be quoted correctly over the phone, it makes a gap visible, and it carries no information that can become wrong. Encoding the department, the supplier or the category into the number is tempting and is a trap, because the moment a department is renamed or an order spans two categories, the number lies. Keep the meaning in the fields and the identity in the number.
The po number on an invoice
When you see a PO number on a supplier's invoice, it is the supplier telling you which of your orders they are billing against. Check three things: that the number exists in your log, that the invoice total is within what was approved, and that what is being billed for was actually received. Where an invoice arrives with no PO number at all, that is worth treating as a finding rather than a formatting issue, because it usually means somebody ordered outside the process and nobody approved the commitment.
Getting the number onto the supplier's invoice
Ask for it, on the order, in a place the supplier's accounts team will see, and repeat the request when an invoice arrives without one. Most suppliers will quote it once they know you require it, and the ones that will not are worth knowing about. The free generator on this site numbers each order it produces, so the number exists on the document the supplier receives rather than only in your log. Consistency does the rest: a business that requires the number every time gets it, and one that requires it sometimes gets it sometimes.
Questions people ask about po number
What is a PO number?
The unique identifier a business puts on a purchase order so that the delivery note and the invoice can point back to the order that was approved. It is what makes an invoice checkable rather than a claim.
How should PO numbers be structured?
Sequential, unique and boring, with at most a year prefix. Encoding department or supplier into the number makes it lie the moment anything is renamed; keep meaning in fields and identity in the number.
What does a PO number on an invoice mean?
The supplier is telling you which order they are billing against. Check the number exists in your log, the total is within what was approved, and the goods were received.
What if an invoice arrives with no PO number?
Treat it as a finding rather than a formatting problem. It usually means somebody ordered outside the process and nobody approved the commitment before it was made.