PO numbers on invoices: what they are and where to put them

A purchase order (PO) is a document a client raises to approve spending money with you. It usually has a unique number, a description of the work, the dates and an agreed amount. The PO number is how their finance team matches your invoice to the work that was approved and the budget it comes from.

Clients name them differently — “PO”, “purchase order”, “order number” or a code with their own prefix. Whatever it’s called, treat it as their reference for your job.

• Near the top, alongside your invoice number and invoice date, with a clear label such as “PO number”.
• Exactly as the client wrote it — including letters, prefixes, dashes and leading zeros. A small typo can stop it matching in their system.
• On the relevant line if one invoice covers work under more than one PO, so each line can be matched to its own order.
• Where they’ve asked for it. Some clients also want the PO in the email subject or the file name — follow their instructions if they give them.

Example: a PO number on an invoice

No. GOV.UK sets out what an invoice must include — a unique invoice number, your name and contact details, the client’s name and address, a description of the work, the supply date, the invoice date, the amounts, any VAT and the total — and a PO number isn’t on that list. If you’re VAT-registered, VAT invoices have extra requirements, such as your VAT number and the VAT shown separately; HMRC’s VAT Notice 700 has the details.

So a PO is a client requirement, not a legal one. Purchase orders tend to be used by organisations with formal purchasing processes — many UK universities and councils, for example, publish “No PO, No Pay” policies for their suppliers. Smaller clients may not use them at all.

For a client that uses purchase orders, the PO is usually how an invoice gets approved: their accounts team checks your invoice against the order — the number, the work and the amount — before paying it. If the PO number is missing or doesn’t match, the invoice may not get through that check.

Some clients spell this out in their supplier instructions. The University of Plymouth says invoices without an official PO number “will be returned to you and may delay payment”, and the Royal Borough of Windsor and Maidenhead says invoices without a valid PO number “will be rejected and returned to you unpaid”. Shakespeare’s Globe’s finance FAQ for freelancers lists the purchase order number first among what to include on an invoice. Not every client is this strict, so check the instructions you’ve been given.

• Ask early. When you’re booked, ask whether they use purchase orders and who will send it. It’s much easier than chasing after the job.
• Check when it arrives. Not every client sends the PO before the work — some raise it afterwards. If you’re waiting, ask when to expect it.
• Confirm if none is needed. If they say they don’t use POs, it’s worth having that in writing, and adding any reference they do give you — the event or job name, or the person who booked you.
• Don’t guess a number. Leave it off rather than invent or reuse a PO; a wrong number is harder to fix than a missing one.

These are three different situations, and they’re handled differently.

A missing or wrong PO number

This is a problem with the client’s process, not a late payment. A missing PO doesn’t, on its own, make a payment overdue or give you a claim — and some clients’ terms say their payment period only starts once they have a compliant invoice. The Royal Borough of Windsor and Maidenhead, for example, says its 30-day payment clock “will only commence once a valid and compliant invoice is received”, and Haringey Council says it is not liable for late payment interest caused by missing or incorrect PO information. Those are the clients’ own terms, so check what you agreed — and get the PO sorted, which is usually the quickest fix.

A disputed invoice

If the client disagrees with the work or the amount, that’s a dispute to resolve with them — ask what they’re querying and keep the conversation in writing. GOV.UK’s late payment guidance doesn’t cover disputed invoices, so if you can’t agree, get advice before claiming interest or costs.

A genuinely overdue payment

If you’ve sent a valid invoice to a business client and the payment date has passed, UK law gives you rights to interest and debt recovery costs. According to GOV.UK:

• An agreed payment date must usually be within 60 days for business transactions (30 days for public authorities); a longer period can be agreed if it’s fair to both businesses.
• If no payment date was agreed, payment is late 30 days after the later of the client receiving the invoice or you providing the service.
• Statutory interest is 8% plus the Bank of England base rate — but not if your contract sets a different rate. To add interest, you send a new invoice.
• You can also charge a fixed sum for recovery costs — £40, £70 or £100 depending on the size of the debt — once for each payment.

• Multi-day jobs: one PO often covers the whole booking. Check the PO dates and amount match the days you actually worked.
• Extra days or overtime: if the job ran longer than the PO covers, ask for the PO to be updated (or a new one) before you invoice the extra.
• Several jobs, one client: some clients issue one PO per show or per job, others one PO for a run of work. Ask which they want on each invoice.
• Keep the PO with the job: save the PO email or PDF where you keep the job details, so you’re not searching your inbox when it’s time to invoice.

Whatever you use, the aim is the same: every job should show its PO number (or “no PO needed”) before you invoice it. A simple spreadsheet works — one row per job with the client, dates, PO number, PO amount and invoice number — as long as you keep it up to date.

CrewSynk is built around the job, so the PO lives with it. You can store the PO number and amount on each job, and the optional PO Finder can search your Gmail for the matching purchase order and attach the PDF to the job — you confirm before anything is applied. When you turn the job into an invoice, the PO number is added to the invoice line for you, and CrewSynk warns you if the same PO is already on another job or invoice. If one PO covers several jobs, you can put them on one invoice.