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Purchase order number on an invoice: where it goes
Put the client's purchase order number in the invoice header, beside your invoice number and date, labeled "PO number" and copied exactly from their purchase order. Their accounts team matches your invoice to that PO before it pays. A filled header reads: "Invoice 1047. Date: October 15, 2026. PO number: 4500018273. Terms: Net 30, due November 14, 2026." Repeat the PO number in the email subject.
Where does the PO number go on an invoice?
In the header block with the invoice number, date and terms, where accounts payable looks first when it matches invoices to orders. If the PO lists line numbers, put the same line number beside each matching line on your invoice. Here is a full header for a $3,000 job billed against one PO:
- INVOICE 1047
- Invoice date: October 15, 2026
- PO number: 4500018273
- Terms: Net 30, due November 14, 2026
- Bill to: [client company], Attn: Accounts Payable, [accounts payable email]
- Email subject: Invoice 1047, PO 4500018273, $3,000
Is a purchase order number the same as an invoice number?
No. The PO number belongs to the client: it is their approval to spend, issued before the work. The invoice number is yours, issued when you bill, and it follows your own sequence. One PO can carry several invoices: a $6,000 website PO billed as a $3,000 deposit invoice and a $3,000 launch invoice puts the same PO number on both.
Keep your own invoice numbers running in one sequence and give the PO number its own labeled field. When a payment arrives, the remittance may quote either number, so record both against it in your books.
What if the client hasn't given me a PO number?
Ask before you start, in writing: "Does your accounts team need a PO number on our invoices? If so, please send it before [start date] so payment isn't held up." If the work is done and the invoice comes back for a missing PO, add the number and resend it with the original invoice date and terms.
Federal agencies spell this out. Under FAR 52.232-25, a proper invoice includes the contract number and the order and line item numbers. One without them is returned within 7 days, and the 30-day payment clock starts only when the corrected, proper invoice arrives, with no late-payment interest owed before then.
What if the job costs more than the purchase order?
Get the PO raised before you bill over it. On a $3,000 PO where the job came to $3,450, the extra $450 needs a change order the client signs and a revised PO, or the whole invoice can stall while their team sorts out the gap. Send the change order the day the extra work comes up.
If the revised PO is slow, offer two invoices: $3,000 now against the original PO, and $450 once the change is approved, with the PO number on both. Use the PO's descriptions and unit prices on your lines so each one matches.
Frequently asked questions
How do I add a PO number in QuickBooks or Xero?
In Xero, type it in the invoice's Reference field. In QuickBooks Online, add a custom field named "PO number" to your sales forms and fill it in on each invoice. In an app with no field for it, put "PO number: [number]" in the header or the first line description.
Who creates the purchase order number?
The buyer. Your client's purchasing or accounts team issues it when they approve the spend, so you never make one up. If you order materials from suppliers with your own POs, those run in a separate sequence from the numbers clients give you.
What is a blanket purchase order?
One PO that covers repeat work over a period, up to a set total. Monthly office cleaning at $850 might run on one 12-month blanket PO for $10,200. Each monthly invoice quotes the same PO number; keep a running total so you know when it's used up.
Sources
- Acquisition.gov: FAR 52.232-25, Prompt Payment · Checked
A proper invoice includes the contract number or other authorization, including order number and line item number; an invoice that doesn't comply is returned within 7 days.
- Bureau of the Fiscal Service: Prompt Payment FAQs · Checked
The payment period starts when the agency receives a proper invoice, and no late-payment interest is due before then; payment is due 30 days after a proper invoice.
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