Blog · Purchase order basics
How to number purchase orders
Give every purchase order a number from one sequence, with a short prefix and a zero-padded counter, such as PO-000123. Never reuse a number, never change one after it’s sent, and record why any number is missing. A revision keeps a visible link to the original. That’s the whole idea; the rest of this guide explains why and how to set it up.
Why the number matters
The purchase order number is the thread that ties a purchase together. The supplier quotes it on their delivery note and their invoice. Accounts payable uses it to find the order an invoice belongs to and to check the price and quantity. An auditor picks a payment and follows the number back to the order, and from the order to the person who approved it.
If the number is missing, duplicated or changed, each of those steps turns into a search. Invoices sit unpaid while someone works out which order they mean, and the same invoice can be paid twice because two orders look alike. A clean sequence costs nothing to keep and saves that work every month.
The rules
Whatever format you choose, four rules keep the numbers trustworthy.
- One sequence. Every order comes from the same counter, so no two people can issue the same number. If several companies need their own sequences, give each a distinct prefix.
- Never reused. A cancelled order keeps its number. Issuing it again to a different supplier makes the old paperwork point at the wrong purchase.
- Never edited. Once an order has gone to the supplier, its number is fixed. Changes are made by revision, not by retyping.
- Gaps are fine, but explained. A number skipped because a draft was abandoned or an order was cancelled is normal. What matters is that you can say why it’s missing.
Formats that work, and formats that cause trouble
A good number is short, unique and easy to read over the phone. A prefix tells everyone it’s a purchase order rather than an invoice or a quote. Zero-padding keeps numbers the same length, so they sort in order in any list. Groups with several companies can add an entity code. Some teams add the year, which helps at year end but means deciding whether the counter restarts.
Three habits cause trouble. Dates alone, such as 20261008, collide as soon as two orders go out on the same day. Initials, such as JS-14, tie the number to a person who may leave and create parallel sequences that clash. Numbers typed by hand, or taken from the next empty row in a spreadsheet, get skipped, duplicated and mistyped.
| Format | Example | When it fits |
|---|---|---|
| Prefix and counter | PO-000123 | One company, one sequence. The simplest choice that holds up. |
| Prefix, year and counter | PO-2026-0123 | You want the year visible and have decided whether the counter restarts each year. |
| Entity code and counter | NTH-PO-00123 | A group with several companies, each ordering on its own paper. |
| Date only | 20261008 | Rarely. Two orders on one day share a number. |
| Initials and counter | JS-14 | Avoid. It breaks when people change and creates parallel sequences. |
Revisions and cancellations
When an order changes after it’s sent, never change it silently. Either keep the number and add a revision suffix, such as PO-000123 rev 2, or issue a new number that names the one it replaces. Both work; pick one and use it every time. The supplier gets the revised order, and the original stays on file marked as replaced.
A cancelled order keeps its number and is marked cancelled, with the date, who cancelled it and why. Tell the supplier in writing. The number is then accounted for, which is exactly what an auditor wants to see when they spot the gap.
Moving numbers out of a spreadsheet
When you move to a system, the orders already issued from the spreadsheet stay valid, and suppliers will keep quoting those numbers on invoices for weeks. Don’t renumber them. Keep the spreadsheet as a read-only record of past orders.
For new orders, avoid any overlap with the old numbers. Either change the prefix or start the new counter well above the last number used, and write down the switch-over date and the last old number. Anyone looking at a number can then tell which record holds it.
Setting your scheme
This takes an hour, and you only do it once.
- Decide who issues numbers: one counter for the whole business, or one per company if each orders on its own paper.
- Choose a prefix for each counter, two to four letters, and check it doesn’t clash with your invoice or quote numbers.
- Choose the counter’s length. Six digits leaves room to grow; pad with zeros.
- Decide whether the year goes in the number. If it does, decide now whether the counter restarts each year.
- Pick one way to show a revision, a suffix or a new number that names the old one, and write it down.
- Note the last number in your old records and set the new sequence to start clear of it.
- Put the rules in your purchasing policy, including how cancellations and gaps are recorded, and ask suppliers to quote the number on every invoice.
Questions
What should a purchase order number look like?
A short prefix and a zero-padded counter, such as PO-000123, suits a typical business. Add an entity code if several companies issue orders, and the year only if you want it visible.
Is it a problem if there are gaps in purchase order numbers?
No, as long as each gap can be explained. A cancelled order or an abandoned draft is a normal reason; keep a note of it so nobody has to guess.
Can we reuse the number of a cancelled purchase order?
No. The cancelled order keeps its number and is marked cancelled. Reusing it means two different purchases share one reference, which confuses invoices and audits.
Should a revised purchase order get a new number?
Either approach works: keep the number with a revision suffix, or issue a new number that references the original. What matters is that the change is visible and the supplier gets the revised order.
Should purchase order numbers restart every year?
Only if the year is part of the number, so PO-2026-0001 and PO-2027-0001 stay distinct. A single running counter is simpler and never collides.