The number you can't just make up when it's asked for
By the end, you'll have your invoice series (full, simplified, and corrective) sequential and without gaps, ready in your invoicing software before your first payment.
By Yodai · Published September 6, 2026 · Updated October 12, 2026
You issue your first full invoice. You mark it F-1. The following week you issue another: F-2. All good, until a customer asks for a simplified invoice and you label it F-3, because that's what makes sense in your head: the next available number.
That small habit, so natural, is the mistake. A full invoice and a simplified invoice can't share numbering. If the tax authority reviews your series of full invoices and sees it jump from F-1 to F-2 and then to F-4 because F-3 is in another folder, it doesn't see a mix-up on your part: it sees a gap. And invoicing regulations require that within each series, the numbering be sequential: a missing number is a number you'll have to explain.
When to issue a full invoice, when a simplified one, and when neither is a separate decision. This lesson is about how to number them: setting up the numbering before your first payment comes in, so you're not deciding it with the customer waiting.
A separate series for each document type, not one for everything
The rule is simple: every type of invoice you issue gets its own series, and within that series the numbers run one after another with no skips and no repeats. Ever. Not even if the gap is because you canceled a sale.
For a small business, the usual setup is two series. One for full invoices, with a prefix that identifies it: F-2026-0001, F-2026-0002... Another for simplified invoices, with a different prefix: S-2026-0001, S-2026-0002... The year in the name isn't just for show: every January 1 you can (and should) start the numbering over, as long as the prefix makes it clear which year it belongs to.
If you also issue credit notes (corrective invoices used to fix or cancel an invoice you've already issued), those get their own series too: A-2026-0001. A credit note never reuses the number of the invoice it's correcting, and it never deletes that invoice: it leaves it where it is and adds the credit note alongside it.
- One single counter: every document, whatever type it is, takes the next number
- Seems simpler to manage: you don't have to think about which series applies
- The day someone asks you to explain a gap, there's no clean way to justify it
- One counter for full invoices, one for simplified ones, one for credit notes
- Each series is sequential within itself: no gaps to explain
- Set it up once in your invoicing software, and you won't have to touch it again.
Which one to pick: Separate series, always. A single series only looks simpler until the first unexplained gap; after that, it's the one that raises the most questions. Set it up once and it's settled for every payment that comes after.
- 1Choose a prefix for each series
F for full invoices, S for simplified, A for credit notes. Add the year: F-2026, S-2026, A-2026. It doesn't have to be exactly this, but it should be recognizable at a glance. - 2Set your starting point
If you've never invoiced before, start at 0001. If you already have loose invoices you made by hand, go through all of them, sort them by date, and start the series from the number right after the last one, not from 0001 again. - 3Let your invoicing software assign the number
Have it assign the number when you issue each invoice, not you by hand afterward. That way there's no room for reusing a number you've already used, or skipping one without noticing. - 4Decide what happens if you cancel a sale
The number doesn't get deleted or reused. You issue a credit note in its own series that references the original invoice. The canceled invoice still exists, just with its credit note next to it. - 5Check each series before your first real customer
In the preview or draft mode of your invoicing software, if it has one, confirm that a full invoice and a simplified invoice each come out with the right prefix and the number 0001. Don't issue test invoices for sales that never happened: once an invoice is issued, it uses up its number and can't be deleted.
With this set up in your invoicing software, every invoice comes out with the series and number it should have: you're not deciding it anymore. Numbering stops being something you figure out on the spot when it's asked for, and becomes something that's already running before your first customer shows up.
- You have a dedicated series for full invoices and another for simplified ones, each with its own prefix and year
- Credit notes cancel without deleting, in their own series, always referencing the original invoice
- Your invoicing software assigns the number when it's issued, never you by hand afterward
- You've checked each series before your first real customer
- What's next: making sure every payment ends up on an invoice without relying on your memory