Invoice numbering: why it must be sequential

The Dutch tax authority has a straightforward rule: your invoices must be numbered sequentially with no gaps. Invoice 1, 2, 3, then 4 — not 1, 2, 5. This rule exists for one reason: to catch missing invoices. A gap suggests you've hidden an invoice, and auditors will ask about it. Here's how to get it right and what to do if you slip up.

The legal requirement#

Under Dutch VAT law, your invoice numbers must form an unbroken sequence. The Belastingdienst doesn't specify a starting point — you can start at 1, 100, or 2026001 — but once you choose, you cannot skip.

The sequence must be unbroken within a single calendar year (most freelancers) or continuous across all years (if you choose that system). See the section on year-by-year vs. continuous, below.

Breaking the sequence — invoice 1, 2, 3, 10 — is a red flag. The auditor will ask: where are 4–9? Did you issue them and forget to record them? Did you void them? If you can't explain the gap, the auditor may assume you're hiding invoices and may open a broader investigation.

Starting your sequence#

You get to choose your starting number. Common approaches:

• Start at 1 each year. Invoice 2024-001, 2024-002, then 2025-001, 2025-002. Simple and resets you each year.
• Start at 1 with a year prefix. Invoice 2024-1, 2024-2, 2025-1. Also clear.
• Continuous numbering. Invoice 1, 2, 3… across all years, so by year 3 you're at 850. Avoids any confusion about what year you're in.
• Start higher for prestige (optional). Invoice 2025001, 2025002. You can start where you like; the key is consistency.

Important: Whatever you choose, stick to it. Switching from year-by-year to continuous mid-stream confuses auditors.

How to number without gaps#

Use software. If your invoicing app auto-increments the number (most do), gaps are nearly impossible. Each invoice gets the next available number.

Use a spreadsheet. Keep a running counter. After you finish invoice 47, the next invoice is 48. Simple, but requires discipline.

Manual system. If you write invoices by hand or in Word, keep a log:

Invoice 1: 12 Jan – ABC Ltd – paid
Invoice 2: 18 Jan – XYZ Corp – paid
Invoice 3: 25 Jan – Private client – pending

Check the log before creating the next invoice and you'll spot a gap immediately.

Year-by-year vs. continuous numbering#

Year-by-year (most common for small freelancers):

• 2025-001, 2025-002, 2025-003… then reset to 2026-001.
• Pro: numbers stay small and readable; each year is a fresh start.
• Con: you must remember to reset on 1 January.
• Requirement: you still need an unbroken sequence within each year.

Continuous (useful if you invoice a lot or want simplicity):

• Invoice 1 (2025), Invoice 2 (2025), Invoice 3 (2026), etc. — one counter across all years.
• Pro: never reset; no chance of accidentally re-using a number.
• Con: numbers grow large; by year 5 you're at invoice 1,200+.
• The Belastingdienst accepts this, but you must prove the sequence is continuous (i.e., you didn't skip year 2026).

Hybrid with month/quarter prefixes:

• 2025-Q1-001, 2025-Q1-002, 2025-Q2-001 (reset by quarter).
• Rarely done and can confuse auditors. Stick with year or continuous.

Pick one system and document it (e.g., "I use year-by-year numbering, reset 1 Jan each year"). If asked in an audit, you can explain your choice clearly.

What happens if you create a gap#

You issued invoice 47, then realised you wrote the amount wrong. You want to reissue it with the correct amount. What do you do?

Option 1 (recommended): Reissue with the same number, marked as corrected.

• Invoice 47 (original, marked "superseded")
• Invoice 47 (corrected) — issued with the right amount
• No gap in the sequence

Option 2 (also acceptable): Void the original, issue a new number.

• Invoice 47 (marked "voided")
• Invoice 48 (the corrected invoice)
• You now have 48 in your system, but 47 is clearly marked voided, so there's no mystery gap. The auditor can see what happened.

You issued invoice 50, but the customer never received it or cancelled the order. Now you want to continue with 51.

Option 1: Mark invoice 50 as "voided" or "cancelled" and keep it in your files.

• Pro: the sequence is technically unbroken (50 exists, it's just voided).
• Con: you must explain the void to an auditor.

Option 2: Issue a credit note against invoice 50 to reverse it.

• Issue invoice 50 for €1,000 (services rendered).
• Customer cancels; issue credit note 50-CR for €1,000 (negative amount).
• Invoice 51 is the next number.

Both approaches work if you document them. The key is: don't silently skip a number and hope no one notices. An auditor will ask.

If you've already made a gap#

You've been invoicing for a year, and you just realised that invoice 23 is missing in your records. Invoices 22 and 24 exist, but 23 is gone. What now?

1. Search your files. Do you have email confirmations, payment records, or a record of what was invoiced? If you find invoice 23, problem solved.

2. Ask the customer. Contact whoever you invoiced that date. Did they receive and pay for an invoice?

3. If it truly vanished: You can issue a formal statement to the Belastingdienst (a brief letter) explaining:

   • "Invoice 23 was issued on [date] to [customer] but my records were lost due to [reason]. I am re-issuing a corrected copy as invoice 23-R" or "I am issuing a replacement invoice 23."
   • Attach the replacement invoice.
   • Most likely, the Belastingdienst will accept it if you're honest and the invoice itself is defensible (i.e., the customer confirms the sale).

4. Going forward: Adopt a system (software, spreadsheet, log) so gaps don't happen again.

If you spot a gap you can't explain, don't panic. Be transparent: contact the Belastingdienst and explain. They're used to honest mistakes and are more forgiving than you'd expect, especially if your bookkeeping is otherwise clean.

Gaps that raise suspicion#

• Regular gaps. Invoice 10, 20, 30 — looks like you're hiding invoices in between.
• Gaps in a high-income quarter. Invoice 50–100 in Q1, then a huge gap before Q2 — auditors may think you're hiding a spike.
• A gap you can't explain. "I don't know where invoice 15 went" is worse than "Invoice 15 was voided because the customer cancelled."

What auditors look for#

When an auditor reviews your invoices, they:

1. Check the sequence — do the numbers form an unbroken line?
2. Spot-check dates — does the invoice date match the transaction date in your bookkeeping?
3. Verify amounts — does the invoice amount match the revenue you recorded?
4. Look for gaps — and if they find one, they ask you to explain.

If your invoices are numbered sequentially, dated clearly, and match your bookkeeping, the auditor moves on. Gaps make them dig deeper.

Best practices to avoid problems#

1. Use software. Invoicing apps handle numbering automatically. It's the safest approach.
2. Number as you invoice. Don't create draft invoices with numbers; assign the number only when you send the invoice to the customer.
3. Keep a log or backup. A spreadsheet of "invoice number | date | customer | amount" gives you a quick audit trail.
4. Never reuse a number in the same calendar year (or ever, if you use continuous numbering).
5. Decide on your system early. Year-by-year or continuous. Document it. Stick to it.

Frequently asked questions#

Sequential invoicing is a cornerstone of clean bookkeeping. It's not a technicality — it's designed to prevent fraud. By numbering sequentially, you prove to an auditor that you're not selectively recording invoices. If you use ZZP Belasting, numbering is automatic, so gaps are impossible and audits run smoothly.

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