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# Set your invoice numbering

Most countries require invoices to carry sequential numbers. Set the format under **Features → Invoices → Numbering**. **Do this before you issue any invoices** — once a format has produced numbers, its starting number is fixed.

## What this does

An invoice number is not decoration. Accountants and tax authorities expect a sequence with no gaps and no repeats.

## Set the format

1. Go to **Features → Invoices → Numbering**.
2. Turn on **Turn on invoice numbering**.
3. Set the parts:

* **Prefix** — Letters at the start, like INV
* **Year format** — Short year, full year, or none
* **Separator** — Dash, slash, or others
* **Starting number** — Where the sequence begins
* **Number length** — How many digits, padded with zeros

1. Check the **Preview** and the **Example sequence**.
2. Save.

## The part to get right first

**Once a format has issued numbers, its starting number is fixed.**

The page tells you which state you are in:

* *"This format has not issued any invoice numbers yet"* — you can still change the start.
* *"This format has already issued invoice numbers, so its starting number is fixed"* — you cannot.

To choose a new starting number after that, you have to change the prefix, year, separator or zero-fill. That starts a **separate sequence**.

**So decide your format before your first invoice.** Changing it later is possible, but you end up with more than one sequence running.

## Each format keeps its own count

You can change the prefix, year, separator and number length at any time. Each combination keeps its own count.

Switching back to a format you used before **resumes where it left off**. That is deliberate — it stops you accidentally reissuing numbers.

**Numbers already printed on invoices never change.**

## The repeat warning

If you re-use a prefix and set a starting number below what that prefix already reached, the page warns you:

||| ***"You have used the prefix INV before and reached 1240. Starting at 1000 can produce numbers that look like ones you have already issued."***

Take that seriously. Duplicate invoice numbers cause real problems in an audit.

## Good to know

**Set this up before you go live.** It is the one invoice setting that is awkward to change afterwards.

**Include the year if you file annually.** Many accountants prefer a sequence that restarts each year, and a year in the format makes that natural.

**Zero-padding is cosmetic but expected.** INV-0001 sorts and reads better than INV-1.

**Check your local rules.** Some countries require strict sequences with no gaps at all. What is acceptable varies.

## Troubleshooting

* **The starting number is greyed out** — Why: That format has already issued numbers — Fix: Change the prefix, year or separator to start a new sequence
* **Numbers restarted unexpectedly** — Why: You changed the format — each keeps its own count — Fix: Switch back and it resumes
* **A duplicate-number warning** — Why: The prefix was used before and reached higher — Fix: Start above the previous highest, or use a new prefix
* **Invoices have no number** — Why: Numbering is off — Fix: Turn on **Turn on invoice numbering**
* **The preview does not match** — Why: Changes are unsaved — Fix: Save, then re-check

## FAQ

### Can I change the starting number later?

Only if that exact format has not issued any invoices yet. Otherwise change the prefix, year, separator or zero-fill, which starts a separate sequence.

### What happens to invoices already issued?

Nothing. Numbers already printed never change.

### Should the year be in the number?

If your accounts run yearly, yes. It makes each year a natural sequence.

### What if I re-use an old prefix?

The count resumes where that format left off, so you do not get duplicates. If you force a lower starting number, the app warns you.

### Do I need invoice numbering at all?

In most countries, yes, if you issue invoices. Check your local rules.

## Related articles

* [Send PDF invoices to customers](/en/article/send-pdf-invoices-to-customers-1j04igi/)
* [Design your invoice template](/en/article/design-your-invoice-template-17s8upk/)
* [Put your tax numbers on invoices](/en/article/put-your-tax-numbers-on-invoices-nfo6mv/)
* [Let customers pay outstanding invoices](/en/article/let-customers-pay-outstanding-invoices-u4gd5/)