Invoice numbering: choose a format and keep it unique

An invoice number has one job: to identify one invoice and no other. Your customer quotes it when they pay, you look it up when a question comes in, and your records stay in order when the numbers do. This guide compares the common formats, shows how the invoice editor works out the next number for each of them, and covers the start of a new year and the mistakes that lead to duplicates.

Common formats

The editor suggests INV-0001 for your first invoice. You can keep it or type your own format; after that, it proposes the next number in the same format:

The counter is the last group of digits in the number. Two exceptions keep the format intact: a year at the end (0042/2026 continues as 0043/2026, not 0042/2027), and a version suffix such as -v2 (INV-0150-v2 continues as INV-0151-v2). The text around the counter is kept, and so are its leading zeros. When the counter needs another digit it grows: INV-99 is followed by INV-100, and INV-0999 by INV-1000.

How the next number is chosen

The editor proposes one above the highest number of the same format that it knows: the invoices saved on this device and the invoices you made in this browser session. It does not fill gaps and it never proposes a number that is already taken.

• With INV-0040 and INV-0041 saved, a new invoice gets INV-0042.
• With INV-0042 and INV-0045 saved, it gets INV-0046, not INV-0043.
• Each format is a series of its own. With INV-0041 and ACME-015 saved, a new invoice continues INV-0042, and a copy of ACME-015 gets ACME-016.
• Capitals and spaces do not make a different number: “inv-0042” and “INV-0042” count as the same.
• If your numbers are written with different widths, such as INV-0008 and INV-9, the next one uses the width most of them have (on a tie, the wider one): INV-0010.

Starting a new year

A year in the number does not change by itself on 1 January. With 2026-0042 as your last invoice, the editor proposes 2026-0043 in January too. To start the new series, type the first number of the year yourself, 2027-0001. From then on the proposals continue from it: 2027-0002, 2027-0003. The 2026 invoices keep their numbers.

The same works for any change of format: type the first number of the new format once, and the following invoices continue it.

When the number is already taken

The editor checks the number of the invoice you are working on against the other invoices saved on this device and the ones made in this browser session. If another one has the same number, Check before you send says “Another saved document of this type already has this number. Use a number that is not taken yet.” Download and Print stay off until you change the number or tick Send with a number already used.

Use that box only when you really mean to send two invoices with one number; normally, change the number.

Numbering step by step

1. Choose the format once: a short prefix, an optional year, and a counter with enough leading zeros for the invoices you expect in a year (four digits allow 9,999).
2. Keep the first suggestion, or type your own starting number (INV-1001 if you want to start above 1).
3. Keep saving on (Keep my documents on this device), so the editor knows the numbers you have used.
4. Back up before you change browser or device (Back up all documents in the list of saved documents), and use Restore from backup… on the new one.
5. At the start of each year, if your format has the year in it, type the first number of the new year.

Common mistakes

• Starting again at INV-0001 on a new device. A browser that has none of your invoices starts at the beginning, and you end up with two INV-0001s. Restore your backup there first, or type your next number.
• Typing the year as the counter. INV-2026 has no separate counter, so the year is the counter: the next proposal is INV-2027. Write the year and the counter as two groups, such as INV-2026-0001.
• Mixing formats by accident. INV-0042, Inv 43 and #44 are three different formats to the editor and to anyone reading your records. Keep the prefix and separator the same on every invoice.
• Reusing a number for a corrected invoice. If an invoice you already sent needs a change, how to correct it (a new invoice, a credit note, a corrected copy with the same number) is a question for your bookkeeper. The editor will warn you if you give two saved invoices the same number.
• Using the customer’s PO number as the invoice number. Their order number goes under Order / PO number in More options; the invoice number is yours. The purchase order vs invoice guide explains which document comes from whom.

Due dates and payment terms are covered in the payment terms guide. See all service job documents.