An invoice number is the unique reference you give each invoice, so you and your client can find, pay and record it. A good format is a short prefix plus a counter that only goes up, like INV-0042 or 2026-0042, with no duplicates and no reused numbers. Some countries also require the numbers to be sequential, covered below.
If you need an invoice number generator, the simplest one is your invoicing tool: it hands out the next number when you send, so nobody types it. Below are six formats with examples, what to avoid, what three tax authorities say, and how LotsInvoice numbers automatically (checked in its docs, settings and code on 24 October 2026). The businesses in the examples are made up.
What a good invoice number looks like
Three parts, all optional except the counter:
- A prefix that says what the document is or whose it is:
INV-,2026-,ES-. - A counter that goes up by one each time and is padded with zeros so it sorts correctly:
0042, not42. - Nothing else. Skip spaces and anything you would have to retype by hand.
Three rules do most of the work:
- Every number is unique. Two invoices must never share a number, even for different clients.
- Numbers are never reused. If an invoice is wrong, you correct it or cancel it with a credit note. You don't hand its number to a new invoice.
- The order is the order you issued them. The next invoice is always the next number up.
Your invoice number is also not your client's PO number. The PO number is theirs and sits next to yours; see Where to Put a PO Number on an Invoice.
Six invoice number formats, with examples
1. Simple sequence
INV-0001, INV-0002, INV-0003
One counter, one prefix, no restarts. Example Studio (hello@example.com) sends INV-0107 to Sample Client Inc. and the next invoice, to anyone, is INV-0108. It is the easiest format to keep clean, and it suits most freelancers and small agencies.
2. Year prefix
2026-0042, 2026-0043, then 2027-0001 (or 2027-0044)
The year in the number tells you and your accountant at a glance when it was issued. You choose whether the counter restarts each January (2027-0001) or keeps counting (2027-0044). If you restart, keep the year in the number, or two years would both have an 0001.
3. Client prefix
SC-001, SC-002 for Sample Client Inc. and NW-001 for another client
Each client gets its own series. It is easy to scan a client's history, but you now keep one counter per client, which is the usual place for a slip. If you use it, write down the code for each client.
4. Monthly reset
2026-11-001, 2026-11-002, then 2026-12-001
The counter restarts every month, so the year and month must sit in the number, or November's 001 clashes with December's. It suits businesses that issue many invoices a month.
5. Per-brand prefix
ES-0107 for Example Studio and EP-0012 for Example Print, two brands of the same owner
Each brand or company has its own prefix and its own counter, so the brands never share a sequence. See Invoicing Several Businesses or Brands From One Account.
6. Credit notes
CN-0001 credits invoice INV-0042
A credit note is its own document with its own series, so credit notes don't use up invoice numbers. It should point to the number of the invoice it corrects.
What to avoid
- Gaps you can't explain. A missing number makes a reader (and an auditor) wonder if an invoice was removed. If a number is skipped, know why.
- Duplicates. Two invoices with the same number usually happen when two people number by hand, or when you copy an old invoice and forget to change it.
- Reusing a number. Never give a cancelled invoice's number to a new one. Cancel it with a credit note and keep the record.
- Editing a sent invoice and keeping no trace. Change a sent invoice as a revision with a reason, or credit it.
- Switching formats without a reason. A new prefix is fine; starting again from 0001 under the same prefix is how duplicates begin.
Does an invoice number have to be sequential?
It depends on where you and your client are. Here is what three official pages say, opened on 24 October 2026. This is what they state, not legal or tax advice, and other countries have their own rules, so ask your accountant or your tax authority.
- United Kingdom (HMRC). Section 16.3.1 of the VAT guide (VAT Notice 700, last updated 25 June 2026) lists what a VAT invoice must show, starting with "a sequential number based on one or more series which uniquely identifies the document". Section 18.2.3 says a valid credit note must show its own identifying number and date. gov.uk/guidance/vat-guide-notice-700
- European Union. The European Commission's explanatory notes on the VAT invoicing rules cover Article 226(2) of the VAT Directive. They say the number may be based on one or more series, may include letters as well as digits, and that using a different series, for instance per branch, per type of supply or per customer, is the business's choice. Commission explanatory notes (PDF)
- India (GST). Rule 46 of the CGST Rules says a tax invoice carries a consecutive serial number of not more than sixteen characters, in one or multiple series, using letters, numerals, a hyphen or a slash, unique for a financial year. CBIC, Rule 46
The pattern across all three: one or more series is fine, each number must be unique, and the numbers follow on from each other. Whatever your country, the formats above (one counter per series, never reused, credit notes numbered too) are built to fit that.
How LotsInvoice numbers automatically
You set the format once and never type a number again.
Numbering is per business and per document type. Each business you invoice from has its own counters, and invoices, estimates and credit notes each have their own prefix, digits and counter. The defaults are INV-, EST- and CN- with four digits (INV-0001), never restarting.
You set the prefix, digits and restart. In Settings > Layout > Titles and numbering, each document type has a Prefix (up to 20 characters: letters, numbers, -, /, _, ., # and {YYYY}), Digits (1 to 10) and Restart (Never or Every year). {YYYY} becomes the year of the invoice's issue date. So INV-{YYYY}- with Restart set to Every year gives INV-2027-0001 in January, and with Never it carries on as INV-2027-0043. Format 5 above is simply a different prefix on each business.
Continue from another tool. Moving from a spreadsheet or another invoicing tool? Under Continue numbering from another tool, pick the document type and type the next number. Numbers only move forward, never below one already used, so you can't collide with an invoice you have already sent. The business owner does this (the tool is set_next_number for an assistant).
The number is given when you send. A draft has no number. LotsInvoice takes the next one at the moment you send the invoice (or mark it as sent because you delivered it yourself), in one step, so two people sending at once can't get the same number. Deleting a draft that was never sent leaves no gap, because it never had a number.
The number locks when sent. A sent invoice is locked. To change it you make a revision, which keeps the number, adds a new version and records your reason. If it has been paid, you correct it with a credit note instead. A voided invoice keeps its number too, so the sequence never loses a number. More on that in Is LotsInvoice Safe?.
Credit notes have their own numbering. Open a sent invoice and choose Issue a credit note; it starts from that invoice's customer and currency, and it takes the next credit note number (CN-0001, CN-0002) when issued, separate from your invoices. Currency doesn't change the numbering either: it is one series per business and document type, whatever currency the invoice is in (see How to Invoice a Client in Another Currency).
Two things to know. The restart option is yearly, by the calendar year of the issue date; there's no monthly restart (format 4), so a month in the number is something you'd add yourself. And numbering is one series per business and type, so a separate series per client (format 3) isn't automatic: turn on the Reference field in Settings > Layout and put the client's code there, or give a separate brand its own business. If you work in Claude, ChatGPT or another assistant connected through lotsinvoice.com/mcp, you can say "continue my invoice numbering from INV-0342" and it uses set_next_number; nothing is sent until you say so.
What it costs, and the free options
Numbering is part of every plan. LotsInvoice is $12 a month per business, with unlimited invoices and team members, and new accounts get 15,000 free credits, valid 90 days, no card (pricing). If you send a handful of invoices a year, a spreadsheet with a counter column works; the risk is two people editing it. Free plans exist too: Zoho Invoice and Wave Starter are $0 (checked on 24 October 2026 on their pricing pages), so check what each lets you set for the prefix and the starting number before you pick.
Common questions
What is an invoice number?
A unique reference you assign to each invoice. It lets you and your client find the invoice, match a payment to it and record it in your books.
What is a good invoice number format?
A short prefix plus a padded counter that only goes up, such as INV-0042 or 2026-0042. Add the year or a brand prefix if you need it, and never reuse a number.
Do invoice numbers have to be sequential?
In some countries, yes. HMRC, the European Commission's notes on the EU VAT Directive and India's CGST Rules each ask for a sequential or consecutive number that uniquely identifies the invoice, in one or more series. Check your own country's tax authority or ask your accountant.
Can I have gaps in my invoice numbers?
Avoid them, or be able to explain each one. In LotsInvoice a draft has no number until it is sent, and a voided invoice keeps its number, so the sequence doesn't skip.
Can I change an invoice number after sending?
Don't. In LotsInvoice a sent invoice is locked, and a revision keeps its number. To cancel it, void it or issue a credit note, then send a new invoice with the next number.
Is there an invoice number generator?
Your invoicing tool is the generator. In LotsInvoice, set a prefix, digits and restart once and every invoice gets the next number when you send it.