# Invoice numbering: formats, rules and examples. By Roy van Oeteren, founder · Updated October 17, 2026 · 6 min read An invoice number is a unique ID for each invoice, and the simplest good format is a prefix plus a running count, such as INV-0001, INV-0002, INV-0003. Never reuse a number, and keep them in order so that you can see at a glance whether one is missing. There is no single legal format in the US. What matters is that every number is unique and that your system is consistent. Below are the common formats, which to pick, and what to do when something goes wrong. ## Why invoice numbers matter The number is how your client refers to the invoice when they pay or ask a question. It is how you match payments to invoices in your books, and how an accountant checks that nothing is missing. In an audit, a clean sequence is a good sign that your records are complete. ## Five common invoice number formats | Format | Example | Pros and cons | | --- | --- | --- | | Simple sequence | 1001, 1002, 1003 | Easy; reveals how many invoices you send | | Prefix + sequence | INV-0001, INV-0002 | Clear it is an invoice; easy to search | | Year + sequence | 2026-014 | Resets each year; shows the date at a glance | | Client + sequence | ACME-003 | Easy to sort by client; the numbers are not continuous overall | | Date-based | 20261017-01 | Self-explanatory; long, and several a day need a suffix | ## Which format should you use? Choose the one you will actually keep up. For most freelancers and small businesses, a prefix and a running number (INV-0001) is enough and never needs rethinking. If you send many invoices a year, a year in the number (2026-014) keeps the lists short. If you bill a few big clients repeatedly, a client code helps you and them. Starting at 1 is fine, but if you do not want new clients to see how small your count is, start at 1000 or add the year. Either way, do not mix systems; a change halfway through the year looks messy. ## Rules for numbering invoices - Every number is unique. Two invoices never share a number, even if one was cancelled. - Numbers go up in order of issue. Keep your sequence continuous, with no gaps you cannot explain. - Use leading zeros (0007) if you want invoices to sort correctly in a list. - Keep the format the same across customers, apart from an optional client code. - Put the number on the invoice, in the email subject and in the file name, such as INV-1042-Juniper-Bakery.pdf. - Use a separate prefix for other documents, such as EST- for estimates or CN- for credit notes, so they do not collide with invoices. ## What to do when you make a mistake If you issued an invoice with the wrong amount or details, do not delete it and reuse the number. The clean fix is a credit note that cancels the original, followed by a new invoice with the next number. Ask your accountant how they want corrections handled. If you have not sent the invoice yet and it was only a draft, you can usually edit it. Once it has gone to a client, treat the number as used. Keep voided invoices in your records, marked as void, so that there are no mysterious gaps. **Correcting INV-1042 without breaking the sequence** > INV-1042 Juniper Bakery $1,550.00 (wrong amount, issued Oct 17) > CN-0003 Credit note for INV-1042 -$1,550.00 (cancels INV-1042) > INV-1043 Juniper Bakery $1,350.00 (corrected invoice, issued Oct 19) ## Numbers across several businesses or sequences If you run more than one business, give each one its own prefix, so numbers can never collide. The same goes for different document types. Recurring invoices, which are generated automatically, should continue the same sequence as the rest; see the [guide to recurring invoices](https://jiffy-invoice.com/guides/recurring-invoices-guide). Jiffy numbers invoices for you: it hands out the next number automatically, even when two are saved at the same moment, and you can set your own prefix and next number in the settings. If a number is already in use, the app does not accept it. You can try the layout first in the [free invoice generator](https://jiffy-invoice.com/invoice-generator). ## A quick way to pick a format Write the format down once, in a note next to your invoice template, so that you or a bookkeeper can keep to it a year from now. - Fewer than 50 invoices a year: INV-0001 is plenty. - Many invoices a year: put the year in, such as 2026-014, and restart each January. - A few large, repeat clients: add a short client code, such as ACME-003. - Several businesses: give each business its own prefix. ## Never worry about the next number again. The Jiffy app numbers your invoices for you and keeps the sequence clean. Try the free plan, or start with the generator. [Try Jiffy free](https://app.jiffy-invoice.com/register?lang=en) ## Questions people ask ### Do invoice numbers have to be sequential? There is no one rule for every business, but sequential numbers are the safest practice. They show that nothing is missing, and accountants and tax authorities like that. ### Should I start my invoice numbers at 1? You can. If you prefer to look established, start at 1000 or include the year, such as 2026-001. ### Can I delete an invoice and reuse its number? No. Mark it void and keep the record. Reusing a number causes confusion in your books and for your client. ### Do estimates and invoices share numbers? Use a separate prefix and sequence for estimates, such as EST-001. See [estimate vs invoice](https://jiffy-invoice.com/guides/invoice-vs-estimate). ## Related guides - [How to write an invoice, step by step](https://jiffy-invoice.com/guides/how-to-write-an-invoice) - [What to include on an invoice: the complete checklist](https://jiffy-invoice.com/guides/what-to-include-on-an-invoice) - [Recurring invoices: how to bill clients on a schedule](https://jiffy-invoice.com/guides/recurring-invoices-guide) [Invoicing guides](https://jiffy-invoice.com/guides) Source: https://jiffy-invoice.com/guides/invoice-numbering