Invoice Numbers: Sequential, Dated and What the Rules Say

What the rules say

United States. There is no federal rule on how invoices are numbered. The IRS says you may choose any recordkeeping system suited to your business that clearly shows your income and expenses, and it names invoices, sales slips and receipts among the supporting documents to keep. Records for gross receipts should show the amounts and sources of the money coming in. Electronic records are held to the same standard as paper.

European Union. For VAT invoices the European Commission lists a unique sequential number identifying the invoice as one of the details of a full invoice. It also says specific national rules apply in some member states, so the country you invoice from adds its own detail.

United Kingdom. GOV.UK puts a unique identification number first in its list of what an invoice must include.

Germany. § 14 (4) number 4 of the VAT act asks for a consecutive number with one or more series of numbers, assigned once by the issuer to identify the invoice. The words "one or more series" matter: the law allows more than one series, for example one per year or per client, as long as no number is assigned twice.

Because the US rule is the loosest, a US business that follows the VAT-style rule (unique and in order) meets both, and its records are easier to check.

Four numbering schemes

• Plain counter. The simplest. None of the rules above says where to start, so 1001 works as well as 1. You never reset it, so the number keeps growing.
• Year and counter. The counter restarts each January, and the year keeps the numbers unique. It shows at a glance which tax year an invoice belongs to.
• Client code and counter. Each client gets its own series. Useful when clients ask for their own sequence, but you need a list of codes and the overall order across clients is harder to see.
• Date and counter. The number contains the issue date. Unique as long as the daily counter never repeats, but long, and a day with many invoices needs a wider counter.

How wide the counter should be

Zero-padding, such as 001 instead of 1, keeps numbers the same length so they sort correctly in a spreadsheet or a file list. Pick a width that covers a whole series. A counter of a given width runs out at:

For a series that restarts every year, multiply your monthly volume by 12:

Add one digit of headroom if your business is growing; going from 999 to 1000 mid-year breaks the neat sorting.

Wrong, cancelled and duplicate invoices

The German rule says each number is assigned once. The safe habit everywhere follows from that:

• Never reuse a number. If an invoice was wrong, keep it and issue the correction or the new invoice under the next free number.
• Never delete a sent invoice. Mark it as cancelled in your records and keep the file.
• Do not number drafts. Assign the number when the invoice is final, so abandoned drafts do not leave holes in the series.
• One tool, one counter. If two people or two programs issue invoices, give each its own series or one shared counter, never two counters that can collide.

One term to know if you invoice in Germany: under § 14 (4) number 10, an invoice issued by the customer on the supplier's behalf (self-billing) must carry the word "Gutschrift".

The other details on the invoice

The number sits next to the rest of the required details. GOV.UK lists your business details, the customer's name and address, a description of what you are charging for, the supply date and invoice date, the amounts, any VAT and the total. § 14 UStG asks for both parties' names and addresses, your tax number or VAT ID, the issue date, the quantity and type of goods or services, the date of supply, the amount by tax rate, the tax rate and the tax amount. Where an invoice is mandatory under § 14 (2), it must be issued within 6 months of the supply. For cross-border EU invoices, see EU reverse charge VAT.

The invoice generator starts with a year-and-counter number such as 2026-001, lets you type your own, and counts up when you start the next invoice. The receipt generator has its own receipt number field for payments you have received and remembers the last number in your browser. Payment deadlines are covered in invoice payment terms.

How long to keep them

In the US the IRS general rule is 3 years from the date you filed the return, with longer periods in some cases, for example 4 years for employment tax records and 6 years if you left out more than 25% of your gross income. In Germany § 14b requires 8 years, starting at the end of the calendar year in which the invoice was issued: an invoice dated October 30, 2026 must be kept until December 31, 2034. How long to keep tax records lists the US cases in full.

Frequently asked questions

Sources

• IRS, What kind of records should I keep (any recordkeeping system that clearly shows income and expenses; invoices among supporting documents; electronic records held to the same standard)
• IRS, How long should I keep records (3 years in general; 6 and 7 years and indefinitely in listed cases; 4 years for employment tax records)
• European Commission, Taxation and Customs Union, VAT invoicing (unique sequential number identifying the invoice on a full invoice; national rules in some member states)
• GOV.UK, Invoices: what they must include (a unique identification number first on the list)
• Germany, Umsatzsteuergesetz § 14 (invoice contents in Abs. 4, number 4: a consecutive number in one or more series, assigned once; issue within six months in the cases of Abs. 2)
• Germany, Umsatzsteuergesetz § 14b (keep invoices eight years from the end of the calendar year of issue)

General information based on the sources above, not tax, legal or financial advice.

Written by Tobias. Updated October 30, 2026.