Invoice Number Generator

Build a sequential invoice numbering format — prefix, client code, date and zero-padded counter — and list your next numbers

Updated 2026-09-30

Prefix (letters or digits)

Client code (optional)

Date block

Separator

Next sequence number

Counter digits (zero padding)

How many numbers to list

Next invoice number

INV-2026-0001

INV-2026-0001
INV-2026-0002
INV-2026-0003
INV-2026-0004
INV-2026-0005
INV-2026-0006
INV-2026-0007
INV-2026-0008
INV-2026-0009
INV-2026-0010

Format: INV-YYYY-0000. A 4-digit counter holds 9,999 invoices per series before it outgrows the padding.

Nothing is stored or sent — the numbers are built on this device.

An invoice number has one job: identify each invoice uniquely, in order, so you, your customer and a tax inspector can find it. Choose a prefix, an optional client code and date block, a separator and how many digits the counter should have, and the generator shows your next invoice number and the ones after it. Copy them or download a CSV to keep with your records.

What the rules actually require

Most tax systems require uniqueness and a sequence, not a particular format. The EU VAT Directive (2006/112/EC, article 226) requires "a sequential number, based on one or more series, which uniquely identifies the invoice". HMRC in the UK asks for a unique invoice number that follows on from the last one. The US IRS sets no format, but expects records that support your income, and gaps or duplicates make that harder to show. Check your own country's rules if you charge VAT or GST.

"One or more series" matters: you may run separate sequences — per year, per branch, or per client — as long as each number is unique and each series has no gaps.

Common invoice number formats

All of these are sequential and unique; pick one and keep it.

Example formats (sequence 1, 4-digit counter, September 2026)
Format	Example	Good for
Prefix + counter	INV-0001	Simplest; one continuous series
Prefix + year + counter	INV-2026-0001	Restart each year, easy to file by year
Prefix + year-month + counter	INV-202609-0001	High volume, monthly bookkeeping
Prefix + client + counter	INV-ACME-0001	A few large clients, each with its own series
Date only + counter	20260930-01	Several invoices a day, no prefix needed

Worked example

With the default settings — prefix INV, the year as the date block, a hyphen separator, next number 1 and a 4-digit counter — your first invoice of the year is INV-YYYY-0001 (INV-2026-0001 in 2026), followed by INV-YYYY-0002 and so on. A 4-digit counter holds 9,999 invoices in each yearly series, which is plenty for most small businesses; the generator shows the capacity for whatever width you pick.

Switching the business to this format mid-year? Start the new series at 1 with the new year, or carry on from your last number (enter it + 1 as the next sequence number) so there is no gap.

Mistakes that cause trouble later

Reusing a number after an invoice is cancelled — void it with a credit note instead and keep the number
Starting at a large number like 1001 to look established is allowed, but be consistent and do not skip ranges later
Letting two people number invoices by hand from separate spreadsheets — duplicates follow
Putting the amount or anything that can change into the number
Changing the counter width part-way through a series, which breaks sorting (INV-999 sorts after INV-1000)

Let the software count for you

The safest numbering system is one you never type. The free invoice tool in ExpressBizTools assigns the next number automatically and keeps paid, overdue and cancelled invoices in one list, so the sequence stays unbroken.

Keep the numbers in a spreadsheet

Ready-made Excel and Google Sheets templates that pick up where this calculator stops. One-off purchase, instant download.

Invoice Template + Invoice Log Spreadsheet
Auto-totalling invoice with tax, discount and balance due, plus an overdue-flagging log.
$7See template
Receipt Template + Receipt Log Spreadsheet
Printable receipt with totals and payment method, plus a numbered log.
$5See template

Frequently asked questions

What should my first invoice number be?

Anything, as long as the next one follows it. INV-0001 or INV-2026-0001 are common. Starting higher, such as 1001, is allowed if you then count up from there without gaps.

Do invoice numbers have to be sequential?

In the EU and UK, yes — VAT rules require a sequential, unique number, though you may run more than one series. The US has no set rule, but a clean sequence makes your records easier to defend.

Can I restart invoice numbers every year?

Yes, if the year is part of the number (INV-2026-0001, then INV-2027-0001) so every invoice stays unique.

What if I void an invoice?

Keep the number in your records and mark it void, or issue a credit note against it. Do not reuse the number for a different invoice.

Can I have a separate sequence for each client?

Yes. Put a short client code in the number (INV-ACME-0001) and keep a separate counter per client. Each number is still unique across the business.

Stop calculating one number at a time

ExpressBizTools tracks cost, price, margin, stock levels, and tax on every product and invoice automatically — free, in your browser.

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As an Amazon Associate, ExpressBizTools earns from qualifying purchases. Product links are affiliate links. Calculations are for general planning and are not tax, legal, or financial advice.