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Invoice number generator

Build an invoice number format that stays unique, sorts in order and reads well. Pick a prefix, a year and a length, then use it in the free invoice generator.

Your next invoice number

INV-2026-0001

13 characters

    How to use the invoice number generator

    Work through the five settings from left to right and watch the preview as you go. When the next few numbers look right, copy the format and use the first number on your next invoice. Nothing is saved to an account, so note the pattern somewhere you will find it again.

    • Prefix: letters that say what the document is, such as INV, RCT, CN or PF.
    • Year: include the year if you like to restart the count each January, or leave it out.
    • Separator: a hyphen, a slash or nothing between the parts.
    • Sequence length: how many digits, padded with zeros. Four digits run from 0001 to 9999.
    • Starting number: where the count begins, useful when you move over from another system.

    Common invoice number formats

    There is no single correct format, but a few patterns cover nearly every small business. Pick the simplest one that will still be unique in five years. The examples below show how each looks on a real invoice and when it suits.

    FormatExampleGood for
    Plain sequence1047Very small volumes, one person billing
    Prefix and sequenceINV-0047Most sole traders and freelancers
    Year and sequenceINV-2026-0047A count that restarts each year
    Year and month2610-007Seeing the billing month at a glance
    Financial yearINV/2026-27/0047India, where numbers must be unique for the financial year
    Branch codeNTH-0047Two locations or two people issuing invoices
    Document typeCN-0009, PF-0014, RCT-0118Keeping credits, proformas and receipts apart

    What the tax offices say about numbering

    Several tax offices make a unique or sequential number a legal requirement on tax invoices. Where it is only recommended, it is still the easiest way to find a document later. Checked against official pages in October 2026.

    • United Kingdom: a sequential number based on one or more series that uniquely identifies the document.
    • Ireland: a unique sequential number.
    • India: a consecutive serial number in one or more series, using letters, numerals, hyphens or slashes, unique for a financial year.
    • UAE: a sequential or unique number. South Africa: a serial number. Singapore: an identifying number.
    • Australia: a unique number is recommended rather than required.

    Good practice that saves trouble later

    The best format is boring and predictable. Your customers should be able to quote it over the phone, and you should be able to spot a missing number by scanning a list. A few habits make that easy, whatever pattern you choose.

    • Never reuse a number, even after canceling an invoice. Keep the canceled one marked void.
    • Use a separate prefix for each document type, so INV-0047 and CN-0047 can never be confused.
    • Give each person or branch its own series, such as NTH- and STH-, rather than sharing one count.
    • Pick a length that will last: three digits run out at 999.
    • Avoid customer names in the number. Names change; numbers should not.

    Yearly resets and moving from another system

    If your format includes the year, you can restart the count each January, because INV-2026-0001 and INV-2027-0001 are still different numbers. Without the year in the number, never restart: carry on counting up, however many years go by.

    Moving from another tool or a spreadsheet? Find the last number you issued and set the starting number one higher, so the series continues without a gap or a repeat.

    Questions, answered.

    Short answers first. Something missing? Ask us.

    What is an invoice number?

    It is the unique reference you give each invoice, so you and your customer can find it, discuss it and match payments to it. It usually appears near the top beside the date. Tax offices such as HMRC, Revenue in Ireland and CBIC in India require one on tax invoices.

    Can invoice numbers include letters?

    Yes. Letters are common as a prefix, such as INV or a branch code. India’s rules expressly allow letters, numerals, hyphens and slashes. Keep to plain letters and digits so the number reads clearly in emails, bank references and spreadsheets.

    What should my first number be?

    Any number you like, as long as the series counts up from there without repeats. Many people start at 0001 or 1001. If you switch systems, start one above the last number you issued. Set it in the starting number box and the preview will show what follows.

    Is it a problem to have a gap in my numbers?

    A gap is not a disaster, but it can raise questions in a tax check, because it looks like a missing invoice. Avoid gaps by never skipping or deleting numbers. If an invoice is canceled, keep it, mark it void, and if it was sent, issue a credit against it.

    Should receipts and credit notes share my invoice sequence?

    It is clearer to keep them apart. Give each type its own prefix and its own count: INV for invoices, RCT for receipts, CN for credits and PF for proformas. Each series then runs without gaps, and nobody mistakes one document for another.

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