Invoice Number Format Singapore: IRAS Rules and a Simple Scheme

What IRAS actually requires on invoice numbers

IRAS lists the information a tax invoice must contain on its invoicing customers page/basics-of-gst/invoicing-price-display-and-record-keeping/invoicing-customers). One of the twelve items is simply "an invoice identifying number". The same page asks for an identifying number on a simplified tax invoice (allowed when the total including GST is S$1,000 or less) and on a credit note, where the example given is "a serial number".

That is the whole rule. IRAS does not say how long the number must be, whether it needs letters, or that it must start from 1. What it does expect is that you can produce your records and support your GST returns. The keeping records page/basics-of-gst/invoicing-price-display-and-record-keeping/keeping-records) lists "serially numbered receipts or cash register tapes" among the income records to keep, and says all records must be kept for at least 5 years from the end of the accounting period.

For the other fields on a GST invoice, see our tax invoice requirements guide. If you are unsure which document you should be issuing at all, start with receipt vs invoice vs tax invoice.

Why sequential invoice numbers are still the safest choice

If the law only asks for an identifying number, why bother with a strict sequence? Because the number is how you, your accountant and IRAS check that nothing is missing. A clean run of INV-0101, INV-0102, INV-0103 shows at a glance that every sale was recorded. A random or reused number makes that check impossible, and you end up explaining each invoice by hand.

Sequential numbering also stops two common mistakes: two invoices sharing one number (which confuses customer payments and input tax claims), and deleted invoices disappearing without a trace. Most accounting tools number invoices for you, so the sequence costs nothing to keep.

A simple invoice number format you can copy

Keep it boring. A prefix that says what the document is, an optional year, and a running number with enough digits for a few years of sales. The examples below all work; pick one and stick to it.

If you reset numbers each year, always include the year in the number. Otherwise INV-0001 from 2025 and INV-0001 from 2026 look identical, which defeats the purpose. Leave room in the running number: four digits covers 9,999 invoices per series.

Running several series: outlets, platforms and credit notes

Separate series are fine as long as each one is unique and sequential on its own. The prefix is what keeps them apart. Common cases for Singapore SMEs:

• Credit notes: always their own series (CN-). Xero uses INV for sales invoices and CN for credit notes by default.
• Online store and counter: Shopify numbers orders from #1001 and lets you add a prefix or suffix, while your POS or accounting tool runs its own series. Give each a different prefix.
• Marketplaces: Shopee, Lazada and similar platforms issue their own order numbers. Record the platform order ID as a reference on your own invoice rather than using it as your invoice number.
• Branches: each outlet gets a prefix (TP-, JE-) so a till in one branch never collides with another.
• Deposits and progress billing: issue normal invoices from the main series and reference the quotation or contract number in the description.

Write the series down in a one-page note for your bookkeeper: which prefix belongs to which channel, who issues it and where the records live. That note is the first thing anyone asks for when the numbers look odd.

Gaps, voids and switching accounting software

A gap is not illegal, but every gap should have an explanation. The cleanest way is to never delete an issued invoice: mark it void in your accounting software so the number stays visible, and issue a credit note when you need to reverse a sale that was already sent to a customer. IRAS lists the situations where a credit note is used, such as returned goods, waived charges or correcting a GST mistake.

Switching software is where most gaps appear. IRAS says you do not have to migrate old transactions into the new system, but you must keep them in the old software (or an export) for at least 5 years and be able to retrieve them when asked.

1. 1

   Pick a cut-over date

   Usually the start of a month or GST quarter. Issue the last invoice from the old system before that date.

2. 2

   Export everything from the old system

   Download invoice, credit note and customer lists as CSV and PDF. Keep the old account readable if the vendor allows it.

3. 3

   Set the next number in the new system

   Continue from the last old number (INV-0458 becomes INV-0459), or switch to a new prefix such as INV-2026-. Do not start at INV-0001 with the same prefix.

4. 4

   Record the cut-over in writing

   A short note: old system, last number, new system, first number, date. Store it with your year-end records.

5. 5

   Check the first week of invoices

   Make sure staff, integrations and your online store are all issuing from the new series and nothing is still posting to the old one.

If you also need to send invoices over InvoiceNow, your numbering carries straight through to the Peppol document. Our InvoiceNow set-up guide covers the rest of that change.

Do it yourself or get help?

Most businesses can sort numbering in an hour: choose a format, set the prefixes and next numbers in your accounting tool, and write the one-page series note.

If your invoices come from several places (Shopify, a POS, Stripe or HitPay and a bookkeeper's spreadsheet), the numbering usually breaks where those systems meet. Our Xero / QuickBooks Integration (S$390) connects your store and payment platform to the accounting software with branded templates and clean number series. This page is general information, not tax advice; check unusual cases with your accountant or IRAS.