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Keep it unique and in order
Use one running sequence so no two invoices share a number. UK VAT guidance, for example, asks for a sequential number that uniquely identifies the invoice, and India's CGST Rule 46 asks for a consecutive serial number of up to 16 characters using letters, numerals, hyphen and slash.
Make it readable
A prefix plus a counter such as INV-0042 is enough. Adding the year (INV-2026-0042) helps if you reset the counter yearly, which some jurisdictions require.
Never reuse or silently delete
If an invoice is wrong, cancel it and issue a credit note, then a corrected invoice with the next number. Keep a note explaining any gap.
Separate series when useful
You can run separate series for invoices, credit notes and quotations. dotinvoice remembers the last number you used for each tool in this browser and suggests the next one.
Check local rules
This guide is general information, not legal or tax advice. Requirements change, so check with your tax authority or an accountant.
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This article is general information, not tax, legal or accounting advice. Rules change and vary by location, so check with your tax authority or an accountant before relying on it.