Start with a plain sequence
INV-0001, INV-0002 and INV-0003 sort properly and leave room to grow. Don’t put private client information in the number.
If you include the year
A format such as INV-2026-001 can help with filing. Decide whether the final counter resets in January, then use the same rule every time.
Correct mistakes without creating a duplicate
Keep a record of the original number and follow the accounting rules that apply to your business. Local requirements differ, so this page isn’t legal or tax advice.