An invoice number is the unique reference that identifies a single invoice, and it is the field an auditor, a client accounts department and your own bank reconciliation all key off. Below are the four numbering systems that work, when each suits, and the errors that quietly break a trail.
An invoice number is a unique identifier assigned to one invoice so it can be referenced unambiguously by you, your client and any auditor. It is the reference a payer quotes when they pay, and the key you match a bank deposit against.
They have to be unique and traceable, and in most accounting frameworks a sequence with visible gaps is expected to be explainable. Sequential is the simplest way to satisfy that, but date-based and client-based systems are equally valid provided no number repeats.
Yes. There is no rule requiring you to start at 1, and nothing prevents you beginning wherever you like — only that you never go backwards and never repeat.
Two documents become indistinguishable in your records and in your client system, payments get applied to the wrong invoice, and reconciliation breaks. Issue a corrected invoice with a fresh number and reference the duplicate in a credit note.
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