Definition
An invoice number is a unique identifier assigned to each invoice so it can be tracked, referenced in payments, and audited. Invoice numbers should be sequential and never reused. Common formats are a plain counter (0001), a year prefix (2026-0001), or a business or client prefix (ACME-0001).
Also called: Invoice ID, Invoice reference
Invoice number in practice
Tax authorities in most countries expect invoice numbers to be unique and traceable, and sequential numbering is the simplest way to show that no invoices are missing. Gaps should be explainable, for example by a voided invoice.
Invoicing software assigns the next number automatically, which removes duplicates and lets you run separate sequences for separate businesses.
Frequently Asked Questions
Do invoice numbers have to start at 1?
No. Many businesses start at 1000 or 0100 so early invoices do not look like a first attempt. Just keep incrementing from wherever you start.
Can I reuse an invoice number after deleting an invoice?
Avoid it. Mark the invoice void and keep the number retired. Reusing numbers breaks the audit trail and can be questioned during a tax review.