How to number invoices: 5 formats and the one to use

Invoice numbers look like admin trivia until a client's accounts team rejects a duplicate or you can't find last March's payment. This short guide is part of our getting paid series.

Five formats compared

The one to use

Year and sequence, with three digits: 2026-001, 2026-002 and so on. It sorts correctly in any spreadsheet, tells you and the client when the invoice is from, and three digits cover up to 999 invoices a year. If you bill more than a few clients, add a prefix that identifies you rather than the client ("SW-2026-014"), because client codes multiply sequences you have to keep unique.

Rules that matter

• Never reuse a number, even after canceling an invoice.
• Don't skip numbers on purpose. If one is skipped by accident, note why in your records.
• Use the same format on every document type you issue as invoices, including deposit and credit invoices.
• For EU business clients, the EU VAT Directive (article 226) expects a sequential number, in one or more series, that uniquely identifies the invoice.
• Keep the full sequence with your records: the IRS expects records that support your income (IRS Publication 583).

Numbering deposits, credits and retainers

Use the same sequence for every invoice you issue, including deposit invoices, milestone invoices and monthly retainer invoices. A deposit invoice is a real invoice and gets the next number; the final invoice then references it ("Less deposit received, invoice 2026-014"). A credit note can use its own short series, such as CN-2026-001, as long as it names the invoice it corrects. Pro forma invoices and quotes stay out of the invoice sequence entirely.

Switching formats mid-year

If you're moving from a simple sequence to year-sequence, start the new format on January 1 rather than in the middle of the year, so each year in your records follows one pattern. If you must switch sooner, note the last old number and the first new one in your records, and never let the new format produce a number you've already used.

If a client's system rejects your number

Some accounts-payable systems reject invoice numbers that are too long, contain slashes or repeat a number they've already seen from another vendor. Keep numbers under about 15 characters, use only letters, digits and hyphens, and add your prefix so your 2026-014 can't collide with another supplier's.

What to do after you void an invoice

Mark it void (or canceled), keep it, and issue a new invoice with the next number. If invoice 2026-014 had the wrong amount and you've already sent it, cancel 2026-014 and send 2026-015 with a note: "Replaces invoice 2026-014, canceled." To reduce an invoice that's already been paid, issue a credit note or a refund rather than editing the original.

Making one invoice without an account? The free invoice generator lets you type any number; start your sequence there and keep it going. The difference between an invoice and a receipt (which can carry the invoice number) is covered in invoice vs receipt.

In SoloWOS

SoloWOS Invoicing numbers invoices automatically in one sequence per workspace, with a prefix of 1 to 8 letters or numbers you set in Settings > General (the default looks like INV-1001). The number is reserved when you create the invoice, so two invoices can never share one. Canceling an invoice keeps it, with its number, in your records; the client sees it as canceled and can't pay it. A draft that was never sent can be deleted, which leaves a gap in the sequence, so cancel rather than delete if a number has already gone to a client.