The short answer: for most small businesses, a simple prefixed sequence is best: INV-0001, INV-0002, INV-0003. It is unique, sorts correctly in every file browser and spreadsheet, satisfies auditors, and never needs a lookup table to decode.

Why numbering matters more than it seems

  • Uniqueness is expected almost everywhere. Duplicate numbers cause double payments, rejected imports into your client's accounting system, and awkward audit questions.
  • Gaps look like deleted income. Many tax authorities expect a broadly sequential series. Jumping from INV-0042 to INV-0090 invites questions.
  • Everyone references it. "Re: INV-0041" is how clients, banks and bookkeepers identify a payment. Make it easy to quote.

Three schemes that work

1. Plain sequence (recommended)

INV-0001, INV-0002, ... Zero-pad to four digits so alphabetical sorting matches numerical order. This is the default suggestion in Fast Invoice Maker, which remembers your last number and suggests the next one when you are signed in.

2. Date-based sequence

2026-0041 or INV-202607-03: a year (or year-month) prefix plus a counter. Useful if you like knowing the period at a glance or want the counter to reset every year. Keep the counter padded and never reuse a full combination.

3. Per-client prefixes

ACME-0007, PPL-0012: a short client code plus a counter. Helpful for agencies with a handful of big clients and many invoices each. The cost is bookkeeping overhead, since you now track several counters.

Mistakes to avoid

  • Starting at 1 visibly. Nothing wrong legally, but INV-0001 tells your first client they are your first client. Starting at a number like INV-0041 is common and harmless as long as you go up sequentially from there.
  • Encoding the amount or job details in the number. Numbers are identifiers, not databases. Keep the meaning in the line items.
  • Unpadded numbers. INV-9 sorts after INV-10 alphabetically. Pad to a fixed width.
  • Changing schemes mid-year. Pick one in January (or at incorporation) and switch, if you must, at a clean boundary like a new financial year.

Bulk billing and numbering

If you generate invoices in bulk from a spreadsheet, assign the numbers in the spreadsheet itself (a simple fill-down series in Excel) so your records and the generated PDFs always agree. Each PDF is named after its invoice number, which keeps the zip tidy too.