The rules that matter
- Unique: no two invoices may share a number. [1] [2]
- Sequential: numbers follow an order within a series, so a missing invoice is noticeable. The EU requires a sequential number based on one or more series. [1]
- You choose the start: in the UK you can start the series at any number, as long as it then runs in sequence. [2]
- Several series are allowed: for example one per branch, per type of document or per year, as long as each is sequential. [1]
Common formats
| Format | Example | Good for |
|---|---|---|
| Prefix and counter | INV-0001, INV-0002 | Most small businesses |
| Year and counter | 2026-001, 2026-002 | Seeing the year at a glance; restart each year as a new series |
| Year, month and counter | 2026-10-001 | High volumes, monthly reconciliation |
| Client code and counter | ACME-001, ACME-002 | Few clients with many invoices each |
| Document type prefixes | INV-0042, QUO-0042, RCT-0042 | Keeping invoices, quotes and receipts apart |
Leading zeros keep numbers aligned and sorted in file names, but they do not change the sequence. Avoid formats that reveal more than you want to, such as starting at 0001 when a client may read it as your first sale.
Pick one format and keep it. Changing format mid-year is allowed if the new series is still unique and sequential, but it makes your records harder to follow, and your accountant or a tax inspector will want to see that no invoice is missing from either series.
Mistakes and gaps
Once an invoice has been sent, do not reuse or delete its number. If it was wrong, issue a credit note that refers to it and then a new invoice with the next number; amending documents must refer clearly to the original invoice. [1] If a number was skipped by accident, keep a note of why, so the gap can be explained if your records are checked.
Numbering in BillCrisp
Each document type has its own prefix: INV- for invoices, QUO- for quotes, RCT- for receipts and PRO- for proforma invoices. When you start a new document, BillCrisp continues from the last number of that type saved in this browser, keeping the prefix and the leading zeros, so INV-0041 is followed by INV-0042. You can type any number you like instead. The history lives only in this browser, so if you invoice from several devices, check the last number before you start.
Next: what to include on an invoice and payment terms.
Sources
- [1] European Commission, Taxation and Customs Union: VAT invoicing rules (accessed 2026-10-02)
- [2] HM Revenue & Customs: VATREC5010 – VAT invoice: Details which must be shown on a full VAT invoice (accessed 2026-10-02)