# Number series: how to set up document numbering

Source: https://lucanto.eu/guides/number-series
Published: 2026-09-14
Updated: 2026-09-15

A number series in Lucanto decides how your documents get numbered. It is built from a mask — prefix, year, optional month, type code and counter, for example FAK{YY}-{T}-{CCCC}. The mask also sets when the counter resets: a month token means monthly, a year alone yearly. Every document type has its own series.

A number series is the rule Lucanto follows when it numbers a document at the moment you issue it. It is not decoration: a sequential number is what makes a document traceable, and a series that skips or repeats numbers is the kind of thing you end up explaining during an audit. This guide covers what a number is made of, what you can configure, and what Lucanto deliberately will not let you do.

## What numbering has to get right

Whatever format you pick, four things always hold — and Lucanto takes care of all four:

- **No gaps.** Numbers follow one another unbroken, which is why a number is assigned at issue time rather than when a draft is created.
- **No duplicates.** Two invoices can never share a number; database indexes enforce it rather than your attention.
- **Separate series.** Invoices, credit notes, advance invoices and expenses each get their own series, so their numbers cannot cross.
- **Period reset.** At the start of the year — or the month — the counter returns to one, depending on what your mask declares.

## Where to find them

Number series live under **Account settings → Number series**. You do not have to create them — a new company starts with the full set, one series per document type: invoice, credit note, advance invoice, quote, expense, delivery note, orders, internal document and cash receipt.

**Image:** The number series list: columns for Number series name, Document type, Format, Next number and Usage with a capacity meter. Every row carries a Default badge.

The most useful column on the screen is **Next number** — not a formula, but the actual number the next document will get. Beside it, **Usage** shows how many numbers in the series' range are gone and how many are left.

Names and prefixes come from your **company country**, not from the interface language. A Slovak company gets `Faktúra` with the `FAK` prefix, a Czech one `Faktura` with `FAK` too, a Romanian one `Factură` with `FACT` — even when the person is reading the app in English. Numbering is data about your documents rather than interface text, so switching languages must not change it.

## Setting one up, step by step

1. Open **Account settings → Number series**.
2. Click **New number series**, or **Edit** on one that already exists.
3. Pick the **document type** the series covers — invoice, expense, advance invoice and so on.
4. Assemble the **number format**: the year, optionally the month, a divider, a prefix and roughly how many documents a year you expect. The preview under the form shows the result as you go.
5. Continuing from another tool, fill in **First number of series**. Then save.

The rest of this guide explains what the choices in steps three and four actually mean.

## What a number is made of

The number format is written as a **mask** — a string in which placeholder tokens sit in curly braces and everything else is copied literally. Prefixes come from your country, so the exact letters differ; for a Slovak company the default invoice mask looks like this:

```
FAK{YY}-{T}-{CCCC}   →   FAK26-1-0001
```

The form walks you through it without making you write the mask by hand: you pick whether the number carries the year and in which shape, whether it carries the month, which divider and prefix you want, and roughly how many documents a year you expect — that last one decides how wide the counter is. Above all of it runs a live preview that labels each segment.

**Image:** The new number series form, Number format section: Year, Month, Estimated number of documents, Divider and Type code option groups plus a Prefix field, with a live preview of the resulting number and its segments below.

The tokens a mask understands:

| Token    | Meaning                                                          |
| -------- | ---------------------------------------------------------------- |
| `{YYYY}` | the full year, e.g. `2026`                                       |
| `{YY}`   | two-digit year, e.g. `26`                                        |
| `{MM}`   | month as `01`, `02`, …                                           |
| `{CCCC}` | the counter; the number of `C`s sets the digit count, up to ten  |
| `{T}`    | document type code — more on this below                          |

Besides tokens, a mask may contain letters **A–Z**, digits, a hyphen and an underscore. No spaces. **The counter is mandatory** — without it the mask could not number a second document.

**Image:** The expanded mask syntax reference inside the form: the list of tokens with explanations, the allowed characters, and the ten-digit warning about payment matching.

When the option groups are not enough, expand **Number series mask (advanced)** and write your own. One limit is worth knowing: the whole number should stay within **ten digits**, otherwise it cannot serve as a variable symbol and automatic bank payment matching stops working. Lucanto warns you in the form rather than forbidding it, because large shops match payments some other way, by order ID for instance.

## When the counter resets

This is where Lucanto differs from most invoicing tools: **there is no reset setting — it is derived from the mask.**

- The mask contains `{MM}` → the counter resets **every month**
- The mask contains only a year → it resets **every year**
- The mask contains neither → it **never resets**, and the counter runs as a lifetime sequence

The reason is simple: the reset and the format must not drift apart. Configured separately, you could build a series with a yearly reset and a mask without a year — and after New Year it would start producing numbers that already existed. Deriving it from the mask makes that combination impossible.

It also means the capacity figure next to a series knows which period it is talking about: a monthly mask reads “documents per month”, a yearly one “per year”, and a mask with no date shows the capacity of the whole series.

## The `{T}` type code and variable symbols

At issue time the `{T}` token resolves to a digit that depends on the document type:

| Digit | Document type                        |
| ----- | ------------------------------------ |
| 1     | invoice                              |
| 2     | credit note                          |
| 3     | debit note                           |
| 4     | proforma / advance invoice           |
| 5     | tax document for a payment received  |
| 6     | correction of that tax document      |

The numbering follows the ISDOC standard. What matters is **where** the token sits: placed between the year and the counter, it becomes the first digit of the variable symbol. That splits the variable symbols of each document type into separate bands so they can never land on the same value — invoice `FAK26-1-0042` and credit note `DOB26-2-0042` carry different variable symbols despite sharing a position.

Without that split, a bank could not reliably tell which document a payment belongs to, and matching would go wrong exactly where it hurts most: on corrective documents.

## How many documents fit in a series

The width of the counter sets the ceiling. `{CCCC}` means 9,999 documents per period, `{CCCCC}` means 99,999. For a one-person business this is a non-question; for a shop, or for a monthly series, it is a real one.

Lucanto keeps an eye on it. Each series has a usage meter, and when one approaches its ceiling the row is highlighted and a **Schedule counter widening** action appears in the “⋯” menu. It adds a digit to the mask.

What matters is what **does not** happen: numbers already issued stay as they are, and the widening does not take effect immediately. It is scheduled, and the mask only stretches once the current capacity is genuinely used up — 9,999 continues straight into 10,000. To an accountant or an auditor the change then reads as a natural consequence of outgrowing the range, not as numbering rewritten after the fact.

Widening the counter is also **the only mask change Lucanto permits on a live series**. Everything else locks after the first issued document — the format and the document type both. What stays editable is the series name and the default line values. If you need a different format, you create a new series.

## Continuing numbers from another tool

Moving from a spreadsheet or another invoicing tool, you do not want to start at one. That is what the **First number of series** field is for — set it to the number you want to continue from and Lucanto carries on from there.

It is the one migration step worth doing carefully: renumbering documents you have already sent is not possible. The whole move is covered in the article on [invoicing in Excel](/blog/invoicing-in-excel).

## The default series and picking one at issue time

Each document type has one **default** series, used automatically when you issue a document. In the list it carries a _Default_ badge.

When a document type has more than one series, you can pick a different one while issuing. The picker shows each series' pattern and its next number, so you are not guessing what a given series will produce.

Some document types have no series of their own and **borrow from a relative**: a credit note falls back to the invoice series, and a correction of a tax document falls back to the credit note. One exception is deliberate — **a cash receipt never borrows the invoice series**. If it did, it would consume invoice numbers and leave gaps in the invoice sequence.

## What about a gap in the sequence

A number series should be continuous, and Lucanto takes care of that itself: a number is assigned **at issue time**, not when a draft is created. A half-finished document you end up discarding therefore consumes nothing — [a draft has no number yet](/guides/invoice-statuses).

Cancel a document after it was issued and its number **stays taken**. That is correct: a cancelled document is still traceable and its number is part of the history. When to cancel and when to issue a credit note instead is covered in [its own guide](/guides/cancel-invoice-or-credit-note).

For the same reason, a series with documents attached cannot be deleted. If you no longer want to use it, you **suspend** it — it disappears from pickers and lists while the documents it produced stay untouched.

## How many series your plan includes

**Free and Starter** give you **one series per document type**, which covers ordinary invoicing including credit notes and advance invoices.

Extra parallel series — per project, per site, per brand, or however you split your work — are a **PRO and Business** feature, and there the count is not capped. The details are in the [pricing](/pricing).