# BT-1 Invoice Number

A unique identification of the Invoice. The identification scheme shall provide uniqueness within the business relationship between the Seller and the Buyer.

> BT-1 (Invoice number) is the unique identifier the seller assigns to the invoice. It is mandatory under EN 16931 and German UStG §14, must be unique per supplier per fiscal year, and is the primary key both buyer and tax authorities use to reference the document. Example: 2024-INV-0042.
>
> BT-1 · EN 16931 / XRechnung 3.x

## BT Reference Card

Canonical EN 16931 / XRechnung facts for this business term — citable from this anchor.

- **BT code**: BT-1
- **Name**: Invoice Number
- **Cardinality**: Mandatory (M)
- **Business group**: Document
- **Data type**: Identifier
- **Example value**: 2024-INV-0042
- **Common error**: BR-2: An Invoice shall have an Invoice number. The number must be unique per supplier per fiscal year (German UStG §14 sequential numbering).

## Quick answer

A unique identification of the Invoice. The identification scheme shall provide uniqueness within the business relationship between the Seller and the Buyer.

> **Invoice basics**: Core document identifiers, dates, process identifiers, and high-level invoice context.

## In plain language

A unique identification of the Invoice. The identification scheme shall provide uniqueness within the business relationship between the Seller and the Buyer.

A unique identification of the Invoice. The identification scheme shall provide uniqueness within the business relationship between the Seller and the Buyer.

The invoice number serves as the primary identifier for the invoice and is essential for accounting, payment processing, and legal compliance. It must be unique within the business relationship.

## What you usually enter here

Alphanumeric string without spaces or special characters except hyphens

Typical valid values include: INV-2024-001, RE20240327001, R-2024-03-15-001, 2024-INV-12345

Mandatory according to EN 16931-1. German tax law (UStG) requires invoices to have consecutive numbering within one or more series.

## What goes wrong often

Avoid values like: ,  , INV 001, INV/001/2024

Relevant rule codes: BR-2

## Examples in context

The invoice number serves as the primary identifier for the invoice and is essential for accounting, payment processing, and legal compliance. It must be unique within the business relationship.

Mandatory according to EN 16931-1. German tax law (UStG) requires invoices to have consecutive numbering within one or more series.

## Valid Examples

- INV-2024-001
- RE20240327001
- R-2024-03-15-001
- 2024-INV-12345
- F-20240327-001

## Invalid Examples

- INV 001
- INV/001/2024

## Related Fields

The invoice number serves as the primary identifier for the invoice and is essential for accounting, payment processing, and legal compliance. It must be unique within the business relationship.

- [BT-2 Invoice Issue Date](/resources/xrechnung/bt-2-invoice-issue-date)
- [BT-3 Invoice Type Code](/resources/xrechnung/bt-3-invoice-type-code)
- [BT-25 Preceding invoice reference](/resources/xrechnung/bt-25-preceding-invoice-reference)
- [BT-26 Preceding invoice issue date](/resources/xrechnung/bt-26-preceding-invoice-issue-date)

## XML mapping and field facts

Open the technical reference if you need XML paths, validator rule codes, legal wording, or schema-specific mapping details.

### Field ID:
BT-1

### Data Type:
text

### Cardinality:
1..1

### Requirement:
Mandatory

### Max Length:
35 characters

### Since Version:
XRechnung 1.0+

## XML Implementation

### UBL 2.1 Syntax
```
/Invoice/cbc:ID
```

### CII (UN/CEFACT) Syntax
```
/CrossIndustryInvoice/ExchangedDocument/ram:ID
```

## Validation Rules

### BR-2
BR-2

## Why the standard cares about this field

### Business Context
The invoice number serves as the primary identifier for the invoice and is essential for accounting, payment processing, and legal compliance. It must be unique within the business relationship.

### Legal Requirement
Mandatory according to EN 16931-1. German tax law (UStG) requires invoices to have consecutive numbering within one or more series.

## FAQ

### Can I use the same invoice number for different customers?
Yes, the invoice number only needs to be unique within the business relationship between you and each specific customer.

### What are the requirements for invoice numbering in Germany?
German tax law requires consecutive numbering within one or more series. The numbering system must be designed to prevent gaps and duplicates.

## Official sources and tools

- [KoSIT Validator](https://erechnungsvalidator.service-bw.de/)
- [Invoice Converter](/pdf-to-xrechnung)
- [XRechnung Standard](https://xeinkauf.de/xrechnung/)
- [Test Suite](https://github.com/itplr-kosit/xrechnung-testsuite)