# Reference Number of the Invoice

Standard defines:

- Length of reference number
- Position of check digit
- Algorithm for check digit calculation

Reference number is numeric transfer clearance issued by invoicing party witch is corresponding to the present standard.

Requirements of standard

- The length on reference number is between 2 and 20 characters.
- The last character of reference number is check digit (the invoicing company is free to generate its reference numbers at 1…19 characters of length, the check digit is calculated using 7-3-1 method.
- To ensure better readability the characters of reference number may be presented as four-digit groups, groups being separated by one empty character space. The spaces are not used in the electronic form of reference number.
- Check digit is calculated using 7-3-1 method.

**The calculation of check digit using 7-3-1 method:**

- The digits in the basic reference data (1…19 numeric characters) to be verified are multiplied by the weights 7, 3, 1, 7, 3,1 … from right to left.
- The multiplied sums are added up.
- The sum is subtracted from then nearest figure ending in zero.
- The resulting difference is the check digit, which is entered as the last digit in the reference number.

The implementation of standard

The banks of Estonia are verifying the reference numbers accordance to standard since 01.02.1997, the inadequate reference numbers are not accepted.

**Example for the calculation of the check digit**

**
**

Basic reference data (1) | 1 | 2 | 1 | 3 | 1 | 2 | 9 | 5 | |

Weights from right to left (2) | 3 | 7 | 1 | 3 | 7 | 1 | 3 | 7 | |

The products arrived at are added together (3) | 3 | 14 | 1 | 9 | 7 | 2 | 27 | 35 | 98 |

The following number ending in zero (4) | 100 | ||||||||

from which the added sum is subtracted (5) | 100-98 | ||||||||

Difference (6) | 2 |

**In the example, the reference number with its check digit is 121312952 and it is printed from left to right in four-digit groups, the groups being separated by one empty character space. The spaces are not used in the electronic form of reference number.**