# SEPA CT - MOD11

Paying with a bank transfer (SEPA CT) contains the risk of not correctly submitting the (invoice) data in the transaction. The correct provision of the data is necessary in order to be able to report the transaction as paid. This reconciliation of the SEPA CT can be supported by using the mechanism that is referred to within the NL banking environment as the Payment Reference.

Within the NL bank environments, the entered Payment Reference is checked by means of the MOD11 test. By applying this MOD11 calculation to the invoice number, incorrectly entered invoice numbers are already rejected in the banking environment. In that case, the debtor will first have to check and correct the invoice number before this transfer can be completed in the banking environment.

In the transfer request, the Merchant can be asked to enter the invoice number in the Payment reference field within the bank environment in the e-mail, letter or SMS. This reduces typos.

### Explanation MOD11 usage

Building up the invoice number in accordance with the MOD11 calculation can be done by calculating a check number. See the figure below for visual guidance. The steps are as follows:

- Compose a unique invoice number of 15 digits. An existing sequence may be completed with leading zeros. See the blue blocks below.
- Multiply the individual numbers by the weighting number*. See the green blocks.
- Add the results of all multiplications from the purple blocks as a result to one total.
- Divide the total of the numbers by 11 and keep only the whole number
- Calculate the difference between the total and 11 * the whole number. This difference can also be described as the remainder of the division by 11 above.
- Calculate the difference between 11 and the rest of the previous division. That's the check digit.
- Add the check digit to the front of the invoice number sequence of 15 numbers.

- Note: The weighting numbers for the calculation within the MOD11 have a fixed series. They are defined from right to left as follows: 2, 4, 8, 5, 10, 9, 7, 3, 6, 1, 2, 4, 8, 5, 10.

- Note: The MOD11 format of a payment reference has also been used for years on bank giro credit slips in the Netherlands. That is a printed slip where the payment info was already filled in. After signing the bank giro credit slip, this could then be handed in to the bank, which made the payment to the beneficiary on behalf of the customer.