The TAC number (the first 8 digits) allocated by the Reporting Body plus the serial number (6 digits) assigned by the TAC holder gives 14 digits of the 15 digit IMEI number.
The fifteenth digit is the Check Digit. This is calculated from the other fourteen digits by the TAC holder using the Luhn formula. An example of the calculation is given in 3GPP specification TS 23.003.
Currently, the IMEI structure is as follows: