Article 30 of the Mexican Social Security Law
In order to determine the base daily salary of contribution, the following shall apply:
When, in addition to the fixed elements of the salary, the worker regularly receives other periodic remunerations of previously known amount, these will be added to said fixed elements;
If, due to the nature of the work, the salary is integrated with variable elements that cannot be previously known, the total income received during the two immediately preceding months will be added and divided by the number of days of salary earned in that period. In the case of a newly hired worker, the probable salary that corresponds to him/her in said period shall be taken, and
Reformed fraction DOF 20-12-2001
In those cases in which the salary of a worker is integrated with fixed and variable elements, it will be considered of a mixed nature, therefore, for contribution purposes, the average obtained from the variable elements will be added to the fixed elements in terms of what is established in the preceding fraction.