A universal method to determine the geometry of burned-gas zone in SI piston engine is presented. Although the background of the method has been previously used by other authors the applicational details have not been exposed, therefore the mathematical details of the method are introduced. The short description of the computer code is given. The computer code has been divided into two files. First file, run as independent program, calculates proper set of data being later used as an input to the second file. The second file, the quick-response one, can be used as a subroutine in a quasi-dimensional SI engine cycle model and to be called at each consecutive step of calculations.