This paper presents a simple analytical approach to define cut geometry of multi-pass rough milling during a free-form surface milling. The shape of in-process workpiece surface was identified using the coordinate of corner points that are found in every step of stair-surface. In every instantaneous tool location, the workpiece sections that have possibility intersecting with the cutting edge were identified based on the coordinate of cutter location point. The algorithm was developed for machining using indexable flat end-mill by considering the effect of helix angle to the cut geometry. The proposed method was successfully used to determine the length of cut and generate the shape of cuts. The implementation test also demonstrated that helix angle tends to produce larger cut.The validation of the accuracy was carried out by comparing the length of cut measured using CAD software with those generated by the proposed approach. The results showed that the differences were very small or less than 0.4%. Therefore, it can be taken into conclusion that the method was accurate. The comparison test on computational time was conducted. ABS took only 1.63 second for calculating cut geometry during one tool pass, while Z-mapping method spent 23.21 second. This result proved that ABS is computationally more efficient.