Mitochondrial swelling due to permeability transition is an important biological phenomenon with a lot of open questions. We give a new mathematical approach to model the volume change during the process. As opposed to the existing models we do not use differential equations, just basic assumptions about the swelling of a single mitochondrion and its cumulative effect. The results accurately describe the experimental swelling curves and are in accordance with previous findings. They also provide more insight into the nature of the positive feedback in the process. Due to its simplicity the method can be used for other biochemical or population dynamical processes as well.