A system for monitoring ablation size is provided and includes a power source including a microprocessor for executing at least one control algorithm. A microwave antenna is configured to deliver microwave energy from the power source to tissue to form an ablation zone. An ablation zone control module is in operative communication with a memory associated with the power source. The memory includes one or more data look-up tables including data pertaining to a control curve varying over time and being representative of one or more electrical parameters associated with the microwave antenna. Points along the control curve correspond to a value of the electrical parameters and the ablation zone control module triggers a signal when a predetermined threshold value of the electrical parameter(s) is measured corresponding to the radius of the ablation zone.