The basis of cryosurgery is the assumption that complete tumor necrosis occurs only when target tissue has dropped below a tumoricidal temperature. It is the cryosurgeon's goal to conform the critical isotherm to the volume of the target tissue while minimizing peripheral damage caused by iceball intrusion into surrounding health tissue. We introduce the novel concept of the ablative ratio, defmed as the volume enclosed by the critical isotherm divided by the total iceball volume, as a means of quantifying the killing efficiency of an iceball. A high value of the ablative ratio implies effective kill and minimal collateral injury. It is shown that the ablative ratio decreases with increasing iceball size since the volume enclosed by the critical isotherm increases at a rate slower than the total volume of the iceball. Calculations of isotherm locations using the infinate cylinder approximation are shown to yield over-estimations of the ablative ratio. This concept permits objective evaluation of technical and procedural modifications to increase both the efficiency and safety of cryosurgery.