We address the problem of computing critical area for missing material defects in a circuit layout. The extraction of critical area is the main computational problem in VLSI yield prediction. Missing material defects cause open circuits and are classi ed into breaks and via-blocks. Our approach is based on the L1 medial axis of polygons and the weighted L1 Voronoi diagram of segments. The critical area problem for both breaks and via-blocks is reduced to a weighted L1 Voronoi diagram of segments. This reduction results in a plane sweep algorithm to compute critical area in one pass. The time complexity is O(nlogn) in the case of breaks and O(nlogn + K) in the case of via-blocks, where n is the size of the input and K is bounded by the number of interacting vias (in practice K is small). The critical area computation assumes square defects and re ects all possible defect sizes following the D(r) = r20=r3 defect size distribution. The method is presented for rectilinear layouts.