The essential work of fracture (EWF) is the most straightforward method to assess the toughness of ductile polymer films and sheets in which per se plane-stress conditions prevail. The interest is focused, however, on the determination of the plane-strain fracture toughness being a material parameter. It was demonstrated that the plane-strain essential work of fracture (determined in dynamic tensile impact tests) agreed well with that of the yielding-related specific essential work of fracture concluded from static mechanical tests for an amorphous copolyester sheet. This agreement still held when the plane-strain essential work of fracture derived by considering the experimental data in the plane-stress/plane-strain transition range (laying below the lower threshold ligament for which a linear regression was adopted) for a thicker sheet of the same polymer has been considered.