The current ASME code flange design addresses flange rotation by introducing the concept of an effective gasket width in the calculation of the bolt load instead of the full gasket contact width. Although it is accepted widely and is relied on, it has no theoretical or fundamental basis and above all it has not been verified experimentally. In addition, with the new design rules based on the new gasket constants obtained from ROTT tests conducted on rigid platens, there is a need to reconsider and verify the validity of the effective gasket width concept.The main objective of the present work is to investigate the effect of the non-uniform gasket contact stress due to flange rotation on the leakage behavior of sheet gasket materials. Both experimental and numerical investigations will be carried out to quantify this effect. Particular attention will be focused on the applicability of the effective width concept and its variation with flange flexibility and load.