In a watertight concrete pipeline with O-ring rubber gasket joint, the gasket is the sole element of the joint depended on to provide watertightness. An under-sized gasket could cause leakage in the joint, and an oversized gasket could induce excessive stress in the concrete bell and cause eventual failure, in addition to laying difficulties during field installations. A method is provided to enable the engineer to accurately compute the stretched rubber gasket diameter so that a sound and watertight joint can be designed. The various methods of calculating stretched gasket diameter now specified by the Federal and State government agencies and technical societies should be standardized.