I gave the basic idea above about how to measure barn door hardware. Let me explain a bit more again. The core idea is to select the track length in a way so that it is enough to fit the opening you want to cover and the width of the door. That means if you want to buy a single door or bypass door hardware kit, then the track length = length of the opening + the width of the door + some extra space on both sides (4 to 6 inches). Similarly, if you want to buy a double door hardware kit, then the track length = length of the opening + the width of the left door + the width of the right door + some extra space on both sides (4 to 6 inches).