The initial effective area is the most important parameter to a piston pressure gauge. The initial effective area and uncertainty of the piston pressure gauge are calculated with a dimensional method, by aid of the dimensional measurement results of the piston and cylinder of the primary piston pressure gauge. The calculation result of the initial effective area of the piston pressure gauge is 1961.0276 mm2, with an extended uncertainty of 4.3 ppm (k=2, 1 ppm=10-6); The initial effective area of the piston obtained with a pressure balance method is 1962.0279 mm2, with an extended uncertainty of 15 ppm (k=2). Therefore, the uncertainty of the piston effective area obtained with the dimensional method is the smallest, and the piston initial effective area is 1961.0276 mm2.