Piston-cylinder assemblies are used to create a calculable pressure in a container, which can then be used for calibration of other instruments. For this purpose one needs to calculate the pressure in the container so accurately that both imperfections in the piston, and the leakage of fluid or gas through the small space between cylinder and piston have to be taken into account. Because of these effects, the piston behaves as if its area was slightly larger than it actually is. This slightly larger area is called the effective area of the piston-cylinder assembly, and its computation is the subject of this report. We derive a formula for this effective area, which under some simplifications leads to the formula used by four European metrological institutes. The formula used by NMi is based on a further simplification. We conclude with some recommendations to NMi concerning which formula to use and how to compute the uncertainty in the results. Keywords: effective area, piston-cylinder assemblies, pressure balance, thin film approximation
