Fluid continuity theory is used to describe the dynamic response of open Micro-Electro-Mechanical-System (MEMS) devices. For a packaged device, at low pressure, the fluid continuity theory is no longer valid and a rarefication theory based on a Knudsen number is used. In an open MEMS device, the characteristic length which determines the Knudsen number is represented by the gap between the MEMS bridge and underneath actuation electrodes. On the other hand, for a packaged device, effective characteristic length is modified with the packaging cavity height. In this paper, for a packaged MEMS device, effective characteristic length with reference to the packaging height is derived. © 2016 Author(s).