This paper presents a simple dynamic model to estimate the ejected droplet volume of a pneumatic printing system. The printing system has a thin flexible membrane with precisely controlled pneumatic actuation is used to produce various droplet volumes. The dynamic model considers the membrane deflection at the liquid chamber and fluid flow at the outlet nozzle using lumped parameters. To estimate the ejected droplet volume for the applied pressure, we obtain a second order differential equation and numerically solved the equation. Simulated results were compared with measured droplet volumes with applied pressures for varied design parameters such as the liquid chamber diameter and the outlet channel width. We confirmed that the dynamic model follows a trend to saturate the ejected volume above certain pressure within a designed chamber size. The simulated results also show that the positive pressure at the saturated volume is decreased with outlet channel width.