This paper suggests a simple approach to determining the screw characteristics for a three0dimensional flow in a channel with a finite aspect ratio(ratio of a width to a depth, W/H) by introducing a Total Shape Factor($F_t$) to correct a two-dimensional flow analysis for a channel with an infinite aspect ratio. In the present study, the Total Shape Factor($F_t$) was defined as a ratio of a net flow rate obtained by the three-dimensional analysis to that by the two-dimensional analysis. In the proposed approach, the quantity, $ frac{{partial}F_t}{partial(H/W)}$ turns out to be almost constant and to play an important role in understanding the effects of the flights. Therefore, $ frac{{partial}F_t}{partial(H/W)}$ are extensively reported in this paper in terms of several dimensionless parameters. This simple approach with such database will be very useful for extruder designers to predict the screw characteristics.