Using Wide Field Camera 3 (WFC3) data from the Hubble Space Telescope (HST) archival project "SKYSURF", we model completeness with respect to the exposure time and background of an image. This is accomplished by adding simulated objects with varying magnitudes and sizes into these HST images, and determining the matching rate for each set of parameters. The fifty percent completeness results then can be compared to the Exposure Time Calculator (ETC), in order to assess the differences between it and our analysis of the archive data. We find that for larger objects and exposures the ETC predicts higher completeness magnitudes, while for smaller objects, the ETC predicts lower magnitudes.