Risk of infection with most tick-borne pathogens depends directly on the duration of tick attachment. To determine the duration of attachment of nymphal and adult female Ixodes scapularis Say, previous studies have used polynomial regression to model tick engorgement with a fixed time of initial tick attachment. We studied the impact of the time of day of initial tick attachment on engorgement rates in the nymphal stage, and found that ticks attaching early in the photophase engorged at significantly slower rates than ticks attaching later. We propose an improved method of predicting duration of tick attachment that incorporates this variability in tick engorgement rates. A graphical method is presented to facilitate prediction.