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Summary The well-collision-avoidance separation rule presented in this paper is a culmination of the work and consensus of industry experts from both operators and service companies in the SPE Wellbore Positioning Technical Section (WPTS). This is the second of two papers and complements the first paper, SPE-184730-PA (Sawaryn et al. 2018), which described the collision-avoidance management practices. These practices are fundamental in establishing the environment in which a minimum allowable separation distance (MASD) (in m) between two adjacent wells can be effectively applied. A standardized collision-avoidance rule is recommended, complete with parameter values appropriate to the management of health, safety, and environment (HSE) risk, and benchmarks for testing it. Together, these should help eliminate the disparate and occasionally contradictory methods currently in use. The consequences of an unplanned intersection with an existing well can range from financial loss to a catastrophic blowout and loss of life. The process of well-collision avoidance involves rules that determine the allowable separation and the management of the associated directional planning and surveying activities. The proposed separation rule is dependent on the pedal-curve method and is expressed as a separation factor, a dimensionless number that is an adjusted center-to-center distance between wells divided by a function of the relative positional uncertainty between the two. The recommended values for the rule's parameters result from a comparison of various industry models and experience. The relationships between key concepts such as the MASD and allowable deviation from the plan (ADP) are discussed, together with their interpretation and application. The dependency on the error distributions of the survey-instrument performance models used to establish the tolerance lines is also discussed. The consequences of implementing a standardized separation rule across the industry are far-reaching. This affects slot separations, trajectories, drilling practices, surveying program, and well shut-in. We show how the MASD can be related to a probability of crossing and being in the unacceptable-risk region of an offset well. We show why this qualification is required for safe drilling practices to be preserved. Examples are presented in Appendices A through D to help the reader validate the calculations and the directional-drilling software necessary to perform them. The geometrical and statistical limitations of the methods are explained and areas are highlighted for further work. The methods outlined here, taken together with SPE-184730-MS, will improve efficiency in planning and executing wells and promote industry focus on the associated collision risks during drilling. The WPTS also supports the current development of API RP 78, Recommended Practices for Wellbore Positioning. Mathematical derivations or references are shown for all the calculations presented in the paper.