This report presents a new method for estimating the effects of deviations from a true circular form on external pressure capacity of cylinders. The new, improved method is based on analytical work and test results that have become available since earlier Code rules were developed. The new method is based on a more rational basis than earlier rules, and the new method provides better correlation with test results. The method could be adopted by ASME Code Committees as a replacement for the existing rules and as a means of providing some flexibility on specified tolerances, so that a reduced allowable external pressure may be calculated if tolerance limits are exceeded.