The traditional half-brick, one-brick and standard cavity wall have been designed successfully for many years using the design guidance contained in the various recent editions of BS Codes of Practice. The method, however, does have some limitations. These stimulated the search for a new prediction equation. The suggested approach, which is based on a pin-ended model, is developed using a modified Rankine theory. Capacity reduction factors are derived which are similar in value, but not identical with, the P values in BS 5628: Part 1. The approach appears to better predict the behaviour of masonry walls than the present Code guidance when the full range of slenderness ratios, for which some test results are available, is considered.