Cross-section classification is one of the key concepts for structural design in metallic materials. Current design specifications for aluminium alloys, such as Eurocode 9 (EC9), provide clear definitions and discrete design treatments for different class categories.The purpose of the present paper is to re-evaluate the yield (Class 3) slenderness limits for internal elements and outstands on the basis of substantial, recently generated experimental data on compressed aluminium alloy cross-sections. In this study, approximately 350 stub column test results have been gathered; the section types include square and rectangular hollow sections, channels and angles. The members were extruded from a variety of aluminium alloy tempers with a wide range of yield and ultimate strengths. Following analysis of the available data, the existing slenderness limits in EC9 were assessed and new slenderness limits were proposed. In conjunction with the proposed Class 3 limits, corresponding effective thickness formulae have also been established for Class 4 sections. The suitability of the proposed limits has been demonstrated through reliability analysis for both the conventional design methods in EC9 as well as the alternative methods given in Annex F of EC9. The revised limits enable more precise predictions of capacity.