Carbon fibre reinforced polymer (CFRP) strengthening of structures has been gaining increasing interest, traditionally applied to concrete structures, and more recently applied to steel structures. This paper describes 20 experiments on short, axially compressed square hollow sections (SHS) cold-formed from G450 steel and strengthened with externally bonded CFRP. The SHS were fabricated by spot-welding and had plate width-to-thickness ratios between 42 and 120, resulting in plate slenderness ratios between 1.1 and 3.2. Two different matrix layouts of the CFRP were investigated. It is shown that the application of CFRP to slender sections delays local buckling and subsequently results in significant increases in elastic buckling stress, axial capacity and strength-to-weight ratio of the compression members. The experiments are an extension of a previous study [Bambach MR, Elchalakani M. Plastic mechanism analysis of steel SHS strengthened with CFRP under large axial deformation. Thin-Walled Structures 2007;45(2):159-70] in which 25 commercially produced SHS with plate slenderness values between 0.3 and 1.6 were strengthened with CFRP in the same manner. A design method is developed whereby the theoretical elastic buckling stress of the composite steel-CFRP sections is used to determine the axial capacity, and is shown to compare well with the 45 test results. A reliability analysis shows the method to be suitable for design.