The strain concentration factors were determined for aluminum alloy 2026 in the T3511 temper using multi-hole structural coupon specimens. Samples of the alloy were evaluated for both the 6.25 mm (0.25 in.) thick and 10 mm (0.4 in.) thick specimens and having widths of 50 mm (2 in.) and 100 mm (4 in.), respectively. For the case of the specimens that were 50 mm in width the mechanical tests were conducted for both the open hole and filled hole conditions and the corresponding strain concentration value was determined. Threaded fasteners having collars were used for the case of the filled hole specimens. The fasteners posses a shank diameter that was slightly larger than the nominal hole size in order to provide for some interference. The strain concentration values were evaluated at both the failure strain (εf) and the strain at maximum load (εmax). The average strain concentration value was then used to predict the results for the stack-up tests.