In this study, the influence of size and capping type of cylindrical specimens on compressive strength of concrete is investigated. For this purpose, eight series of concrete mixtures were designed to have water/cement ratios of 37%, 42%, 47%, 48%, 55%, 62%, 71% and 77% (by volume). Three hundred and eighty-four cylindrical specimens having dimensions of 150/300 and 120/200 mm were casted. At the end of 28-day standard curing period, the uniaxial compressive strength of capped and uncapped specimens was determined. Cement, gypsum and sulphur are used as capping materials. The results of the experimental study reveal that the suitable average conversion factor (the ratio of compressive strength of 100/200 mm cylinder to 150/300 mm cylinder f(c100)/f(c150)) can be taken as 103%. Linear and nonlinear regression analyses were employed between f(c100) and f(c150). Linear and nonlinear regression analyses exhibited better performance. (c) 2006 Elsevier Ltd. All rights reserved.