The compressive strength of concrete is highly influenced by the properties of test specimens, such as size and moisture content. This paper presents the results of compression tests on more than 650 test specimens made with four different concrete types, which are mainly air‐entrained. The differences in compressive strength between different types of concrete specimens were investigated with similar compaction and curing conditions. The tested specimens were 50 × 50 mm, 80 × 80 mm, 100 × 100 mm, and 150 × 300 mm cores, which were drilled from the cast cylinders. In addition, 150‐mm cubes, 100 × 100 mm and 150 × 300 mm cylinders were included in the test programme. The ratios of compressive strength between different core sizes and core strength comparability to the cast specimens were found to be strongly dependent on the concrete type. Drilling was found to have a clear weakening effect on obtained compressive strength. The conversion factors for the compressive strength between the core and the same size cast specimen was proposed for 150 × 300 mm and 100 × 100 mm specimen sizes.