Architectural infill walls interact with the surrounding reinforced concrete load bearing members of the building under lateral earthquake loads and can change the system behavior significantly. Therefore, lateral strength calculation of these walls may become mandatory for the designers. These infill walls often formed with horizontally-hollow clay based bricks in many regions of the world. However, strength of these bricks depends on the loading direction and bricks remain in the diagonal compression strut of an infill wall are subjected to biaxial compression. Thus, testing procedures investigate the strength only in the direction perpendicular to the bed-joint seems insufficient to reflect actual behavior. Accordingly, it is confusing for designers to determine or consider compressive strength of hollow clay bricks in order to model and calculate the strength of an infill wall. To clear up this confusion, the seismic strength of plastered infill walls made up of horizontally-hollow bricks were tried to be estimated by various prism tests. The effects of loading direction, plaster, and presence of a single or double brick on prism strength were experimentally investigated. Six reference walls and more than 170 prism specimens were tested in accordance with this scope. The correlation between prism and infill wall strength was established with a valid and practical method. Results showed that diagonally loaded plastered prism group successfully and adequately estimated the experimental infill wall strength. In addition, if diagonal testing is not possible to conduct, the average strength of the vertically and horizontally loaded prism tests may be used instead.