Due to the structural measures on existing buildings engendered by the increasing density of urban building as well as the preservation of listed buildings, it is now crucial to be able to assess the quality of the existing historic brickwork. The primary substantial issue for structural inspection is the compressive strength of the existing brickwork, which is composed of brick compressive strength and mortar compressive strength. In Austria, the testing of historic masonry is regulated by norms present in ÖNORM B 1996-3 and which are primarily designed for the testing of masonry made of small-format solid bricks. As for masonry made of hollow clay masonry units, there are so far no adequate test methods that operate without strongly interfering with the structural loadbearing integrity. Therefore, this paper develops a methodology which allows to determine the brick compressive strength thanks to samples of the hollow clay masonry unit. The experimental programme is divided into two main sections: to start with, an examination of material-specific behaviour of small-sized brick samples in the punch compression test is conducted. For this purpose, a total of 430 small plate-shaped samples are extracted from 21 solid bricks, and four drilled cores are obtained from each stone in order to determine the individual material compressive strengths. Subsequently, a comparison of the punch compression strength of the small test specimens with the material compressive strengths of the bricks is established, and a regression curve is calculated. In the second part, two drilled cores are taken from 15 different hollow clay masonry units from the company Wienerberger Ziegelindustrie GmbH. Their percentage of voids is examined by means of three different methods, and they are then broken down into 284 small specimens. Following the method employed in the first part, a punch compression procedure is conducted and the compressive strengths of the individual masonry units are evaluated. The determined percentages of voids can thus ultimately help convert the material compressive strengths into the brick compressive strength of hollow clay masonry units. The results of the conducted tests show that it is possible to establish a plausible correlation between the brick compressive strength of a masonry unit and the punch compression strength of a small test specimen obtained from it. Based on those results, this paper presents a concept for a non-destructive test method which allows the determination of the brick compressive strength in existing buildings built with hollow clay masonry units.