<p>Index tests are widely implemented for in-situ evaluation of rocks. In case of most rock types, the universal conversions are well suited for rock strength estimation. However, this approach can potentially lead to significant deviations for less typical types such as porous volcanic rocks. Derivation of specialized conversions is commonly conducted using the regression analysis of index and strength values obtained from tests executed on same specimens. In some cases (e.g., for destructive index tests), the data cannot be easily analysed, as they are obtained separately. The proposed procedure aims to address these cases by a suitable data preparation – after statistical evaluation, a set of representative values is calculated and paired at regular cumulative frequency intervals, allowing for subsequent regression analysis. The method was evaluated on volcanic rock datasets, allowing for direct comparison with the standard procedure. The conversions designed according to the proposed method yielded superior estimation.</p>