At sufficiently small soil clay content, the clay contributing to the soil aggregates usually contains so-called lacunar pores that are essentially larger than the clay matrix pores. A recently introduced parameter, the soil lacunar factor, determines the volume fraction of the clay matrix pore decrease at shrinkage that is transformed to the lacunar pore volume increase inside aggregates. The lacunar factor essentially influences the soil shrinkage and is a fundamental soil property that can be found independently of a measured shrinkage curve. The aim of this work was to theoretically derive and experimentally validate an expression to estimate the soil lacunar factor through the actual soil clay content and critical soil clay content (when the actual soil clay content is higher than critical one the lacunar pores lack). To validate the approach the available data of sixteen soils were used.