The aim of this research was to finding methods of calculating the content of water unavailable to plants in the total soil mass based on the water properties of particular grain sizes. Fractions <0.02 mm bind from 90.1 to 98.8% of total water unavailable to plants in soils, and the clay fraction (<0.002 mm) alone binds from 52.1 to 80.1% of this water. Binding water in fractions <0.02 mm significantly depends on the mineralogical composition of fractions. The presence of illite, chlorite or kaolinite causes a decrease of capacity of water unavailable to plants by even 40% in relation to fractions composed of smectites, vermiculites, and humus. Due to high variability of the capacity of water unavailable to plants in particular grain size fractions <0.02 mm resulting from the variable mineralogical composition of this fraction, coefficients allowing to calculate the capacity of water unavailable to plants in soil microcapillaries below 0.2 μm show very high oscillation and could not be applicable.