This paper mainly deals with the description of the initial distribution of bulk density and porosity at the moment a compost pile is built or rebuilt. A relationship between bulk density and vertical position in a pile is deduced from theoretical and empirical considerations. Formulae to calculate the air filled volume fraction and the true densities of the solid phase and of the organic matter are derived. The true density of dry matter is used in the computation of porosity distributions. The relationships between bulk density and height and between air-filled volume fraction and height are shown to be valid for composting material consisting of chopped wheat straw and chicken manure. The check of this validity is limited to total bulk density values ranging from 150 to 950 kg/msuperscript 3, with values of dry matter content varying between 18 and 28% (w.b.). Moreover, the gravimetric dry matter content must be constant throughout the total cross section of the pile. The error in the calculated bulk densities and air-filled volume fractions was found to be 12% at a reliability level of 95%. It was concluded that the presented equations will give reasonable results for other values of dry matter content and other kinds of chopped fibrous materials as long as the gravimetric dry matter content remains independent of height.