<p>Silica glass-fiber materials are manufactured by acid treatment of glass-fiber materials of a special composition. Such treatment selectively removes basic components, leading to an increase in the mass fraction of high-temperature resistant oxides (SiO₂ and Al₂O₃) to almost 100% and formation of a porous fiber structure. The resulting silica fibers have a unique property set, i.e., high temperature resistance (operating temperature 1000-1100°C, melting point 1650°C); low thermal conductivity; excellent dielectric characteristics; and chemical resistance to the action of H₂O, acids (except HF and H₃PO₄), weak bases, and radiation. These properties are responsible for their wide use in metallurgical and energy sectors for fabricating various types of high-temperature thermal protection and as a filler in composites. The filler density is a key parameter for preparing composites. The density of silica fibers after acid treatment cannot be found directly, in contrast to glass fibers (grade E, VMP), the density of which corresponds to the density of the starting glass determined by a hydrostatic method. As a rule, it is calculated by the Appen additive method. The present work reports the densities of silica fibers calculated by additive methods and experimental data obtained on a Poromer apparatus. The results were found to be comparable, which allowed the additive method for calculating the density of silica fibers to design composite materials to be recommended.</p>