From the Chemical Engineers'Resource Page www.cheresources.com

Use this calculation procedure to perform preliminary sizing of storage bins for bulk solids.
With minimal inputs, the volume of the final bin will be calculated.

References:
Steve,Edward H.,Sizing Up the Storage Bin,Chem.Eng.,p.84,July 2000
Precision Stainless Website,www.precisionstainless.com
Walas,Stanley M.,Chemical Process Equipment: Selection and Design, McGraw-Hill, 1990

Angles of slide given in the table are the minimum at which the various materials will slide on a steel plate, under best conditions for determination of friction.

Materials are assumed to be dry, sized, and without dust. The minimum slide angle will increase as size of particles decreases and with an increased moisture content

Source:Walas, Stanley M., Chemical Process Equipment:Selection and Design, Butterworth-Heinemann, 1990

If your solid is not shown in the table, choose '_other': this will assume a minimum slide angle of 40° for preliminary bin sizing.