Landscape Mulch

landscapecalculator.com - Mulch Calculator

        try
        4 ft. by 5 ft coverage of 1 bag
        on a 4 bag by 4 bag grid
          = 16 ft by 20 ft
          = 320 sq. ft with 16 bags
        
        output: 13.3 2 cu. ft. bags
        
        so bags on a 4 ft. by 5 ft. grid
        results in just over 1 inch depth.