The diagram below shows the tradeoff between regular pallet racking and double deep pallet racking.

Wider aisles are required for double deep pallet racking. In order to reach the second row of pallets, the forklift needs longer forks, or extensions on the forks. The turning radius for a forklift with longer forks is going to be significantly larger, leading to wider aisles.

The exact calculation for aisle width is dependent on a few factors, including the base turning radius of the forklift, the size of the pallets and the depth of the double deep pallet racking.

If increased density is your goal, then double deep racks usually create more room for pallets compared to standard racks. However, the trade-off in aisle width needs to be calculated, as well, the forklift drivers need to be well trained, since operating a forklift with longer forks brings additional safety risks.