Box Liner Calculation

How to find proper bag size for a square box liner:

Width of bag: Width (W) + Depth (D) + 2” for a looser fit
Length of bag: Height (H) + ½ of Depth (D) + 5” for overhang 

For example, Box size is 16”(W) x 6”(D) x 20”(H), then
Width of bag: 16”(W) + 6”(D) + 2” = 24” Wide
Length of bag: 20”(H) + 3” (1/2 of (D)) + 5” = 28” Long

Bag size needed is 24" x 28"