Fill depth is not wall height
Measure the empty space that will receive new soil. A 12-inch-tall bed with 8 inches of existing fill needs a different calculation from an empty bed. Use the depth you intend to add and account for any space you want to leave at the top.
Use inside measurements
Outside dimensions include the walls. Measure inside length and width to avoid counting the space occupied by lumber or other framing. For sloped sides or irregular bases, divide the space into approximate sections and check the estimate on site.
Adding several beds
Add one area for each bed or section. Each may use a different depth. Garden soil is totaled independently from mulch, so the two materials never become one shopping quantity.
Does this include settling?
No settling factor is imposed. The optional allowance is a user-chosen percentage. Soil blends, compaction, moisture, and existing fill affect the quantity actually needed; check your product and installation requirements.