Window Box Soil Volume
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It multiplies the box's interior length, width, and depth to get total volume, then typically suggests filling to about an inch below the rim to leave room for watering without soil spilling over the edge.

Window boxes are mounted or hung, so weight matters more than in a ground-level bed, and a lightweight, well-draining potting mix reduces strain on the mounting brackets while also draining faster than dense garden soil, which is important given how quickly a shallow box can become waterlogged.

Yes, shallow boxes (under 8 inches deep) limit you to shallow-rooted plants like herbs, lettuce, or trailing annuals, while deeper boxes can support small vegetables with more substantial root systems, so soil volume and plant selection are closely linked in a window box.

Window boxes are elevated and exposed to air and wind on more sides than ground soil, and their limited volume heats up and dries out faster, often requiring daily watering in summer even though a similar volume of soil in the ground would retain moisture for several days.

Potting mix is strongly preferred over garden soil for window boxes; garden soil compacts in a confined container, drains poorly, and is heavier, while potting mix is formulated specifically for the drainage and weight needs of container growing.