Medium-sized planter boxes from a planter box factory are usually built to hold a volume of potting mix rather than a fixed weight of soil. In most production lines, a medium box falls between roughly 24 and 36 inches long, 10 to 16 inches wide, and 10 to 16 inches deep. That size range commonly holds about 1.5 to 4 cubic feet of loose potting soil. In practical terms, this is around 40 to 120 dry quarts, 11 to 30 gallons, or 42 to 113 liters.

The exact soil volume depends on the box's internal dimensions, drainage design, and how high the soil is filled. Manufacturers often leave 1 to 2 inches of clearance below the rim to prevent overflow when watering. A sloped or tapered box may also hold less than its outside measurements suggest. For this reason, the best method is to multiply the inside length, width, and planned soil depth in inches and divide by 1,728. The result is the volume in cubic feet.

Medium planter boxes are typically filled with a lightweight potting mix, not heavy garden soil. Potting mix provides drainage, aeration, and moisture retention, which matters because a medium box can become surprisingly heavy when wet. If the box is used for vegetables, herbs, or flowering plants, a mix with compost, peat or coir, and perlite is often suitable. Add a slow-release fertilizer or renew nutrients during the growing season.

When buying from a planter box factory, ask for the internal volume in cubic feet, quarts, or liters. That figure is more useful than the external size alone. As a general guide, a medium box with about 2 cubic feet of soil can support compact vegetables, herbs, or small shrubs; a 3- to 4-cubic-foot box offers more root room for larger plants. Always confirm the factory's specification, because "medium" is not a universal measurement.