Find the inside planting depth
A planter advertised as 36 inches tall may include legs and a storage shelf. Measure the inside length and width, then the depth to your intended soil line. Reservoirs, sloping bottoms and dividers can reduce usable capacity. A manufacturer’s fill-volume specification is helpful for complex designs.
Example: a 36 × 18 inch planting box
With an inside fill depth of 8 inches, the volume is 36 × 18 × 8 = 5,184 cubic inches. Divide by 1,728 to obtain 3 cubic feet. At 6 inches deep the same box needs 2.25 cubic feet. These are geometric examples, not specifications for a particular product.
Turn that volume into bags
A 3 cubic foot target needs two 1.5 cubic foot bags exactly. With 2 cubic foot bags, buy two: the purchased volume is 4 cubic feet, leaving 1 cubic foot above the target. Rounding to whole bags and adding an allowance are different operations.
Use the raised-bed calculator
Choose inches and enter the inside box dimensions, with the number of identical planters. The rectangular geometry works for elevated boxes too. For differently sized compartments, calculate them separately. Transfer the resulting volume to the bag calculator and use the capacity printed on your chosen product.
Check load and growing needs separately
Volume is not a wet-weight estimate. Check the planter’s load limit and installation requirements before filling. The right growing medium and root depth depend on the plants; the calculation does not decide either. Keep reservoirs and other non-soil spaces outside the measured fill depth.
Frequently asked questions
Does total planter height equal soil depth?
Often it does not. Measure the planting box.
Should I count a water reservoir?
Not as soil volume. Use the actual growing-medium space.