Measure usable space
Record inside length, width and fill height. Exclude the unfilled rim, water reservoirs and false bottoms. If the manufacturer supplies a usable potting-mix capacity, you can enter that volume directly in the bag calculator rather than deriving it from outside dimensions.
Example: a 24-inch window box
An inside space measuring 24 × 8 inches, filled 7 inches deep, holds 1,344 cubic inches. That is about 0.778 cubic feet. Three identical boxes need about 2.333 cubic feet. Two 1.5 cubic foot bags supply 3 cubic feet, above the combined target.
Watch for tapered sides
A narrower bottom means the rectangular calculation using top dimensions overestimates capacity. Our rectangular mode models straight walls; it does not automatically correct a tapered window box. Use the manufacturer’s capacity or clearly treat the rectangular result as an approximation.
Root balls occupy space
The empty-container estimate is not always the amount of fresh mix needed for planting. Existing root balls displace some of the volume. Subtract their space only when you can estimate it reasonably, and do not use a plant’s weight as a volume conversion.
Round the combined purchase
Select rectangular pots, inches and the number of identical containers in the pot calculator. Combine all required volume before calculating whole bags. Rounding each small box separately can inflate the shopping list. Keep any additional allowance separate from the leftover caused by whole-bag rounding.
Frequently asked questions
Does a 24-inch planter have a standard capacity?
No. Width, fill depth and shape also matter.
Can I use outside dimensions?
Inside measurements or a stated usable capacity are preferable.