Planter & Pot Volume Calculator
Find the potting mix for any container — round, tapered, rectangular, or an odd shape you measure yourself. Get the volume in cubic feet and quarts, scaled to how many pots you're filling, so you can buy the right bags.
The classic flower-pot shape — wider at the top than the base.
Window boxes and trough planters.
For odd shapes — estimate the opening area and the fill depth.
Potting mix is sold by cubic feet or quarts — confirm on the bag (1 ft³ ≈ 25.7 qt).
Advanced: price
Formula & how it works
Round: π × (dia ÷ 2)² × height. Tapered: (π × h ÷ 3) × (R² + R·r + r²) with top radius R and bottom radius r. Rectangular: L × W × H. Custom: opening area × depth.
Volumes are converted to cubic feet (÷ 1,728 from cubic inches) and dry quarts (1 ft³ ≈ 25.71 qt), then multiplied by the number of containers.
Worked example
A 12 in round pot 10 in deep holds π × 6² × 10 ≈ 1,131 in³ = 0.65 ft³, about 17 quarts. Taper it to an 8 in base and it drops to about 0.46 ft³ — the narrower bottom really does hold less. Filling three 2 ft² custom planters 6 in deep needs 3 ft³, roughly 77 quarts, or two 1.5 ft³ bags.
Filling containers
Shape matters more than you'd think
A tapered pot can hold noticeably less than a straight-sided one of the same top diameter, because the volume narrows toward the base — using the cylinder formula for a flower pot overbuys soil. Measure the top and bottom widths for tapered pots so the frustum math can account for the slope.
Quarts is the shopping unit
Bagged potting mix is usually labeled in quarts for small bags and cubic feet for large ones, so the calculator shows both. When you're comparing an 8-quart bag against a 1.5 cubic foot bag, converting them to the same unit is the only way to see which is the better deal for your pots.
Leave headroom and count displacement
The number here is the pot filled to the brim, which you don't actually want — leave about an inch below the rim so water soaks in instead of spilling. Large plants also arrive with a root ball that takes up space, so for a single big container you can buy slightly under; when filling many small pots, round up so you don't run short.