Estimate mix for containers that taper
Most round nursery pots and decorative containers are wider at the opening than at the base. Treating them as straight cylinders can overstate their capacity. This calculator uses the volume of a truncated cone, also called a frustum, to model straight tapered sides. Measure inside the pot, not across its outer rim. The top diameter is taken at the intended soil fill line, while the depth runs from the inner base to that line. Leaving watering space is therefore handled through your dimensions rather than an automatic deduction.
Subtract the space occupied by the plant
Pot capacity = π × fill depth × (top diameter² + top × bottom + bottom diameter²) ÷ 12The equation uses consistent centimetres or inches for all three dimensions. Cubic centimetres are divided by 1,000 for litres, and cubic inches by 1,728 for cubic feet. Root-ball volume is subtracted from the capacity of each pot because that space already contains the plant's existing growing medium. The remainder is multiplied by the whole-number pot count, then increased by the optional extra-mix percentage. Bags are rounded up only at the end. The displayed per-pot mix remains before allowance, so you can see the geometric requirement separately from the quantity planned for shopping.
Four pots with thirty-centimetre fill diameters
Each pot has a 30 centimetre inside diameter at its fill line, a 20 centimetre base diameter and a fill depth of 25 centimetres. Its capacity is π × 25 × (900 + 600 + 400) ÷ 12 = 12,435.47 cubic centimetres, or 12.435 litres. An existing root ball occupying 2 litres leaves 10.435 litres of fresh mix per pot. Four pots require 41.742 litres before allowance, and a 10 percent extra-mix allowance brings the shopping quantity to 45.916 litres. Two 25 litre bags cover that amount, leaving a small balance for adjustment.
Measure unusual containers and plan the fill
A pot with curved walls, a bulging belly or an internal reservoir is not a straight frustum. Use a measured capacity or divide its interior into approximate sections when the shape differs substantially. Account for any permanent insert by reducing usable depth or including its displaced volume, without subtracting the same feature twice. A large root ball must fit within the estimated capacity; a greater entered displacement is rejected. This worksheet counts volume rather than choosing a mix recipe for the plant. Read the potting product's stated bag volume, and use cubic feet rather than gallons in the imperial bag field. Weight cannot be inferred from litres alone because moisture and mix composition vary. Extra mix is editable and applies to the combined fresh-fill requirement, not to the root-ball volume.
Frequently asked questions
Where do I measure the top diameter?
Measure the inside diameter at the soil fill line, below any watering headspace. The entered depth ends at that same line.
Can I calculate a straight round pot?
Yes. Enter identical top and bottom diameters. The frustum equation then becomes the ordinary cylinder volume equation.
Why subtract a root ball?
Its existing medium already occupies part of the container. Subtracting its approximate volume estimates the additional fresh mix needed around it.
Does bag count include extra mix?
Yes. The chosen allowance is applied to the fresh mix for all pots, then the total is divided by bag volume and rounded up.
What about a rectangular planter?
Use a box-volume or raised-bed calculator for that shape. This page models round containers with straight tapered sides.
Last reviewed: 2026-10-03 · How we calculate