Home Container gardening & soil volume
How much potting mix fits a tapered round planter?
Published by Potting Ledger
Measure the inside diameters of the bottom and the soil line, then halve them to get radii. For straight sloping sides, liters equal π × fill height × (top radius² + top radius × bottom radius + bottom radius²) ÷ 3000, with lengths in centimeters.
A 30 cm pot that holds less than a cylinder
Imagine a planter 30 cm across at the chosen soil line, 20 cm across its flat interior bottom and 24 cm from that bottom to the line. The radii are 15 and 10 cm. The three terms in parentheses are 225, 150 and 100. Multiply their sum of 475 by 24 and π, then divide by 3000. The pot takes about 11.94 liters.
Using only the top diameter in a cylinder formula gives about 16.96 liters. That is roughly 5 liters too much for this particular shape. For three identical empty pots the tapered estimate is 35.82 liters. With hypothetical 10-liter bags, the four-bag purchase leaves about 4.18 liters.
When the shape breaks the formula
A planter may be wide at the shoulder, narrow at the mouth or stepped inside to take a liner. Measure the liner if that is what you will fill. For a pot made of two straight-sloped sections, measure each section’s end diameters and height and add their volumes.
For a strongly curved bowl, a known-volume scoop gives a better check than a long list of approximate diameters. Count level scoops as you fill, then note the soil-line height and scoop size on the plant label. You can reuse that measurement for the next identical pot.