HOW IT WORKS
Measure inside, up to the fill line
Measure the inside diameter where the mix will reach, the inside base diameter and the height between those levels. A flared rim above the fill line should not be included. For a straight-sided rectangular planter, use its inside length, width and fill height.
Use a tapered-pot model
The calculator uses a truncated cone: V = π × h × (D² + D × d + d²) ÷ 12. D and d are diameters, not radii. Equal top and base diameters make it a cylinder. The calculator converts the resulting volume to liters, cubic feet or cubic yards; it does not assume a trade gallon equals a US liquid gallon.
Example: 12 inches across the top, 8 at the base, 10 high
The tapered volume is about 0.461 cubic feet, or 13.04 liters. A 12-inch-wide cylinder of the same height would hold about 0.654 cubic feet. Measuring the base avoids that overestimate. Choose inches for all three dimensions; the quantity field lets you plan several matching pots.
Allow for the plant already in the pot
A root ball takes up space, so a full cavity estimate is not the exact amount of new mix required for repotting. Subtract occupied volume if you can estimate it reliably. Keep drainage holes clear and consult plant-specific growing advice; a layer of rocks is not a required part of this calculation.
Frequently asked questions
Does this work for a cylinder?
Yes. Enter the same top and base diameter.
Can volume tell me the weight of the mix?
No. Weight also depends on the material, moisture and density.