Two circles, one useful approximation
Look at the inside of the container from the side. If straight walls slope inward toward a smaller circular base, a truncated cone is a useful geometric approximation. The mathematical name is a frustum. It needs a top diameter, a bottom diameter and a height. Rounded sides or a stepped base will still make the result approximate.
Treating the full opening as a cylinder assumes the widest circle continues all the way down. That describes a different cavity and normally overestimates the capacity of a pot that narrows toward the bottom.
A concrete example
Imagine an empty pot with a 12-inch internal top diameter, an 8-inch internal bottom diameter and a 10-inch internal depth. Filled to the rim, the frustum estimate is about 13.0 liters. A 12-inch cylinder of the same height would be about 18.5 liters. These are invented measurements for explaining the calculation, not specifications for a product.
Now leave 1 inch empty at the top. At that lower fill line the diameter is 11.6 inches, not 12. The filled depth is 9 inches, giving about 11.3 liters. The calculator first finds that smaller fill diameter and then computes the filled frustum.
Use the simple model honestly
Measure across the internal base, not across the feet underneath. If a central mound or insert takes up substantial room, the geometry alone cannot describe it. Prefer the manufacturer’s stated growing-media requirement for a purpose-built reservoir system.
You do not need to force every planter into this model. Record an uncertain measurement as uncertain and compare estimates using plausible dimensions before purchasing. Do not convert the result into a structural load rating: water, container material, supports and the building itself require different information. The calculator is a shopping-volume aid, not an engineering approval.
Sources for factual statements
Planning methods and fictional worked examples are our own analysis. No physical product testing or harvest results are claimed. Use guidance for your actual plant, location and equipment. Read our research method.