A ‘10-inch pot’ does not tell you its soil volume

Measure inside the container

Measure the internal diameter or length and width where the soil will sit. Then measure only to the intended soil line, leaving watering headspace. External dimensions include wall thickness and decorative rims that hold no mix.

For a strongly tapered pot, the calculator's single-diameter cylinder is deliberately a rough upper estimate when you enter the internal diameter at the soil line. An exact tapered-vessel estimate needs both diameters and the frustum formula, which this tool does not claim to provide.

Drainage volume is not filler volume

A drainage hole lets excess water leave; a deep layer of gravel does not turn a poorly drained pot into a well-drained one. Container mix selection, hole area, and watering practice matter more than adding a generic rock layer.

Buy for the whole group

When filling several pots, measure one of each size, multiply by count, and add a modest reserve. Keep old potting mix decisions separate: whether it can be reused safely depends on crop health, structure, salts, and how it will be refreshed—not on volume math.