Understanding your result
Capacity is the geometric space below the fill line. New mix subtracts the entered root-ball or occupied volume. The total combines identical pots and your additional allowance before rounding to bags.
For rectangular outdoor beds rather than tapered containers, use the Raised Bed Soil Calculator .
The formula
A tapered round pot is modeled as a conical frustum. The diameter form of the formula includes division by four to convert diameters to radii, division by three from the frustum volume, and division by 1,000 to convert cubic centimetres to litres.
Worked example
A pot measuring 30 cm across the fill line, 20 cm across the bottom, and 25 cm deep holds about 12.435 L. Subtracting a 2 L root ball leaves 10.435 L. Four pots with 5% extra need about 43.829 L, so buy three 20 L bags.
How to use this calculator
- Measure both inside diameters and the vertical height between them.
- Enter the occupied volume per pot and number of matching containers.
- Set an extra allowance and copy the bag volume from its label.
Measure the soil surface, not the rim
When the fill level sits below the rim, measure the top diameter at that lower level. Using the rim diameter while reducing only the height can overestimate the volume of a tapered pot. The height is vertical, not the slanted wall length. Equal top and bottom diameters correctly describe a cylinder.
Root balls and settling
Existing roots and soil take up space that does not need new mix. Enter their estimated volume rather than treating the container as empty. The allowance is extra material for purchasing; it is not a horticultural prescription or an exact settlement model. Pot shape, internal reservoirs, curved walls, and inconsistent bag filling can all affect the final quantity.
Assumptions & limitations
What this calculation assumes
- Round cross-sections change diameter linearly over the fill height.
What to keep in mind
- Does not model curved, square, or irregular containers, or select a suitable growing mixture.
Common questions
Can the bottom be wider than the top?
Yes. The frustum formula works in either direction as long as both diameters and the height describe the actual interior.
Does the result specify drainage material?
No. Subtract any genuinely occupied space separately. The calculation does not recommend adding a drainage layer or changing the potting mixture.