Two pots sold under the same size name can hold different amounts of medium. One may have straight sides while the other narrows sharply towards its base. A diameter printed on a label is therefore not enough to calculate the fill you will actually use.
Open Pot Volume Calculator and choose the shape that matches the container. The round cylinder uses one inside diameter, the tapered pot uses both top and bottom diameters, and the rectangular planter uses inside length and width. All heights in this tool are internal measurements in centimetres.
The tapered pot example
Suppose a round pot measures 30 cm across its inside top, 20 cm across the bottom and 30 cm high. Leave 3 cm unfilled below the rim. The medium then occupies 27 cm of height, and the diameter at that fill line is 29 cm rather than the full 30 cm top diameter.
Using that shortened frustum, the calculator returns approximately 12.872 litres per pot. Six matching pots need about 77.231 litres. Ignoring the taper or filling to the rim would produce a different order, even though all three versions might be described casually as a 30 cm pot.
For comparison, a straight cylinder with a 30 cm inside diameter and the same 27 cm fill height holds about 19.085 litres. A rectangular planter 60 cm long, 25 cm wide and filled to 27 cm holds 40.5 litres.
Measure where the medium will sit
Use the inside diameter, not the outside width across a thick decorative rim. For a taper, measure the bottom where the growing space begins. A solid pedestal, raised internal base or substantial moulded insert changes that usable shape.
Headspace is the empty distance above the fill. It must be smaller than the total inside height. Leaving this field at zero means the calculation fills the modelled pot to its top; it does not mean the container has no watering or overflow requirements.
An existing root ball occupies space too. The calculator does not subtract that displacement. When repotting, use the result as the capacity of the container and separately account for the medium that is already arriving with the plant.
Turn the capacity into a batch
Container count multiplies one pot volume. It works well for a matched set of balcony pots, but it should not combine several sizes under one average diameter. Calculate each shape separately and add the litres for the order.
Once the useful volume is clear, Potting Mix Ratio Calculator can split your batch into chosen ingredients. Keep the final mix target distinct from the volume of complete packages you must buy, especially when a small batch leaves substantial leftovers.