Two quantities to write down
The filled cavity of the new pot and the amount of new potting mix you need are not always the same. A transplant brings an existing root ball and its media. If the destination cavity is estimated at 20 liters and the existing root ball occupies about 3 liters, the simple difference is 17 liters. These numbers illustrate subtraction; they are not a recommended pot size or repotting step.
Our calculator keeps the displacement adjustment separate from container dimensions. Enter it per pot. The container count then multiplies the remaining new-mix requirement, so every pot in that run must use the same assumptions.
Avoid a misleading shortcut
Do not use the outside volume of the old decorative pot as if it were the root-ball volume. Walls, empty space and a separate inner nursery container can all affect that comparison. A nursery size label is also not a guarantee that the actual contents occupy exactly that many liquid gallons.
If you cannot make a useful estimate, leave the adjustment at zero and label your result “empty-container estimate.” That is clearer than inventing a precise correction. Do not compress or disturb roots just to obtain a measurement for a shopping calculator.
Keep the record useful
For different-size transplants, run separate calculations and note each result. The calculator accepts identical pots in a batch; it does not model a collection of different root balls inside different containers. If displacement equals or exceeds the available cavity, the tool flags the input for review instead of presenting a negative purchase quantity.
Save the original measurements and record what you actually used after the task. A note such as “estimate 17 liters; some mix remained from a labeled 20-liter bag” is useful without claiming laboratory accuracy. Keep planting-depth and repotting decisions separate from this accounting exercise, using guidance for your actual plant.
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.