Separate container and root-ball volumes
First estimate the new pot’s volume up to the planned fill line. Then estimate the space taken by the root ball you will retain. Their difference approximates the fresh mix required. Soil lost during removal or root work adds uncertainty; the calculation cannot predict it.
Example with known volumes
A new pot with 0.8 cubic feet of usable fill space and a retained root ball of approximately 0.3 cubic feet needs about 0.5 cubic feet of fresh mix. Three similar repottings need 1.5 cubic feet. One bag marked 1.5 cubic feet matches that geometric target before any optional allowance.
The old pot is only a proxy
Its stated capacity is a useful approximation only if it was filled and you retain almost all of its contents. Missing mix, voids and an irregular root ball change the answer. Do not present an old-pot capacity as an exact root-ball measurement, and do not substitute weight for volume.
Use the calculators in two steps
Calculate the new and old pots separately using the pot-volume tool and the actual fill heights. Note the results and subtract the retained volume manually; the current calculator does not perform this subtraction. Enter the combined fresh-mix target in the bag calculator.
Growing decisions are separate
The appropriate pot size and repotting method depend on the species, roots and growing conditions. This guide estimates a purchase, not whether a plant needs repotting. Check plant-specific guidance rather than treating a fixed increase in pot size as suitable for every plant.
Frequently asked questions
Must I buy the full new-pot capacity?
Not if a retained root ball occupies some of it.
Does the calculator subtract root balls automatically?
No. Calculate the two volumes and subtract them manually.