Pots and potting mix
Repotting Soil Calculator: How Much Extra Mix?
Choose the current pot, the larger pot, and whether the root ball stays intact or loses some old mix. The estimate updates instantly.
Calculator
Compare the current pot with the new pot
Choose how much of the old root ball stays together. The result updates instantly as the old-mix scenario changes.
Before you calculate
What this estimate can tell you
The amount of new potting mix depends on both the size difference between the containers and how much old mix still moves with the roots. This calculator compares the filled volumes, then adjusts the occupied root-ball volume using one of three transparent planning scenarios. The scenarios estimate supplies; they do not recommend removing old mix.
How the additional potting mix is estimated
Each tapered pot is estimated with the frustum formula and multiplied by the fill percentage. Estimated retained root-ball volume = old filled volume × selected retention percentage. Additional mix per plant = new filled volume − estimated retained volume. Plant count and the optional allowance are applied last.
Assumptions and limits
- The scenario estimates how much of the old pot's occupied volume still moves with the roots. It is not a weight percentage or the percentage of roots retained.
- The 100%, 80%, and 50% settings are transparent supply-planning assumptions, not recommendations for removing old mix.
- Quick choices use reference dimensions from one nursery-pot product line. Nominal sizes do not guarantee the same height, taper, or capacity across brands.
- The estimate excludes drainage layers, raised pot bases, root shape, compaction, and volume changes caused by root pruning.
Worked example
Example: moving from a 4-inch to a 5-inch nursery pot
At a 90% fill level, the 4-inch reference holds about 0.50 L and the 5-inch reference about 1.22 L. An intact root ball requires about 0.19 US gal of new mix. Choosing the lightly loosened 80% scenario increases the estimate to about 0.22 US gal, or about 0.24 US gal with a 10% allowance.
FAQ
Common questions
Why don't I need the full volume of the new pot?
The existing root ball moves into the new container and occupies part of it. When the root ball remains close to the old pot's shape, the difference between the two filled volumes is a useful starting estimate for new mix.
Which root-ball scenario should I choose?
Choose mostly intact when the root ball moves with little disturbance, lightly loosened when a small amount of outer mix falls away, and much of the old mix removed only when the occupied root-ball volume is visibly reduced. These settings change the supply estimate; they are not care instructions.
Does 80% retained mean 20% of the old mix by weight was removed?
No. It is an occupied-volume assumption based on the old pot, not a weight conversion. It cannot measure how much mix remains between individual roots.
Does this calculator tell me whether to repot or remove old mix?
No. It compares the container and root-ball scenarios you select. It does not diagnose roots or decide whether a particular plant should be repotted, bare-rooted, or root-pruned.
Do all pots with the same nominal size hold the same amount?
No. Nominal size usually describes the top opening, while height and taper vary. Use the references for planning; when inside dimensions are available, use the full potting soil calculator instead.
Calculation references
- Botanically Unique: Clear Nursery Pot DimensionsUsed for the editable 3-, 4-, 5-, and 7-inch nursery-pot reference dimensions; nominal pot sizes are not universal capacities.
- NIST: SI Units — VolumeUsed to verify the relationship among cubic centimeters, liters, and milliliters.
- Penn State Extension: Repotting HouseplantsUsed to verify that repotting moves the existing root ball into a larger container and that pot choice should not be reduced to maximizing volume; the calculator therefore compares amounts without prescribing a size.