How to use this calculator
Choose dimensions for empty, round tapered pots, or measure the new loose compost added to one representative pot when repotting around a retained rootball. In geometry mode use inside diameters at the bottom and intended fill line, and the height between them. Enter the identical-pot count and actual bag volume. Open Compost already on hand to deduct usable stock. The result compares complete pots fillable now with bags needed for the full target; both start from the same stock.
The formula
A worked example
The original 40 cm top, 30 cm bottom and 30 cm fill height still give 29.06 L for one empty pot, requiring 15 bags of 2 L when stock is zero. For 25 identical repotting jobs measured at 0.6 L of new compost each, the total is 15 L. With 5 L already available, one 10 L bag covers the shortfall exactly and leaves zero; the original stock alone fills eight complete pots.
Assumptions & limitations
Geometry models an empty circular pot with straight tapered sides; equal diameters form a cylinder and a zero bottom diameter forms a pointed cone. It does not subtract roots, drainage material or internal curves. Measured mode assumes the same pot, retained-rootball size and filling method for every pot. Use comparable loose volumes for measured compost, stock and bags. There is no universal rootball or settling deduction; no automatic allowance is added. Current-stock capacity is not capped at your target and is an alternative to, not a preliminary step before, the full purchase plan.
Common questions
Why measure at the fill line?
The pot may be wider at the rim than where the soil ends. Using the rim diameter could overestimate volume.
How do I account for a retained rootball?
Measure the new loose compost actually added to one representative repotting job and use Measured new compost. Do not assume the entire new pot must be filled with fresh material, or subtract a universal rootball percentage.
Should I round bags separately for each pot?
No. Combine the volume for all identical pots, subtract usable stock, then round the purchase shortfall up once. For two pots needing 2.5 L each, one 5 L bag covers both.
Why can pots fillable now exceed my target?
This reports the full capacity of your existing stock. The bag order and final leftover are calculated for the target pot count separately; do not deduct the same stock twice.
Will this work for a rectangular planter?
Geometry mode does not. Use the raised-bed calculator for a rectangular container, or enter a representative measured new fill in this tool.
Supporting reference
The method above states this calculator’s assumptions. This reference provides related definitions and technical context.
RHS: How to repot a plant ↗Formula and worked example checked against automated test cases. About our methodology.