A pot containing some mix and an open bag in the shed both reduce what you need to buy, but they belong in different places in your plan. The pot’s retained amount reduces that pot’s new fill. The loose bag reduces the combined shopping requirement once.
The Multi-planter Soil Inventory spreadsheet keeps those two quantities separate. It supports ten groups of matching containers, with a quantity, target fill and retained volume for each group.
Decide what each number means
Target fill is the total growing-medium volume up to your intended level, not necessarily the container’s advertised capacity. A gap below the rim helps prevent compost spilling during watering; the RHS describes this when explaining how to plant containers. RHS planting guide.
If you need an estimate from internal dimensions, use Planter Fill first. For a box, the calculation is internal width × length × fill depth. Round and tapered pots need their corresponding shapes. Copy the result in litres per container into the inventory sheet.
Retained volume means usable mix that will stay inside each container. Loose stock means suitable mix outside those containers that is available for this job. The sheet does not judge growing-medium suitability or whether old material should be reused. Choose that separately: the RHS notes that some plants need a different compost from general-purpose container mixes. RHS planting guide.
Keep separate files for different mixes. This is an inventory method, not a recipe for blending them. For seed sowing in particular, the RHS advises fresh compost rather than treating older compost as interchangeable. RHS seed-sowing guidance.
A five-container shopping example
Suppose you are preparing three matching troughs and two matching pots. These volumes, prices and the allowance are invented examples:
| Group | Quantity | Target each | Retained each | New fill for group |
|---|---|---|---|---|
| Troughs | 3 | 24 L | 6 L | 3 × (24 − 6) = 54 L |
| Pots | 2 | 18 L | 4 L | 2 × (18 − 4) = 28 L |
| Total | 5 | 82 L |
The 24 L trough figure could come from internal measurements of 80 × 20 × 15 cm to the chosen fill line: divide 24,000 cm³ by 1,000 for litres. The example assumes these targets already describe the volume you intend to fill with mix. It does not estimate space occupied by root balls or inserts.
With a chosen 5% allowance, plan for 82 × 1.05 = 86.1 L. Subtract 17 L of loose stock to get 69.1 L to buy. At 20 L per bag, buy four bags. If each costs $5.75, the purchase costs $23, and the estimated surplus is 17 + 80 − 86.1 = 10.9 L.
The allowance is editable. It is not a universal rule for settling, compaction or planting technique.
The double-counting trap
There are 26 L already retained across these containers: three lots of 6 L plus two lots of 4 L. Those litres were deducted in the group calculations. Do not add them to the 17 L of loose stock as well. Doing that would falsely reduce the shopping requirement to 43.1 L and suggest three bags, leaving you short under the original assumptions.
Also check whether an amount is per container or per group. “6 L retained each” in three troughs means 18 L retained across that row. If the troughs contain different amounts, give them separate rows instead of using an unlabelled guess.
Check the open bag before leaving
If your supposed 17 L loose stock is actually only 6 L, the requirement becomes 80.1 L. Four 20 L bags would be slightly short; the whole-bag estimate becomes five. Check uncertain quantities rather than rounding them until the answer looks convenient.
Before saving your shopping copy:
- Enter 0 retained litres for an empty container; leave an entire unused row blank.
- Correct any retained amount larger than its target fill.
- Use litres throughout; bag count is calculated separately.
- To remove a group, clear its blue cells in columns A–D and keep the formulas.
- After the job, update loose stock from what remains. Do not carry an old estimate forward as a measurement.
Bag costs exclude tax and delivery. Keep product choice and plant-care decisions alongside this volume plan.
How this guide was prepared
Prepared by AXO Labs with AI-assisted writing and calculation checks. The worked quantities and shopping arithmetic were checked on . Dimensions, prices and allowances are invented examples, not measurements from a customer project or current shop prices.
The linked Royal Horticultural Society guidance supports watering headroom and choosing an appropriate growing medium. Our example checks retained-soil, loose-stock and whole-bag arithmetic; it does not establish that old compost is suitable for your plants. Read our editorial approach and report a correction.
Keep the plan with the project.
Use the editable sheet for your own measurements and stock. The browser tool is there when you need to explore another setup.