Outside bed size is not soil volume
A 2,400 × 1,200 mm outside frame with 100 mm thick walls has an inside footprint of 2,200 × 1,000 mm. Filling that space 300 mm deep requires 660 L before any allowance. That is 33 bags at 20 L, 17 at 40 L, or 14 at 50 L. These are volume comparisons, not a recommendation for one growing medium.
Combine identical beds before rounding bags
Three of those 2,200 × 1,000 mm inside beds with 300 mm of new fill require 1,980 L. At 40 L per bag, one shared order needs 50 bags and leaves 20 L against the plan. Buying 17 bags separately for each bed would use 51 bags. The sleeper-to-soil link deliberately leaves fill depth blank: timber courses cannot tell how much existing soil, base material or headroom is in the bed.
Published cell capacity and loose compost can differ
Containerwise lists 60 cells of 21 cm³ for the CD60 tray: 1.26 L of nominal cell space. In a separate fill trial, Charles Dowding used 2 L with firming and filling above the dividers. Those are different measurement conditions. A trial with your own tray and compost is the clearest basis for purchasing.
Containerwise: CD60 specifications ↗
Charles Dowding: tray filling trial ↗
Use the compost you already have
For ten 60-cell trays at 21 mL per cell and a 10% allowance, the plan needs 13.86 L. If 5 L of usable loose compost is already available, the remaining requirement is 8.86 L. One 20 L bag covers it and leaves 11.14 L after the target. Separately, the original 5 L can fill three complete trays now. These are alternatives from the same starting stock, not two consecutive batches.
Repotting needs new compost around the retained rootball
An empty pot’s geometric capacity is not the fresh compost needed when the existing rootball occupies part of it. RHS repotting guidance describes adding compost around that rootball. Measure the new loose compost used in one representative pot rather than subtracting a universal percentage. If each of 25 identical pots needs 0.6 L, the total is 15 L. With 5 L on hand, one 10 L bag covers the remaining requirement exactly. Separately, the original stock can fill eight complete pots using that measured amount.
Check the product, not a supposed standard size
A 2.4 m sleeper and a 20 L compost bag are editable examples. Timber section, usable stock length, tray capacity and bag volume vary. Use the actual wall thickness in the orientation you plan to build; confirm the supplier’s construction guidance separately from the arithmetic.
These tools plan cuts and quantities. They do not choose a safe raised-bed construction, compost recipe or regional planting date.