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.
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 ↗
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.