How deep the soil needs to be
The RHS says 30 cm is deep enough for dwarf bulbs, salad leaves and strawberries, but most plants, especially fruit bushes, shrubs and small trees, need 45 cm or more. The height of the bed is not always the usable depth: count any good soil beneath an open-bottomed bed, and subtract drainage layers.
Choose the mix
For a standard bed, the RHS suggests three parts organic matter, two parts sharp sand and seven parts topsoil, and doubling the sand for plants that need very good drainage. Pure compost is not a good fill on its own. Match the mix to the crops and to your soil.
Calculate the internal volume
Use the inside dimensions, not the outside of thick boards or blocks. The calculator’s layer mode handles beds filled in layers. A 2.4 × 1.2 m bed with 25 cm of soil and 5 cm of compost, plus a 10% settlement allowance, needs 792 L of soil (20 bags of 40 L) and 158.4 L of compost (4 bags of 40 L).
Fill, settle and top up
The RHS advises filling to about 5 cm below the rim and leaving the bed to settle for a couple of weeks, topping it up if needed before planting. Firm each layer lightly as you go, but avoid heavy compaction.
Practical checklist
- Do not bury drainage outlets or block overflow routes.
- Keep soil below untreated timber edges where appropriate.
- Plan delivery access before ordering bulk material.
