What this calculator works out
This calculator gives the volume needed to fill a raised bed to a chosen depth, in litres and in bags.
Raised beds consume far more material than their footprint suggests, because depth multiplies quickly. A bed that looks modest can swallow the best part of a tonne.
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Before you order
- Use the internal dimensions of the bed, not the external size including the timber.
- Fill depth is usually the full internal depth for a bed on a hard surface, or slightly less for one on soil where roots can reach through.
- Allow for settling. Loose fill drops by 10 to 20% after watering and a season's weathering, so most beds need topping up.
- Compost alone is not a good fill. A mix with topsoil holds structure and does not slump.
Compost is decomposed organic matter and continues to decompose in the bed, so a bed filled with it alone loses volume steadily and slumps. It also dries out fast and holds nutrients poorly once the organic matter has broken down. Mixing with topsoil — commonly around 60 to 70% soil — gives a bed that holds its level and its structure.
How this calculator works
A volume converted to litres, then to bags:
Litres = length × width × (depth ÷ 100) × 1,000
Bags = litres ÷ bag size, rounded upThe same arithmetic as the compost calculator, applied at greater depth. That difference in depth is what turns a manageable bag count into a bulk delivery.
Worked example: a 2.4 m by 1.2 m bed
Using the default figures — 2.4 m by 1.2 m filled to 30 cm, in 50 litre bags:
- Volume: 864 litres
- Bags: 18
Eighteen bags to fill one standard raised bed, and that is before allowing for settling. At bagged prices this is comfortably more expensive than a bulk delivery, and it is around 400 kg of material to carry through a garden. Anyone building two or three beds should be pricing bulk topsoil and compost separately rather than buying bags.
Common mistakes
- Using external bed dimensions. The timber thickness reduces the internal volume noticeably.
- Filling with compost alone. It slumps and dries; mix with topsoil.
- Not allowing for settling. Budget for topping up after the first season.
- Ordering in bags for a large job. Bulk is cheaper and involves less carrying.
- Filling the bottom with rubble. It impedes drainage rather than helping it.
Frequently asked questions
What should I fill a raised bed with?
A mix of topsoil and compost, commonly around 60 to 70% topsoil to 30 to 40% compost by volume. For a deep bed, the lower portion can be bulked out with less refined material — the hügelkultur approach uses logs and woody material at the base, which breaks down over years and reduces the volume of bought soil substantially.
Do I need to line a raised bed?
Line the timber sides with a permeable membrane if you want to extend the wood's life, but do not line the base — you want roots and worms moving between the bed and the soil below. On a hard surface, the base needs to drain, so a solid liner would create a waterlogged container.
How deep should a raised bed be?
15 to 20 cm suffices for salads and shallow-rooted crops on top of decent soil. 30 cm suits most vegetables. 45 cm or more is worthwhile for deep-rooted crops, on a hard surface, or where the underlying soil is poor — and for gardening at a comfortable working height without bending.
Should I put rubble in the bottom for drainage?
No, despite the persistent advice. A layer of coarse material under finer soil creates a perched water table, where water accumulates above the boundary rather than draining freely. Uniform fill drains better. Broken pots over drainage holes in containers are a different case and also largely unnecessary.
Is what I enter stored?
No. Measurements are processed in your browser and never transmitted or retained.
Related tools
References
- Royal Horticultural Society — raised bed construction, soil mixes and depth guidance
- GOV.UK — soil, peat and garden waste guidance
Sources are checked at publication and can change — how I choose and check references.