Raised Bed Soil Calculator
Works out how much soil it takes to fill a raised bed. Enter the internal dimensions — raised beds swallow far more than people expect, and the figure is usually the surprise.
Includes the waste allowance — this is the figure to order.
Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.
The formula
volume = length × width × depth × (1 + waste) weight = volume × bulk density
The geometry is trivial; the bulk density is where the answer actually comes from. Soil mix runs about 800 kg per cubic metre loose, so a cubic metre weighs roughly 0.80 tonnes. Suppliers quote one or the other, which is why both are given.
What to watch out for
Two things catch people out:
- The volume is larger than it looks. A 2.4 × 1.2 m bed, 300 mm deep, takes nearly 0.9 m³ — about 18 bags of 50 litres. Beds are routinely under-ordered by half.
- It settles. Expect to lose 10–20% over the first season. Fill to about 50 mm below the rim, then top up next spring rather than mounding it now.
A sensible mix is roughly 60% topsoil, 30% compost and 10% grit or sharp sand for drainage — not pure compost, which collapses and dries out. For a deep bed, the bottom third can be coarser or part-filled with woody material that will break down, which cuts both cost and weight.
Frequently Asked Questions
How much soil do I need for a raised bed?
Length × width × depth. A 2.4 × 1.2 m bed at 300 mm deep needs about 0.86 m³ before waste — roughly 18 bags of 50 litres, or most of a bulk bag.
What is the best soil mix for a raised bed?
About 60% topsoil, 30% compost and 10% grit or sharp sand. Pure compost is the common mistake: it grows well for one season, then compacts and dries out.
How deep should a raised bed be?
300 mm suits most vegetables. Root crops such as carrots and parsnips want 450 mm. If the bed sits on open soil rather than a hard surface, roots can go deeper than the frame, so shallower works.