How much soil a raised bed needs

A raised bed takes more soil than it looks: a 240 × 120 cm bed 30 cm deep holds almost a cubic metre. This calculator starts from the inside dimensions, adds a margin for settling and tells you how many litres and bags to buy, split between compost and soil according to the mix you choose.

How the calculation works

• Volume: inside length × width × depth, in litres (1 m³ = 1,000 litres), times the number of beds.
• Settling: fresh soil compacts with the first rains and waterings; an extra 10-20% saves topping up straight away.
• Bags: total litres divided by litres per bag, rounded up.
• Mix: the chosen compost share, the rest soil.

Measure the inside dimensions: board thickness can take away a lot of volume.

Volumes for common beds

With 15% settling:

Depth matters as much as area

A 240 × 120 cm bed, 15% settling:

Doubling the depth doubles the soil. Very deep beds are often filled at the bottom with branches, logs and coarse plant material to save soil; over time this layer breaks down and sinks.

Which mix to use

A common mix is garden soil or topsoil with 30-50% compost. More compost makes the soil richer and lighter, but it settles more. Avoid using compost alone: it is too rich and compacts.

The reading bands

• Below 0.25 m³: a small volume, bags are the most practical option.
• 0.25 to 1 m³: a medium volume, quite a few bags to carry.
• 1 m³ and above: bulk soil delivered by truck is often more practical.

Limits of the calculator

The calculation applies to rectangular beds. For L-shaped or irregular beds, split them into rectangles and add the results. Bags state a nominal volume, which can vary with the product's moisture. Real settling depends on the material: light soil rich in organic matter sinks more.

Frequently asked questions

How much soil for a 240 × 120 cm raised bed?

At 30 cm deep, about 864 litres; with 15% settling about 994 litres, or 20 bags of 50 litres.

Why add a margin for settling?

Because fresh soil compacts with rain and watering: an extra 10-20% saves topping up straight away.

How many litres are in a cubic metre?

1,000 litres.

How much compost should I add?

Often 30-50% of the mix, the rest soil or topsoil. Avoid using compost alone.

Is bulk soil worth it?

For volumes over a cubic metre it often is: truck delivery is more practical than dozens of bags.

How do I fill a very deep bed?

Many people fill the bottom with branches and coarse plant material and put soil on top: it saves soil, but the layer sinks over time.

Should I measure inside or outside the bed?

Inside: board thickness reduces the volume.

Does it work for planters and containers?

Yes, for rectangular ones: enter the inside dimensions.

Why do compost and soil bags add up to one more?

Because of rounding: each component is rounded up to a whole bag, so separately you may need one bag more than the total.