Raised Bed Soil Calculator
Calculate the soil volume needed to fill a bed or container.
Volume grows faster than the bed looks
A raised bed 2m by 1m by 30cm deep needs 600 litres of soil — around 24 of the 25-litre bags sold at most garden centres, or about 0.6 cubic metres. People consistently underestimate this by a wide margin, because the numbers on the side of a bag feel large and the bed does not look deep. Buying in bulk by the cubic metre is dramatically cheaper than bagged compost once you pass roughly 300 litres, which is a bed of quite modest size.
Soil settles, and then settles again
Freshly filled beds drop noticeably. A mix heavy in compost and organic matter can lose 20 to 30% of its volume over the first season as the organic fraction decomposes and the structure compacts under watering. Overfilling slightly at the start, and planning to top up each spring, is normal practice rather than a sign anything went wrong. This is also why beds are usually filled proud of the frame rather than level with it.
What to fill with matters more than how much
Pure compost is a poor filler despite being what many people reach for: it decomposes rapidly, holds too much water, and can be too high in nutrients and salts for seedlings. A durable mix is roughly half topsoil for structure and mineral content, a third to a half compost for fertility and water-holding, and a portion of coarse material such as grit or bark for drainage. Deep beds can be part-filled at the bottom with coarse woody material to reduce the volume of bought soil, though that layer will sink substantially as it breaks down.
Container mixes are a different product entirely — they are formulated to stay open and drain freely in a small volume, and garden soil packed into a pot compacts and waterlogs.
How to use it
- Enter the internal length, width, and fill depth of the bed.
- Read the result in litres, cubic metres, and standard bag counts.
- Add roughly 10% for settlement, and plan an annual top-up.
- Compare bulk pricing against bags once you need more than about 300 litres.
Before you plant
How many bags of compost is a cubic metre?
About 40 of the 25-litre bags, or 25 of the 40-litre bags. Bulk delivery is usually far cheaper once you need more than roughly 300 litres.
Should I fill a raised bed with pure compost?
No. Pure compost decomposes fast, sinks substantially, holds excess water, and can be too rich for seedlings. Roughly half topsoil, a third to a half compost, and some coarse grit or bark makes a more durable mix.
Why did my bed sink over the winter?
Organic matter decomposes and the mix compacts under watering — losing 20–30% of the volume in the first season is normal. Overfill slightly at the start and top up each spring.
Can I put wood or branches in the bottom?
Yes, in a deep bed it reduces the volume of bought soil and adds organic matter over time. Expect that layer to sink considerably as it decomposes, so keep it well below the root zone.
Disclaimer: These are general planning estimates. Seed packets, product labels, and regional frost, water, and pesticide guidance always take priority over any figure produced here. Local soil, climate, and variety differences can change the result substantially.