Do not fill a raised bed with compost
This is the most expensive mistake in raised bed gardening and almost everyone makes it once. Bagged multipurpose compost looks like exactly what a new bed wants: dark, fine, full of nutrients. Fill a bed with it and the first season is superb.
The second season is not. Compost is largely organic matter, and organic matter decomposes. A bed filled with pure compost drops by a quarter to a third in the first year as the material breaks down, leaving you topping it up annually forever. It also has almost no mineral content, so it holds water badly once it dries out — and peat-free composts in particular become genuinely difficult to re-wet after a dry spell.
The mix this calculator gives you is roughly half topsoil, a third compost and a fifth grit or bark. The topsoil provides mineral structure that does not decompose. The compost provides fertility and biology. The grit keeps it draining. That mix settles a little in year one and then holds its level.
| Bed size | Depth | Volume | Bulk bags |
|---|---|---|---|
| 4 × 2 ft | 10 in | 6.7 ft³ | 0.22 |
| 8 × 4 ft | 10 in | 26.7 ft³ | 0.89 |
| 8 × 4 ft | 18 in | 48 ft³ | 1.60 |
| 2.4 × 1.2 m | 250 mm | 720 L | 0.85 |
| 3 × 1.2 m | 400 mm | 1,440 L | 1.69 |
Depth is a crop decision
Bed depth is usually chosen from what timber was available, which is the wrong way round. It should come from what you intend to grow and what is underneath.
Over open soil, a bed only has to hold the top of the root zone, because roots will go straight down into the ground below. Six inches of good soil above workable earth grows almost anything. Over concrete, paving or heavy compacted clay, the bed is the entire root zone and it has to be deep enough on its own: twelve inches minimum, eighteen for root crops and anything perennial.
The other consideration is reach rather than roots. A bed you can only reach from one side should be no more than about two feet wide. From both sides, four feet is the practical maximum for most people. Wider than that and you end up standing on the soil, which compacts it and undoes the main advantage of building a raised bed in the first place.
Bags or bulk
The calculator gives you both counts because the crossover point surprises people. Bagged compost is convenient and roughly three to four times the price per litre of the same material delivered loose or in a bulk bag.
Around two thousand litres — a little under two cubic metres — bulk delivery is almost always cheaper even after the delivery charge, and that is only about two and a half 8 × 4 ft beds at a foot deep. If you are building more than one bed, ring a landscape supplier before you fill a trolley at a garden centre.
Two practical notes on bulk. It arrives on a pallet and has to be dropped somewhere a lorry can reach, so check access before ordering. And a bulk bag of wet topsoil weighs close to a tonne, which is well beyond what a wheelbarrow and one person move comfortably — allow a day and, ideally, help.
Common questions
How much soil for an 8 × 4 ft raised bed?
At 10 inches deep it needs about 26.7 cubic feet, which is roughly 0.9 of a bulk bag or 14 bags of 2 cubic feet. At 18 inches deep it is 48 cubic feet.
Can I fill the bottom with logs or branches?
Yes — the technique is called hügelkultur and it works, but the wood decomposes and the bed will sink noticeably over two or three years. Expect to top it up, and do not use it under shallow-rooted annual crops you want level.
What is the best soil mix for a raised bed?
Roughly 50% topsoil, 30% compost and 20% grit or bark. Pure compost is the common mistake: it looks ideal, sinks by a third in the first year, and dries out badly.