Do not fill a raised bed with compost
Nothing else in raised bed gardening costs more to get wrong 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.
Timing counts for as much here as quantity
Compost is the one material in the garden where timing changes the result. Spread in autumn it protects bare soil through winter, and worms take it down through the cold months so the bed is ready without being dug. Spread in spring it holds moisture through summer — but only once the soil has warmed.
That last condition is the one people get wrong. A thick layer over cold March soil insulates it, and insulation works both ways: the ground stays cold, germination is slow, and everything sown into it sulks for a fortnight. Wait until the soil is workable and warm to the hand, then mulch.
The other timing rule is around planting. Compost against a stem holds moisture where the plant least wants it, so pull it back a hand-width from anything with a woody base. The ring of bare soil looks unfinished and prevents collar rot, which is a fair trade.
Questions from the plot
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.
Where to go next
Part of soil, compost and beds. These three connect directly to what this page works out: