Soil volume in plain numbers
Volume is simply area × depth, but the units trip people up. A cubic metre holds 1,000 litres, so a bed 2.4 m long, 1.2 m wide and 30 cm deep holds 0.864 m³, or 864 litres, before any allowance for settling. If your supplier works in imperial units, one cubic yard is about 0.765 m³ and one cubic foot is about 28.3 litres. The calculator shows all of them side by side so you can compare quotes without a conversion chart.
Allow for settling
Freshly mixed soil and compost are full of air pockets. After a few heavy rains and a season of worms and roots, the level drops noticeably, and compost-rich mixes drop the most as organic matter breaks down. A settling allowance of 10–20% covers most blends. Choose the lower end for heavier topsoil-based mixes and the higher end for fluffy, compost-heavy fills. Expect to top up with a few centimetres of compost each spring anyway; it is part of keeping a raised bed productive.
What to fill a raised bed with
Most growers use a blend of screened topsoil and well-rotted compost. Topsoil gives structure and holds moisture; compost adds nutrients and life. Pure compost is tempting but it shrinks heavily and can be too rich for some crops, while pure topsoil can compact and drain poorly in a box.
For very deep beds, some gardeners place logs, branches and woody prunings in the bottom third, a technique inspired by hugelkultur. It reduces the amount of soil you need to buy and holds moisture as the wood rots, but it also settles more over time, so plan on regular top-ups. Containers and pots are different: use a purpose-made potting compost, as garden soil tends to compact in pots.
Bags or bulk delivery?
| Option | Best for | Advantages | Drawbacks |
|---|---|---|---|
| Small bags | Top-ups, pots, one small bed | Easy to carry, store and pick up in a car | Usually the highest cost per litre; plastic waste |
| Bulk bag | Several beds at once | Lower cost per litre; one delivery | Needs a drop spot with vehicle access |
| Loose load | New gardens and large projects | Often the lowest cost per litre | Needs space, a tarp and plenty of barrow trips |
Ask suppliers what their bag or load size actually holds, as labels may quote litres or cubic feet. Enter that figure in the bag size box for an exact count.
Choosing a mulch
| Mulch | Typical depth | Breaks down | Best for |
|---|---|---|---|
| Bark chips | 5–8 cm | Slowly | Shrub borders and paths |
| Wood chip | 5–10 cm | Moderately | Paths, fruit trees and perennial beds |
| Straw | 5–10 cm | Fairly quickly | Strawberries and vegetable rows |
| Garden compost | 2–5 cm | Quickly | Vegetable beds; feeds as it mulches |
| Leaf mould | 3–5 cm | Moderately | Woodland plants and soil improvement |
| Gravel | 3–5 cm | Does not | Alpines, herbs and dry gardens |
Applying mulch well
- Water the soil thoroughly before mulching; mulch locks moisture in, not out.
- Keep mulch a few centimetres clear of stems and tree trunks to avoid rot.
- In spring, wait until the soil has warmed before laying a thick layer.
- Use a thin compost layer on seedbeds, as thick mulch smothers seedlings.
Organic mulch
- Feeds soil life as it breaks down
- Keeps soil cooler in summer
- Needs renewing every year or two
Inorganic mulch
- Lasts for years
- Great for drainage-loving plants
- Adds no nutrients and is hard to remove