Depth is the dimension that costs you money
Length and width are usually fixed by the space. You have the bed you have. Depth is the one number you actually choose, and it is the one that multiplies straight through to what turns up on the driveway.
The relationship is direct and unforgiving. Volume is area times depth, so doubling the depth doubles the soil. A bed 4 metres by 1.2 metres is 4.8 square metres, which at 100 mm deep is 0.48 cubic metres and at 300 mm deep is 1.44 cubic metres. Same footprint, three times the delivery, three times the barrowing and roughly three times the bill.
That is worth knowing before you specify a depth out of habit. It is also why guessing 50 mm too generously across a large area is a surprisingly expensive rounding error.
What the plants actually need
Depth should follow what is being grown, because roots have a working range and there is no benefit in providing depth beyond it.
- Lawn topdressing: a thin layer, typically 10 to 25 mm at a time. This is levelling and feeding, not building a soil profile, and burying grass under too much material kills it.
- New lawn from seed or turf: around 100 to 150 mm of decent topsoil over prepared ground. Turf roots into what is under it, so the quality matters more than the thickness.
- Annuals, salad and shallow-rooted vegetables: roughly 150 to 200 mm. Lettuce, radish and most herbs work happily in this range.
- Most vegetables: around 300 mm. Beans, brassicas, courgettes and the like want room to spread.
- Deep-rooted vegetables: 450 mm and up, for parsnips, carrots and anything else that puts down a long tap root. This is where raised beds get genuinely expensive.
- Shrubs: 450 to 600 mm, depending on what you are planting.
- Trees: a tree needs a rooting volume rather than a bed depth, and it will send roots far beyond any bed you build. Do not try to solve a tree with soil depth alone.
A mulch layer is a separate calculation and follows a different logic — you are covering the surface, not filling a root zone. That is covered in the guide on mulch types.
The bed does not have to be all soil
Here is the thing that saves the most money on deep beds. A raised bed 600 mm high does not need 600 mm of bought topsoil in it. What the plants need is the top portion; below that, you need bulk and drainage.
- Build up from the bottom: coarse material at the base, then a rougher soil or subsoil, then good topsoil in the top layers where the roots actually live.
- Existing ground counts: if the bed sits on soil rather than on a hard surface, roots will grow down into it. You are adding depth to what is already there, not building a container.
- Compost is a component, not the whole bed: a common blend is topsoil with compost mixed through the upper layers, rather than filling a bed with pure compost, which slumps dramatically and holds too much water.
- Beds on paving or a roof are different: with no ground below, the bed genuinely has to hold the entire rooting depth, and weight becomes a structural question rather than a gardening one.
Measuring beds that are not rectangles
Real gardens have curves. The soil calculator handles rectangular, circular and triangular shapes, and between them you can cover most layouts by breaking the bed into pieces and adding them up.
- Circular and oval beds use the ellipse area, which is why the calculator takes a length and a width rather than a radius — an oval bed is just one where they differ.
- Triangular is half of length times width, which handles corner beds and wedges.
- A curved border is close enough to a long rectangle at its average width. Measure the width in several places and take the average rather than the widest point.
Once you have the finished volume, the settlement and waste allowances go on top, as covered in the compaction guide. On the 4 by 1.2 metre bed at 100 mm, the finished 0.48 cubic metres becomes about 0.61 with the standard 15 percent settlement and 10 percent waste — which is 13 bags at 50 litres each, or a small bulk bag.
Bulk or bags
That last conversion is worth doing early, because it changes what you order.
Bagged soil is convenient, easy to carry through a house, and expensive per cubic metre. Bulk is far cheaper and arrives in one heap on your drive, which someone then has to move. The crossover is usually somewhere around half a cubic metre — below that, bags; above it, bulk starts winning quickly.
Check access before ordering bulk. A bulk bag needs somewhere for a lorry with a crane to reach, and loose tipped material needs somewhere it can sit without blocking anything. Neither is a problem you want to discover on delivery day.