How Much Soil Do I Need for a Raised Bed? Formula, Mix & Worked Examples

Planning estimate — measure the inside of the bed, and fill proud of the rim. Bed soil is volume plus settling plus a mix split. This guide gives you the method, then run your bed through our raised bed soil calculator.

The formula

Cubic feet = inside length × inside width × depth in feet. Cubic yards = cubic feet ÷ 27. Bags = ceiling(cubic feet with settling allowance ÷ 1.5). A 4 × 8 ft bed at 12 inches deep: 4 × 8 × 1 = 32 cu ft = 1.19 cu yd; with 10% settling allowance, 35.2 cu ft, or 24 bags of 1.5 cu ft. Bulk orders use cubic yards; bagged orders use the bag count — a cubic yard is about 18 bags of 1.5 cu ft.

Measure the inside

Boards steal volume. A bed built from 2-inch boards loses 4 inches off each outside dimension, so an 8 ft outside bed holds about 7 ft 8 in of soil, and a 4 ft outside width holds about 3 ft 8 in. On a 4 × 8 ft bed that error alone is roughly 8% of the order. Measure inside face to inside face, at the depth you will actually fill to — not the board height, if you plan to fill below the rim.

Depth by crop

12 inches is the practical minimum for most vegetables: roots need room, and shallow beds dry out fast. 18–24 inches suits deep-rooted crops like carrots, parsnips, and potatoes, and beds built over concrete or poor ground where roots cannot grow down into native soil. If the bed sits on good, loose garden soil, roots will grow down past the bed into it — but do not count that ground as bed volume unless it is already loose and fertile.

The 60/30/10 mix

The standard raised-bed recipe by volume is 60% topsoil, 30% compost, 10% perlite (or coarse vermiculite): topsoil for bulk and structure, compost for nutrients and water holding, perlite for drainage and air. Pure topsoil compacts into a brick in a bed; pure compost shrinks dramatically as it breaks down and costs far more. For the 4 × 8 ft bed above with allowance, the split is about 21.1 cu ft topsoil, 10.6 cu ft compost, 3.5 cu ft perlite.

Settling and topping up

Fresh fill settles 10–20% after the first waterings as air gaps collapse. Fill proud of the rim — mounded slightly above the boards — and top up after a week of watering. The settling allowance in the calculator covers this first drop; after that, beds lose a little volume each season as organic matter breaks down, so plan a yearly compost top-dressing of an inch or two.

Worked examples

1. 4 × 8 ft, 12 in, 10% allowance: 32 cu ft net, 35.2 cu ft with allowance, 24 bags of 1.5 cu ft (21.1 topsoil / 10.6 compost / 3.5 perlite).
2. 4 × 4 ft, 6 in, 0% allowance: 8 cu ft = 0.30 cu yd, 6 bags of 1.5 cu ft — a shallow herb bed.
3. 3 × 6 ft, 18 in, 10% allowance: 27 cu ft net, 29.7 cu ft with allowance, 20 bags — a deep carrot bed.

Frequently asked questions

The rest of the garden

Beds sit in a garden — the topsoil calculator covers open-ground topsoil, and the mulch calculator covers the mulch that goes on top of the beds.