Raised Bed Soil Calculator

How the Raised Bed Soil Calculator works

Raised bed soil calculator: volume and bags

A raised bed takes more soil than most people expect. A modest 2.4 by 1.2 metre bed filled 30 cm deep needs 864 litres, which is over twenty standard 40-litre bags, and the mix will settle by a tenth or more in the first few weeks. Working the figure out before you buy avoids two trips to the garden centre or, worse, a bulk delivery of twice what you needed.

The calculation. Volume is length × width × depth, measured on the inside of the frame. The dimensions accept metres and centimetres (1.2 or 120cm) or feet and inches (4 or 4'6"), and the depth is best given in centimetres or inches. Multiply by the number of beds for a whole plot. Cubic metres convert to litres at 1,000 to 1; a cubic yard is 27 cubic feet or about 765 litres.

Settling. Fresh soil and compost are fluffy and shrink as they are watered and as the organic matter breaks down. Ten percent is a sensible allowance for a bought topsoil and compost mix; use 15 to 20 percent if the mix is mostly compost, leaf mould or bark, and less if you are using screened topsoil alone. You can fill to the brim and top up later, but most gardeners prefer to order enough the first time.

What to fill it with. Pure compost is too rich and too light for a whole bed; pure topsoil is heavy and slow to warm. The usual advice is roughly 60 percent topsoil to 40 percent compost by volume, which the result shows, or a three-way mix of topsoil, compost and a coarse material such as sharp sand or bark fines for drainage. Deep beds over 40 cm can have the bottom third filled with woody prunings, old turf or straw, which saves soil and breaks down into humus over a few seasons.

Weight. Topsoil weighs about 1.1 to 1.3 tonnes per cubic metre when damp; compost around half that. The weight line matters for beds on decking, balconies and roofs, and for whether a bulk delivery will fit in a car or needs a trailer. A single bulk bag is about a cubic metre and weighs close to a tonne.

Bags versus bulk. Bagged soil is convenient up to a few hundred litres; beyond that, a bulk bag or a loose tipper delivery is far cheaper per litre. The result shows the count of 40-litre bags and, over a thousand litres, the number of bulk bags. Bag sizes in the table cover the usual 25, 40 and 50-litre sizes and one-cubic-foot to three-cubic-foot bags in the imperial view.

How deep should the soil be? Salad crops and herbs manage in 15 to 20 cm; most vegetables want 30 cm; root crops, brassicas and shrubs prefer 40 to 45 cm. If the bed sits on open soil, roots can go deeper than the frame, so 30 cm is usually enough. On concrete or a membrane, go deeper and make sure the base drains.

Soil for raised beds

One bed per row, with the default allowance for settling.

Soil volume and 40 L bags by raised bed size and depth

Raised bed soil worked examples

The formula: soil volume = length × width × depth × number of beds; order = volume × (1 + settling allowance); litres = m³ × 1,000.

First example. A 2.4 × 1.2 m bed filled 30 cm deep holds 0.864 m³. With 10% for settling, order 0.95 m³ (950 litres), which is 24 bags of 40 L and roughly 1.05 tonnes of topsoil mix.

Second example. A 1.8 × 0.9 m bed filled 40 cm deep holds 0.648 m³, so with 10% settling order 0.71 m³ (713 litres), or 18 bags of 40 L.

How many bags you need is the litres to order divided by the bag size, rounded up.

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