Garden Topsoil Calculator: m³, Tonnes and Bags

How the calculation works

How to calculate garden topsoil: volume, bags and cost

Preparing a garden bed or lawn starts with working out how much topsoil or compost you need. The most common mistake, and the most expensive, is ordering too little. When soil arrives in bulk by lorry, a second delivery often costs as much as the first, and the extra charge for a short load can be significant.

The formula: simple, with one detail that matters

The volume of soil comes from three measurements: length, width and the depth of the layer you are adding. A vegetable plot 5 metres long and 4 metres wide, filled to 30 centimetres, needs 5 × 4 × 0.30 = 6 m³ of soil. The calculator takes the depth in centimetres and converts it to metres, so a 30 cm layer is 0.30 m.

The bag count is the volume multiplied by 25, because a 40-litre bag is 0.04 m³ and there are 25 of them in a cubic metre. The result is rounded up, since suppliers sell whole bags. The total weight is the volume multiplied by the density of the substrate, expressed in tonnes per cubic metre.

The detail that many people overlook is settling. Loose topsoil occupies more space when it is laid than it does after a few weeks of rain and foot traffic. For bulk topsoil, allow a generous margin above the calculated volume. For bagged multi-purpose compost, which is more uniform and settles less, a smaller margin is usually enough. The calculator gives the exact volume, so the margin is a decision you make when you place the order.

Bags or bulk delivery: which is more economical?

The choice between bags and bulk delivery depends mainly on the quantity and on access to the site. For volumes up to about 1 m³, 40-litre bags are the practical option. They are easy to store, need no lorry access and let you work at your own pace, although a full cubic metre means several car trips. The price per cubic metre of bagged compost is, however, considerably higher than for bulk.

Between about 1 and 2 m³, either option can make sense, depending on access and the quotes you receive. From about 2 m³ onwards, bulk delivery usually makes more economic sense. Topsoil supplied by local landscape suppliers, garden centres with bulk yards or builders' merchants arrives by tipper lorry, and the price per cubic metre is far lower than for equivalent bags, even after adding the delivery charge. For a vegetable plot of 20 m² at 30 centimetres deep, which is 6 m³, the difference can be large.

Before ordering a bulk load, check that a tipper lorry can reach the site and that there is somewhere to tip it. A load left on a narrow path or a gravel drive can block access and then has to be moved by hand, which takes a long time and adds real cost.

Types of substrate and what to choose for each job

Three categories are common in the UK.

Topsoil: soil dug from natural ground, screened and sometimes enriched with compost. It is the heaviest of the three, and it is the cheapest when bought in bulk. It suits filling large volumes, raising vegetable plots and levelling ground. BS 3882:2015 is the British specification for topsoil, so ask the supplier whether the material is supplied to it. Soil pH depends on the parent material: soils over chalk and limestone tend to be alkaline, while many peaty soils are acidic. For intensive vegetable growing, measure the pH before planting, using a method such as the one in ISO 10390.

Multi-purpose compost: a mix of peat or peat-free materials, horticultural grit, compost and slow-release fertiliser. It is lighter than topsoil, porous and good at holding moisture. It is a common choice for domestic beds, containers and borders. Peat-free versions are widely available, and their density can differ from peat-based products, so check the manufacturer's data sheet on the bag or on the supplier's website.

Premium or ericaceous compost: formulated with a higher proportion of sphagnum peat and extra nutrients, and intended for acid-loving plants such as rhododendrons, azaleas and blueberries. For an ordinary garden, the performance gain over multi-purpose compost rarely justifies the extra cost, unless you grow plants that need acidic conditions.

Depth for each type of planting

The depth of new soil decides how much you need to order. For sown lawns, 10 to 15 centimetres of prepared topsoil over levelled ground is enough for the usual grass mixes, such as fescue and perennial ryegrass. Turf laid in rolls needs about 10 centimetres of levelled topsoil underneath.

For flower beds, annuals and perennials, 20 to 25 centimetres gives roots enough room to grow without reaching compacted subsoil. For vegetable plots, 30 centimetres is the minimum for crops such as tomatoes, peppers and courgettes. Carrots and other root crops need 40 to 50 centimetres of loose soil. In a London terrace, where the ground may be rubble or builders' waste left from earlier work, raised beds with timber or brick edging are often more practical than digging out the existing ground.

For shrubs and hedging, such as privet, bay or dwarf olive, 40 to 50 centimetres of good quality soil at planting avoids root problems in the first few years, especially on heavy London clay.

Drainage and compaction: what the tables do not show

New soil compacts over the first months after it is laid. This is normal. The problem arises when the ground underneath does not drain: water collects at the boundary between the new layer and the old soil, causing waterlogging and root rot.

On heavy clay, the simplest solution is to loosen the base soil with a fork or a subsoiler before adding the new layer. Mixing coarse horticultural grit or sharp sand into the new soil can improve drainage. Do not use sea sand, which contains salt. In formal gardens or on slopes, a geotextile membrane laid on the base stops the two soils mixing while still letting water drain through.

Common mistakes

The most frequent errors are ordering exactly the calculated volume with no margin, mixing up centimetres and metres for the depth, forgetting that bulk topsoil and bagged compost have different densities, and planting into compacted subsoil with only a thin layer on top. A further mistake is ignoring access: a lorry that cannot reach the site turns a good price into a poor one.

When not to use this calculator

The calculator sizes the material; it does not design the garden. It assumes a rectangular area with level ground and a uniform depth. For irregular shapes, split the area into rectangles and add the volumes together. It does not cover sloping ground that needs grading, complex raised bed shapes, or soil that must be removed before the new layer goes down.

When to bring in a professional

For projects above about 100 m², sloping ground, automatic irrigation, integrated paving or work involving building elements, a chartered landscape architect or an experienced garden designer usually saves money over the long term. Membership of the Landscape Institute, the professional body for landscape architects in the UK, is a useful way to check qualifications. Even so, review each professional's references and past work before appointing them.