How Much Soil Do I Need? A Guide to Calculating the Right Amount

Knowing How Much Soil You Need

Whether you're filling a raised bed, topping up borders, or starting a new lawn, working out how much soil you need is essential for planning and budgeting. Underestimating means running short mid-project, while overordering adds unnecessary cost and effort. The right amount of soil depends on the space you’re filling and the depth required for healthy plant growth. Getting accurate measurements at the start saves time and avoids waste later on.

Measuring Your Space

To calculate soil volume, you need to know the length, width, and depth of the area you’re filling. Measure in metres to keep things consistent, and aim for the final depth of soil you want after compaction and settlement. For raised beds, this is usually between 20 and 30 centimetres. For lawns or borders, a layer of topsoil between 10 and 15 centimetres is generally sufficient unless you’re replacing poor existing soil.

Once you have the measurements, multiplying the length by the width by the depth gives you the volume in cubic metres. This is the standard unit most suppliers use when selling bulk soil.

Understanding Volume and Bulk Bags

Soil is usually sold in bulk bags or by the cubic metre. A typical bulk bag holds around 0.7 to 0.9 cubic metres, depending on the supplier and the moisture content of the soil. This means a full cubic metre is slightly more than one bulk bag, and you’ll need to round up if you're close to the edge of your calculation.

For example, if your raised bed is two metres long, one metre wide, and 30 centimetres deep, that equals 0.6 cubic metres of soil. In this case, one bulk bag would be enough, with a little margin for settlement or topping off.

Soil for Different Applications

The depth of soil required varies depending on what you're planting. Shallow-rooted plants such as herbs or bedding flowers only need a small layer of soil, while shrubs and vegetables need more depth to establish properly. Lawns generally require a base of 10 to 15 centimetres of good-quality topsoil to support healthy growth and root development.

In containers or planters, use the full depth of the vessel for soil unless you’re incorporating drainage layers. In larger landscape areas, you may also need to factor in subsoil or organic amendments as part of your calculation if the existing soil is poor or compacted.

Allowing for Compaction and Settling

Loose soil always settles after spreading, especially if it’s high in organic matter. It’s common to lose up to 10 percent of volume after watering or once plants begin to grow. For this reason, it’s a good idea to slightly overestimate the amount of soil you need — especially in deep beds or planters. Leaving a small mound that will flatten over time is often more effective than trying to top up later.

If you're adding soil over turf or hard ground, it may also compress more quickly than expected, particularly after rain or foot traffic. Allowing for this compression helps maintain the desired soil level long term.

Ordering Tips and Practical Considerations

When ordering soil, consider delivery access, storage space, and how you’ll move it into place. Bulk bags are convenient for large jobs, but loose loads delivered by tipper truck may be cheaper if you need several cubic metres. For small spaces or patios, bagged compost or soil blends may be more manageable.

Always check what type of soil is being delivered — topsoil, compost-rich blends, or loam mixes all serve different purposes, and choosing the right one ensures better results. For vegetable beds, choose a rich, loamy blend. For turfing, a screened topsoil with good drainage is more suitable.

Factoring in Soil Type and Use

Not all soil is created equal, and the type of soil you’re buying affects how much you need. Light, fluffy compost-based mixes often settle more than dense topsoil. If you're using compost-heavy blends for raised beds or vegetable gardens, expect more settling and compaction, which means ordering slightly more volume. On the other hand, denser, screened topsoil used for turfing or borders settles less but can be heavier to handle.

For professional or large-scale landscaping, it’s common to layer different types of soil. For example, a base of subsoil might be topped with a layer of high-quality topsoil or compost. If you’re layering in this way, calculate volumes for each layer separately, then combine them for your total order.

Sloped or Irregular Areas

If your project involves a slope, curved edge, or irregular shape, soil volume becomes harder to estimate accurately. One approach is to divide the area into rough rectangles and calculate each section separately. You can also average out the depth if the slope varies. For example, if one end of a bed is 10 cm deep and the other is 30 cm, work with an average of 20 cm to get a more accurate estimate.

When levelling uneven ground or creating terraces, you’ll often need more soil than expected to fill low areas and bring everything up to grade. It’s better to slightly overestimate in these cases, especially if you're working to fixed heights like paving edges, sleepers, or lawn borders.

Soil Shrinkage When Dry or Wet

Soil volume isn’t static — it behaves differently when wet or dry. Dry soil is more aerated and looks fluffier, but once it’s wet and compacted, it shrinks. If you're measuring from dry, loose soil or using bagged compost that’s been stored indoors, expect it to compress once applied and watered.

Conversely, wet soil from bulk delivery may appear more compact and may not shrink much more after application. This difference is why volume can feel inconsistent depending on the season or the supplier. Always clarify with your supplier whether their bulk bag size is measured loose or compacted.

Using Soil Calculators and Online Tools

Online soil calculators can give quick answers, but they depend entirely on accurate inputs. If you're entering dimensions, make sure you're using metres, not feet, and convert centimetres into decimal values to avoid errors. For example, 25 centimetres should be entered as 0.25 metres.

Some calculators let you toggle between cubic metres, litres, and bulk bags. This is helpful if your supplier lists stock in different units. Just remember that one cubic metre equals 1,000 litres, and most bulk bags hold around 700 to 900 litres depending on density.

Working Around Plants or Existing Features

If you're adding soil around existing trees, shrubs, or permanent structures, the usable volume may be less than the full dimensions suggest. Take into account the space occupied by roots or fixtures like edging, water pipes, or fence posts. You may need to adjust the volume downward slightly or order in stages to avoid waste.

When backfilling around planting holes or raised structures, soil often fills gaps unevenly. Ordering in smaller batches or leaving part of the area unfinished until you've filled the bulk can help manage unexpected shortfalls or overages.

Final Word

Calculating how much soil you need starts with knowing your space and its purpose. Once you have the dimensions, it’s a simple matter of multiplying to find the volume and rounding up slightly to account for settlement. By planning ahead and choosing the right type of soil for your project, you set your plants up for long-term success while avoiding the hassle of running short or wasting money on excess material.