Raised Bed Soil Calculator & Mix

What Goes Into the Numbers

The calculation starts with the physical volume of your raised bed. Multiply the bed length by its width to get the base area in square metres, then multiply by the depth (converted to metres) to get the volume in cubic metres. One cubic metre equals 1,000 litres, so converting to litres gives you a number that maps directly to the bags and bulk sacks you buy from garden centres.

The calculator uses litres rather than kilograms for a good reason. Soil weight changes dramatically with moisture content: a bag of topsoil might weigh 25 kg when dry but over 35 kg when damp. Volume stays consistent regardless of whether it rained last Tuesday, so litres give you a more reliable shopping figure.

Once you have the total volume, the calculator splits it according to your chosen ratio. The default 60/30/10 split (60% topsoil, 30% compost, 10% perlite or grit) comes from RHS guidance for general vegetable growing, and our soil mix guide explores crop-specific ratios in more detail. Each component serves a specific role in the mix:

• Topsoil provides the mineral structure, weight, and trace elements that plants need for steady root development
• Compost delivers organic matter, beneficial microbes, and slow-release nutrients that feed crops through the growing season
• Perlite or horticultural grit creates air pockets in the mix, preventing waterlogging and letting roots breathe

The waste factor (set at 10% by default) accounts for soil settling after watering and natural compaction over the first few weeks. Fresh soil mix in a raised bed typically settles by 10–15% of its original volume within the first growing season, so ordering slightly more than the raw volume saves a trip back to the supplier.

Making Sense of the Output

The total soil mix figure is the combined volume of all three components after the waste factor has been applied. This is the number to work from when comparing prices between bagged and bulk delivery options.

Topsoil is shown in litres and represents the largest portion of your order. Garden centres sell screened topsoil in bags of 40 or 50 litres, while bulk bags (also called dumpy bags or tonne bags) hold roughly 1,000 litres. The topsoil volume calculator can help you convert litres to delivery weight if you are ordering in bulk. For beds needing more than 500 litres of topsoil, a bulk bag is usually cheaper per litre and saves handling dozens of smaller bags.

Compost litres cover well-rotted garden compost, composted green waste, or composted manure — any of these work. Bagged compost typically comes in 40 or 50 litre sacks. If you have a mature compost heap at home, you can substitute your own and reduce the purchased amount accordingly.

Perlite or grit is the smallest volume but plays a critical role in drainage. Perlite comes in bags of 10 to 100 litres. Horticultural grit (typically 2–6 mm grain size) is sold by weight, but a rough conversion is 1 litre of grit equals about 1.5 kg.

Keep in mind that the soil will settle. After the first heavy watering and a few weeks of natural compaction, expect the level to drop by 10–15% from the rim. The waste factor in the calculator accounts for most of this, but if you fill the bed right to the brim on day one, you may still need a thin top-up of compost in autumn.

Practical Tips for Planting

How you fill the bed matters as much as what you fill it with. Tipping all the topsoil in first, then adding compost on top, leaves you with distinct layers that roots struggle to bridge. A better approach is to fill in stages.

Add roughly a third of the topsoil, spread it level, then scatter a third of the compost and perlite over the surface. Fork or rake the layers together lightly, then repeat for the second and third layers. This gives you a genuinely blended mix from top to bottom. Wet each layer with a gentle hose spray as you go. Dry soil mix is hydrophobic and takes a long time to absorb water evenly once the bed is full.

Leave 20–30 mm of space below the rim of the bed. That gap gives you room for a layer of bark or chip mulch on top, which reduces moisture loss and suppresses weeds. Without that gap, watering washes soil over the edges.

If your raised bed is built from untreated timber, line the inside walls with weed membrane or pond liner before filling. The liner slows moisture contact with the wood and can extend the life of a softwood bed from 3–4 years to 7–8 years. Do not line the base, since water needs to drain downward freely.

Different plants thrive in different mixes. The standard 60/30/10 ratio suits most vegetables, but you can adjust the proportions to match what you are growing:

• Vegetables (tomatoes, courgettes, beans): 60% topsoil, 30% compost, 10% perlite. The default mix, well-balanced for heavy feeders.
• Acid-loving plants (blueberries, azaleas, heathers): 40% ericaceous compost, 50% composted bark, 10% perlite. The higher organic content lowers pH naturally.
• Succulents and Mediterranean herbs (lavender, rosemary, thyme): 50% topsoil, 20% compost, 30% grit. The extra grit provides the sharp drainage these plants demand.

These ratios are starting points. If your garden soil is heavy clay, increasing the perlite or grit component by 5–10% helps compensate for poor drainage around the bed's base. Once filled, the plant spacing planner helps you work out how many seedlings fit your bed before planting day.

Factors That Change the Calculation

Bed depth is the single biggest variable. The default 300 mm works well for salad leaves, annual flowers, and most kitchen herbs. Root vegetables such as carrots, parsnips, and potatoes need at least 400 mm of soil depth for unobstructed root growth — stunted roots are almost always a depth problem, not a fertility problem. At the other end of the scale, shallow-rooted crops like lettuce, radishes, and spring onions grow happily in beds as shallow as 200 mm.

If your raised bed sits directly on concrete, a patio, or compacted rubble, you need to plan drainage more carefully. The same area-to-materials logic used here applies to flooring projects indoors, where waste factors and coverage rates drive the final order. Drill or leave drainage holes in the base (one per 300 mm of bed length is a sensible minimum) and consider a 50 mm gravel layer at the bottom before adding your soil mix. Without drainage exits, even a well-mixed soil will become waterlogged after heavy rain.

In areas with heavy clay subsoil beneath the bed, increase the perlite or grit ratio from 10% to 15–20%. Clay holds water at the base of the bed, and the extra drainage material in the mix stops the lower layers from staying saturated. You can test this by digging a small hole next to where the bed will sit, filling it with water, and timing how long it takes to drain. If the water is still sitting after 30 minutes, extra grit is a good investment.

For beds taller than 600 mm, you do not need to fill the entire depth with premium soil mix. The bottom third can be filled with rough organic matter — small branches, straw, or partially composted garden waste — in a method sometimes called Hugelkultur. This material breaks down slowly, improving drainage and releasing nutrients over several years. Fill only the top 300–400 mm with your calculated soil mix and reduce the total order accordingly.

Finally, if you are filling multiple beds at once, round your total order up to the nearest bulk bag increment (usually 1,000 litres). Leftover soil mix stores well in a covered heap and is useful for autumn top-ups when the first season's settling becomes visible.