Raised Bed Soil Calculator (Volume & Cost)

Raised Bed Soil Guide

What do I need to know about Calculating Soil Volume?

The soil needed to fill a raised bed is simply its internal volume: length × width × depth for a rectangular bed. Working in centimetres gives cubic centimetres, which you convert to litres (divide by 1,000) or cubic metres (divide by 1,000,000). For example, a bed 200 × 100 × 30 cm needs 200 × 100 × 30 = 600,000 cm³ = 600 litres = 0.6 m³. Soil is sold in litres (bags) or cubic metres (bulk), so converting to both is useful. A common surprise is how much soil a raised bed swallows — they're deceptively large. A modest 2m × 1m bed filled to 30cm needs 600 litres, which is twelve 50-litre compost bags. Deeper beds (for root vegetables) or larger beds multiply this quickly, which is why buying in bulk often makes sense. This calculator works out the volume and converts it to the number of bags (in your chosen size) or bulk bags, plus the cost — so you can compare buying many small bags against one bulk delivery, which is usually far cheaper per litre.

What to Fill a Raised Bed With?

The fill makes the difference between a thriving bed and a disappointing one. A good general mix balances structure, nutrients, and drainage. A widely-used approach is roughly: topsoil (around 50-60%) for body, structure, and mineral content; compost or well-rotted manure (around 30-40%) for nutrients, organic matter, and water retention; and optionally some grit or sharp sand for drainage if needed. Pure compost alone is not ideal for a deep bed — it's very rich, can be too 'hot' for some plants, settles a lot, and is expensive in the quantities a bed needs. Pure topsoil alone lacks nutrients and organic matter. The blend gives the best of both. For specific crops: vegetables generally want a rich, fertile, free-draining mix; some plants (Mediterranean herbs, alpines) prefer leaner, grittier, low-nutrient mixes. Acid-loving plants (blueberries, rhododendrons) need ericaceous compost rather than standard. The 'no-dig' approach favours building beds up with layers of compost and organic matter on top, improving soil over time. For a large new bed, the bulk of the volume can be cheaper topsoil with a richer compost layer worked into the top where roots are most active.

What do I need to know about Saving Money on Large Beds?

Filling a big or deep raised bed entirely with bagged compost is expensive, so there are sensible ways to economise. Buy topsoil in bulk: a bulk bag (around 1 m³ / 1,000 litres) of screened topsoil is dramatically cheaper per litre than small bags — worth it for anything beyond a small bed, provided you can take delivery and move it. The 'hugelkultur' / part-fill approach: for very deep beds, the lower portion can be filled with bulky organic material — logs, branches, cardboard, turf turned upside down, garden waste, leaves — which slowly breaks down, improves drainage, adds nutrients over time, and reduces how much bought soil you need. Top the bed with a good growing mix where the roots will be (the top 20-30cm matters most for most crops). Make your own compost: home composting provides free organic matter to mix in or top up with over time. Reuse and improve: beds settle over the first season as organic matter breaks down, so expect to top up with compost annually — a little-and-often approach builds fertility cheaply. This calculator helps you buy the right amount first time, avoiding both shortfalls (a half-filled bed mid-project) and costly over-ordering.

What's the key thing to understand about Building and Maintaining the Bed?

A few practical points for a successful raised bed. Depth: most vegetables are happy in 30cm of good soil; root crops (carrots, parsnips) and deep feeders appreciate more (40cm+). Shallow beds (under 20cm) limit what you can grow and dry out faster. Drainage: raised beds drain better than open ground (an advantage), but ensure the base isn't sitting on compacted, waterlogged ground — loosening the soil beneath helps roots and drainage. Most beds are open-bottomed onto soil, which is ideal. Lining: line the sides of wooden beds with permeable membrane to protect the timber and prolong its life, but leave the bottom open to the soil below for drainage and worm access (unless on a hard surface, where you need a different approach with drainage holes). Settling: fresh fill settles over the first weeks and season as it compacts and organic matter decomposes — fill generously and expect to top up. Annual care: top up with compost each year, as organic matter is consumed; mulch to retain moisture and suppress weeds. Watering: raised beds dry out faster than open ground (more exposed surface and better drainage), so they need attentive watering in dry spells, especially when newly planted. This calculator gives the fill volume, bags, and cost so you can plan and budget the project accurately from the start.

What do I need to know about Soil Settling and Topping Up Over Time?

Freshly filled raised beds settle noticeably in their first season as organic material breaks down, air pockets compress under watering, and the mix consolidates — it's normal to see the fill level drop by 10-20% within the first few months, more if the mix is heavy on compost or uncomposted organic matter. This is why it's worth slightly overfilling a new bed (to just below the rim) rather than filling exactly level, since a bed that starts level often ends up noticeably sunken once watered in. Beyond the initial settling, raised beds lose volume every year as organic matter continues to decompose and as plants take up nutrients and structure from the soil, so an annual top-up of a few centimetres of fresh compost or soil improver is standard practice rather than a sign something went wrong. Beds growing hungry crops (brassicas, squash, tomatoes) deplete and compact faster than lightly-used beds, and no-dig growers in particular rely on this annual top-up as their main method of replenishing fertility, rather than digging the existing soil over. Budgeting for roughly 2-5cm of annual top-up material, depending on bed depth and what's grown, avoids an unpleasant surprise each spring when the bed looks emptier than expected.

What should I know about Lining Raised Beds for Drainage and Longevity?

Whether and how to line a raised bed depends on its construction material and what's beneath it. Beds built on existing soil generally shouldn't have an impermeable liner on the base at all, since blocking drainage traps water and causes waterlogging and root rot — a permeable weed-suppressant membrane, or no liner at all, is usually right if the bed sits on soil or lawn. Beds built on a hard surface (patio, decking, concrete) need either drainage holes built into the design or a false, perforated base layer (coarse gravel or crocks) so excess water can escape rather than pooling and souring the soil above. Where lining matters most is on the sides: beds built from untreated softwood, or railway sleepers reclaimed with old creosote-based treatments, benefit from a plastic or butyl liner between the timber and the soil, which slows timber rot by keeping constantly wet soil off the wood, and prevents any residual chemical treatment from leaching into the growing medium — this is particularly important for edible crops. Avoid fully sealing a bed in liner on all sides and base, since good airflow and drainage through at least some of the structure is essential for healthy root growth.

Soil Mix Ratios by Raised Bed Use

The ideal soil blend for a raised bed depends on what's being grown, since vegetables, flowers and fruiting plants each have somewhat different drainage and nutrient needs.

These ratios are starting points rather than fixed rules — heavy clay topsoil benefits from extra grit or perlite regardless of the bed's purpose, and sandy topsoil benefits from extra compost to improve water retention. Testing drainage by watering a filled bed and watching how quickly it drains is a simple practical check that a soil mix, whatever the exact ratio, is actually suited to the plants going into it.