Enter your bed dimensions and choose a soil mix recipe to get the exact volumes — in litres, cubic feet and number of bags — so you never over-order or run short.
Length, width and depth in metres or feet. Add multiple beds with the quantity field.
Pick from Mel's Mix, a standard blend, all compost, or enter your own custom ratio.
We convert the total volume to litres, cubic feet and standard bag counts.
Raised beds drain faster than in-ground soil, which is an advantage — but it also means they require a carefully chosen growing medium that holds enough moisture and nutrients while never becoming waterlogged.
Never fill a raised bed with garden soil alone. In-ground soil compacts under its own weight when placed in a confined space, restricting roots and drainage.
The three most popular mixes each have a different strength. Mel's Mix excels for intensive square-foot gardening. A 60/30/10 blend is more affordable at scale. Pure compost suits shallow beds and containers.
| Mix | Composition | Best for |
|---|---|---|
| Mel's Mix | ⅓ compost, ⅓ peat/coir, ⅓ vermiculite | Intensive growing, square-foot gardens |
| Standard Blend | 60% topsoil, 30% compost, 10% grit | Deep beds, most vegetables |
| All Compost | 100% multi-purpose compost | Shallow beds, salads, herbs |
For most vegetables, 30 cm (12 inches) is the recommended minimum. Root vegetables like carrots and parsnips benefit from 45–60 cm. Salads and herbs do fine in 15–20 cm.
Not as the primary ingredient. Garden soil compacts badly in confined spaces. You can use it as 20–30% of a standard blend, but pair it with plenty of compost and grit to maintain drainage.
A standard bulk bag (1 cubic metre) fills roughly 1,000 litres. Most garden compost bags are 40 or 60 litres. Use the calculator above to find out exactly how many you need.
Raised bed soil shrinks by 10–20% each year as organic matter breaks down. Top up with 5–10 cm of compost each spring before planting. This also refreshes nutrients for the new season.
It helps enormously with moisture retention and aeration, but it is expensive. Perlite is a cheaper alternative. If cost is a concern, replace the vermiculite portion with a 50/50 blend of perlite and extra compost.
The moment you lift soil off the ground and put it inside a wooden frame, its behaviour changes completely. In-ground soil relies on a vast network of microbes, earthworms, and underground moisture reserves to stay healthy. Raised bed soil is a smaller, isolated system — and it needs to be designed from scratch to compensate.
The most critical difference is drainage. A raised bed drains from the bottom, so water moves through the entire profile rather than pooling at a hardpan layer. This is excellent for root health, but it also means the soil dries out faster and leaches nutrients more quickly than in-ground beds.
Raised bed soil also needs to be lighter in structure. Roots don't have the full depth of the earth beneath them to explore — they are confined to the depth of your bed. That means every centimetre of soil needs to be easy to penetrate, well-aerated, and rich in organic matter.
Finally, raised beds warm up faster in spring. Dark, exposed compost-rich soil absorbs heat quickly, giving you a head start on the growing season — but only if your soil mix is loose enough to warm evenly rather than holding cold pockets of moisture.
The calculator above handles the maths for you, but understanding the formula helps you double-check figures and adapt for unusual bed shapes.
The formula:
Volume = Length × Width × Depth
Work in metres for a result in cubic metres (m³). Multiply by 1,000 to convert to litres. Multiply by 35.315 to convert to cubic feet.
For a standard 1.2 m × 2.4 m raised bed filled to a depth of 30 cm (0.3 m):
1.2 × 2.4 × 0.3 = 0.864 m³ = 864 litres = roughly 14 × 60L bags of compost.
Always add 10–15% to your order to account for settling. Compost and soil shrink significantly in the first few months as organic matter breaks down and air is expelled. What looks level when freshly filled will drop by several centimetres within a season.
Depth is the most commonly under-estimated dimension of raised bed design. The right depth depends entirely on what you plan to grow.
| Depth | Crops suited | Notes |
|---|---|---|
| 15–20 cm (6–8") | Lettuce, herbs, strawberries, shallow annuals | Good for ground-level placement on existing soil |
| 30 cm (12") | Most vegetables — tomatoes, peppers, beans, brassicas | Recommended minimum for a productive bed |
| 45 cm (18") | Carrots, parsnips, deep-rooted brassicas, courgettes | More soil volume means better moisture retention |
| 60 cm (24") | Long-rooted varieties, permanent crops, asparagus | Overkill for annuals; ideal for perennials |
If you're placing a raised bed directly on the ground (not on paving), roots can extend downward into the native soil below. In this case, the raised depth matters less — 20–25 cm of quality growing medium on top of loosened ground works well for most crops.
Topsoil forms the mineral backbone of your mix. It provides weight and structure, holds nutrients between waterings, and anchors deep-rooted plants. Choose screened, weed-free topsoil rather than unprocessed garden soil. Quality varies enormously — aim for topsoil with organic matter content above 3%.
Compost is the life of your soil. It feeds soil microbes, releases nutrients slowly as it breaks down, improves drainage in heavy soils and moisture retention in sandy soils, and makes the whole mixture easier for roots to navigate. Use well-rotted multi-purpose compost, green waste compost, or your own home compost. The more diverse the compost sources, the better.
Both are volcanic minerals that create air pockets in the soil, improving drainage and aeration. Perlite is cheaper and better for drainage. Vermiculite holds more moisture and nutrients, making it the better choice for Mel's Mix and intensive growing. Neither breaks down over time, so they remain effective indefinitely.
Added in small proportions (around 10%), grit keeps a topsoil-heavy mix from compacting over time. It is particularly useful in deep beds where the weight of the upper layers tends to press the lower soil together. Use horticultural grit rather than builder's sand — builder's sand contains fine particles that can make drainage worse.
Coir is often used as a sustainable alternative to peat in Mel's Mix and other recipes. It has excellent moisture-holding properties and a neutral pH, and is a renewable resource unlike peat. Coir bricks expand dramatically when wetted — a 650g brick expands to around 9 litres of growing medium.
Raised bed soil depletes and shrinks over time. By the end of a growing season, your bed may have dropped 5–10 cm from its original level, and much of the available nitrogen and phosphorus will have been taken up by plants or washed through by rain.
The simplest annual maintenance routine is to top-dress with 5–8 cm of fresh compost each spring, before planting. Work it gently into the top layer with a hand fork. This restores the volume, feeds soil microbes as it breaks down, and introduces a fresh supply of slow-release nutrients.
Every 2–3 years, do a deeper refresh. Remove the top 15 cm of old soil, loosen the lower layer with a fork, add a layer of well-rotted manure or compost at the base, then refill with a fresh mix. This prevents the lower layers from becoming compacted and anaerobic over time.
If you grow nitrogen-hungry crops like brassicas or sweetcorn, add a slow-release organic fertiliser (such as blood, fish and bone) to the refresh compost. For beds that previously grew legumes, you can often skip the added nitrogen — legumes fix their own and leave residual nitrogen in the soil for the next crop.
Filling multiple raised beds can be surprisingly expensive if you rely entirely on bagged compost from a garden centre. Here are the most effective ways to reduce your soil costs without compromising quality.
Buy in bulk, not bags
A bulk bag (1 m³) of topsoil or compost typically costs 50–70% less per litre than bagged product. For beds larger than 1 m³ total volume, a bulk delivery almost always makes financial sense.
Use the Hugelkultur base method
Fill the bottom 20–30 cm of a deep bed with logs, branches, straw, and garden waste. As this breaks down, it creates a spongy, nutrient-releasing layer and significantly reduces the volume of purchased soil you need to buy. This works best in beds 45 cm or deeper.
Make your own compost
A well-managed compost bin produces rich, free compost from kitchen and garden waste. Even a small bin produces enough to top-dress one or two beds per year. Add a worm bin for faster processing of vegetable peelings.
Check local council green waste compost
Many local councils sell processed green waste compost at very low prices — sometimes free for residents. Quality varies, but it is generally excellent for raised beds and far cheaper than branded alternatives.
Skip Mel's Mix for large beds
Mel's Mix calls for ⅓ vermiculite, which is expensive at scale. For large raised beds, a 60/30/10 blend of topsoil, compost, and grit achieves very similar growing results at a fraction of the cost. Reserve Mel's Mix for shallow pots and small intensive beds.
⚠️ Soil shrinks dramatically after filling
This is normal. Compost-rich mixes can shrink by 15–25% in the first season as organic matter decomposes and air pockets collapse. Order 20% more than your calculated volume, or plan to top-dress in autumn with extra compost.
⚠️ Water pools on the surface or drains too slowly
Usually a sign of too much clay-based topsoil or compaction in the lower layers. Fork the bed deeply to break up compaction, and work in grit or perlite to open up the structure. Adding worms helps long-term.
⚠️ Plants look yellow and pale despite regular watering
Nutrient depletion is common in the second year without amendments. Feed with a liquid seaweed solution for a quick boost, and top-dress with fresh compost. Consider a slow-release organic fertiliser worked into the top layer before planting.
⚠️ Soil becomes hydrophobic (water beads off rather than soaking in)
Peat-based composts can become water-repellent when they dry out completely. Work the surface with a hand fork and water slowly using a rose head rather than a jet. Adding coir-based compost to the mix in future helps prevent this.
⚠️ Weeds appearing despite using "weed-free" compost
Weed seeds are often introduced via compost that wasn't hot-composted at sufficient temperature. Covering beds with a fine mulch or fleece over winter helps. Flame-weeders are useful for clearing seedling weeds before they establish. Always source compost from reputable suppliers.
You can, but be cautious. Existing garden soil often contains weed seeds, compaction issues, and variable pH. If you want to incorporate it, limit it to no more than 20–25% of your total mix and combine it with plenty of compost and grit to compensate for its structure.
Most vegetables prefer a slightly acidic to neutral pH of 6.0–7.0. Blueberries are an exception — they need a pH of 4.5–5.5 and require a completely separate acidic mix. Test your soil pH with an inexpensive probe or test kit before planting, especially if using unknown compost sources.
If your soil drains quickly, dries out fast, and feels very light and fluffy, it needs more topsoil to add weight and slow drainage. If it stays wet for days after rain, compacts easily when you press it, and feels dense and heavy, it needs more compost and grit to open up the structure.
Biochar is a form of charcoal that improves soil structure and acts as a long-term habitat for beneficial microbes. Research supports modest yield improvements in nutrient-poor soils. For most home gardeners with well-composted beds, the benefit is small — but if you want to invest in long-term soil health, a 5–10% addition is worthwhile.
Yes, with some additions. Spent potting compost from pots and containers has broken down further and lost much of its nutrient content, but it still has good structure. Mix it into your raised bed at 20–30% of the total volume alongside fresh compost and topsoil. Don't use it from plants that had disease problems.
A basic soil test kit (pH, nitrogen, phosphorus, potassium) costs around £10–15 and tells you what your soil needs. For a more detailed analysis, send a sample to a professional soil lab — costs vary but results include specific amendment recommendations. Test in autumn so you can improve the mix before spring planting.
Everything you need to plan and grow a productive raised bed garden.
Once you've sourced your soil mix, bring the bed to life in the app. Design the layout cell by cell, log every planting, and track each crop from sowing to harvest.
Visual bed designer
Design your raised bed layout with a drag-and-drop grid planner
Track every planting
Log what you plant in each cell — variety, date, spacing and notes
Garden Overview
Bird's-eye canvas of all your beds showing every planting at a glance (Pro)
Print bed layout
Export a printable plan of your bed and its plantings (Pro)
Bed history insights
See which soil mixes and crops produced the best results each year (Pro)
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