Raised Bed Calculator

How Much Soil Do I Need for a Raised Bed?

Working out how much soil you need for a raised bed is straightforward: multiply the length by the width by the height to get the volume. But most people underestimate the quantity, which means a frustrating trip back to the garden centre mid-project. Our raised bed calculator does the maths for you and adds the recommended 60/30/10 soil mix breakdown so you know exactly what to buy.

The single biggest factor is the depth of your bed. A standard 4x8ft raised bed at 30cm deep needs 864 litres of soil — but the same bed at just 15cm deep needs only 432 litres. Getting the depth right for what you want to grow is critical, and we cover that in the depth guide below.

Here are the exact quantities for the most common raised bed sizes in the UK, calculated at 30cm (12 inches) deep:

Money-saving tip: For beds needing over 850 litres, a bulk bag of topsoil (approximately 850 litres) is typically three times cheaper per litre than individual bags from a garden centre. Most builders merchants deliver for £40–80 including the bag.

Need to calculate soil for non-raised-bed projects? Our topsoil calculator handles borders, lawns and general garden beds. For mulching around your raised beds, use the mulch calculator.

Common Raised Bed Sizes — How Much Soil Do You Need?

Below is a quick-reference table for the most popular raised bed dimensions in the UK. All figures assume a standard 30cm (12-inch) fill depth and include the number of 40-litre bags you would need if buying bagged soil from a garden centre. Costs are based on typical 2026 UK prices of £3–5 per 40-litre bag.

Notice that doubling the depth from 1ft (30cm) to 2ft (60cm) doubles the volume — and the cost. That is why choosing the right depth for what you plan to grow is so important. A 6ft × 3ft bed at 2ft deep needs 972 litres, which tips past the point where a bulk bag (approximately 850 litres for £50–70 delivered) becomes significantly cheaper than buying individual bags.

If your bed dimensions are not listed here, enter your exact measurements into the raised bed calculator above for a precise figure including the 60/30/10 soil mix breakdown.

Raised Bed Soil Calculator — Step by Step

Let us walk through a real example so you can see exactly how the numbers work. Suppose you have a standard raised bed measuring 1.8m long × 1.2m wide × 0.3m deep (that is 6ft × 4ft × 1ft in old money).

Step 1: Calculate the total volume

Multiply length × width × depth:

1.8m × 1.2m × 0.3m = 0.648 m³

Convert to litres (multiply by 1,000):

0.648 × 1,000 = 648 litres

Step 2: Apply the 60/30/10 mix

Using the recommended soil mix ratio, split the 648 litres into three components:

• Topsoil (60%): 648 × 0.6 = 389 litres — that is 10 bags of 40-litre topsoil. Use screened, BS 3882 certified topsoil for the best results. Our topsoil calculator can help if you are buying topsoil for other projects at the same time.
• Compost (30%): 648 × 0.3 = 194 litres — 5 bags of 40-litre compost. Well-rotted garden compost, mushroom compost or peat-free multi-purpose all work well. Use our compost calculator if you need compost for multiple beds.
• Perlite or grit (10%): 648 × 0.1 = 65 litres — 7 bags of 10-litre horticultural perlite, or roughly two 25kg bags of horticultural grit.

Step 3: Estimate the cost

At typical UK garden centre prices (2026), 40-litre bags of topsoil and compost cost £3–5 each. Perlite is roughly £5–8 per 10-litre bag. For our 648-litre bed:

• Topsoil: 10 bags × £4 average = £40
• Compost: 5 bags × £4 average = £20
• Perlite: 7 bags × £6.50 average = £46
• Total: approximately £106

Bulk-buy alternative: One bulk bag of topsoil (850L, ~£60 delivered) covers your topsoil needs with litres to spare, and one bulk bag of compost (~£50 delivered) handles the compost portion. That brings your topsoil and compost cost down to around £110 total — and you will have material left over for topping up next year.

Best Soil Mix for Raised Beds

The soil mix you use in a raised bed makes a bigger difference to plant health and yield than almost any other factor. After years of testing different blends in UK conditions, the 60/30/10 ratio consistently produces the best results for vegetables, herbs and flowers.

The 60/30/10 ratio explained

• 60% quality topsoil — provides the bulk, structure and weight that keeps your bed stable. Topsoil also contains the mineral content and beneficial organisms that plants need. Choose screened topsoil (ideally BS 3882 certified) to avoid stones and weed seeds.
• 30% garden compost — adds the nutrient density and water-holding capacity that topsoil alone lacks. Well-rotted garden compost, mushroom compost or peat-free multi-purpose compost all work well. The compost is what feeds your plants through the growing season.
• 10% perlite or horticultural grit — this is the ingredient most people skip, and it makes a real difference. Perlite (or coarse horticultural grit) creates permanent drainage channels throughout the mix. Without it, the soil can become waterlogged after heavy rain, leading to root rot — especially over a British winter.

Why not 100% compost?

Pure compost seems like the obvious choice — it is nutrient-rich and easy to work with. But compost shrinks dramatically as it decomposes further. Expect to lose 30–40% of the volume in the first year alone. Your 30cm-deep bed becomes an 18cm-deep bed by autumn. Compost also holds too much moisture in wet weather and can become hydrophobic (water-repellent) when it dries out completely. The topsoil in the 60/30/10 mix provides stability and prevents both problems.

Alternative mixes for specific plants

Use the calculator above to work out your total volume, then split the order according to the mix ratio you need. Our compost calculator can help you work out the compost portion separately if you are buying from different suppliers.

Raised Bed Depth Guide

How deep your raised bed needs to be depends entirely on what you plan to grow. Building too shallow wastes your effort — roots hit the base and plants suffer. Building too deep wastes money on soil you do not need. Here is the definitive guide for UK growing conditions.

Our recommendation: If you are building a general-purpose raised bed and want to grow a mix of vegetables, 30cm (12 inches) is the sweet spot. It is deep enough for almost everything except the longest root vegetables and deep-rooted fruit, and it keeps the soil cost reasonable. Remember that soil settles by 10–15% after the first few weeks of rain, so fill slightly above your target depth.

What Depth Raised Bed for Different Plants?

Choosing the right depth saves you money on soil and gives your plants exactly what they need. Here is a detailed breakdown by plant type, based on UK growing conditions where the bed sits on open ground (roots can extend below the bed):

• Salad leaves, herbs and spring onions (15–20cm minimum): Shallow-rooted crops like lettuce, rocket, radishes, chives and basil thrive in just 15cm of soil. A 20cm bed gives them a little extra room and reduces how often you need to water in summer. These are perfect for windowsill-height beds and balcony planters.
• Carrots, parsnips and beetroot (30cm minimum): Root vegetables need depth for straight, unforked growth. Carrots in particular will bend, fork or stop growing when they hit compacted soil or the base of a shallow bed. For long varieties like Autumn King, aim for 40cm. Short-root varieties such as Chantenay or Paris Market can manage in 25cm.
• Potatoes, courgettes and runner beans (45cm minimum): Heavy feeders with extensive root systems need deep, rich soil. Potatoes especially benefit from the extra depth because you can earth them up within the bed as they grow. Courgettes and squash send roots outward as well as downward, so width matters as much as depth for these crops.
• Fruit bushes and perennial herbs (60cm+): Blueberries, gooseberries, currants, rosemary, sage and other perennials stay in the same spot for years. Their root systems expand annually and need generous depth to establish properly. A 60cm bed also provides better frost insulation for roots over winter — important for borderline-hardy plants in northern UK gardens.

Do not forget the drainage layer. If you are adding a 5–10cm layer of gravel or broken crocks at the base of your raised bed (recommended for beds on hard surfaces or in clay-heavy gardens), that layer reduces your usable soil depth. A 30cm bed with a 5cm gravel layer only has 25cm of growing depth. Factor this in when choosing your bed height, and consider building 5–10cm taller to compensate.

Raised beds on hard surfaces

If your raised bed sits on concrete, paving or a patio, you need to add extra depth because roots cannot grow down into the ground beneath. Add at least 15cm to the minimum depths in the table above. A bed on concrete for general vegetables should be at least 40cm deep, and for root vegetables, 50cm or more. Make sure there are drainage holes in the base or a layer of coarse gravel to prevent waterlogging.

How to Fill a Raised Bed on a Budget

Filling a raised bed with quality soil is the single biggest cost of any raised bed project. A standard 4x8ft bed at 30cm deep needs over 860 litres — that is £70–110 in bagged soil. Here are proven ways to reduce that cost by 30–50% without compromising on growing performance.

The Hügelkultur method

Hügelkultur (German for "mound culture") is the most effective way to reduce the amount of bought soil you need. The method is simple: fill the bottom third of your raised bed with logs, thick branches and other woody garden waste. As the wood slowly decomposes over 2–5 years, it releases nutrients, retains moisture like a sponge, and creates air pockets that roots love.

Here is how to layer a raised bed using the Hügelkultur approach:

1. Bottom layer (10–15cm): Large logs and thick branches. Hardwood lasts longer; softwood breaks down faster and gives quicker nutrient release.
2. Second layer (5–10cm): Smaller sticks, twigs, wood chips and bark chippings. Use our bark chippings calculator if buying material for this layer.
3. Third layer (5cm): Leaves, grass clippings, straw, cardboard or newspaper. This provides carbon and prevents the soil above from falling into gaps between the wood.
4. Top layer (15–20cm): Your quality soil mix (60% topsoil, 30% compost, 10% perlite). This is where the plants actually root.

Using this method, a 30cm-deep bed only needs 15–20cm of actual soil mix — cutting your soil cost nearly in half. The bed will settle over the first year as the lower layers compress, so top up with a 5cm layer of compost each autumn.

Filling a Raised Bed Cheaply — Lasagne Method

The lasagne method (also called sheet mulching) is a variation of Hügelkultur that works especially well for beds 45cm or deeper. Instead of large logs at the base, you build up alternating layers of "green" nitrogen-rich material and "brown" carbon-rich material — like layering a lasagne.

Here is how to fill a deep raised bed using the lasagne approach:

1. Bottom third — free bulky materials: Start with a layer of cardboard or thick newspaper (remove any tape or staples). On top of that, add logs, thick branches, and any woody prunings from the garden. This layer costs nothing if you use garden waste and cardboard boxes you already have. It creates air pockets, improves drainage and slowly releases nutrients as it breaks down over 2–4 years.
2. Middle third — green and brown layers: Alternate layers of grass clippings, vegetable kitchen scraps and fresh leaves (green/nitrogen) with layers of dried leaves, straw, shredded paper and small twigs (brown/carbon). Each layer should be roughly 5–10cm thick. This section acts like a slow-release compost factory inside your bed, feeding the plants above for seasons to come.
3. Top third — quality topsoil and compost mix: Fill the top 15–20cm with your 60/30/10 soil mix (60% topsoil, 30% compost, 10% perlite or grit). This is the active root zone where seeds germinate and plants establish. Quality matters here — do not skimp on the top layer even if you are saving money on the lower layers.

The lasagne method can save 30–50% on soil costs for deep beds (45cm+), because you are replacing expensive bought soil with free garden and household waste. A 60cm-deep bed built this way only needs around 20cm of actual soil mix — the rest is free material that decomposes into rich, fertile growing medium over time. The bed will settle by 10–15cm in the first season as the lower layers compress, so build slightly higher than your target depth and top up with compost each autumn.

Other budget tips

• Buy bulk bags over small bags. A bulk bag of topsoil (850 litres) costs £50–70 delivered. The same volume in 40-litre bags from a garden centre would cost £65–105. The saving is even bigger for compost.
• Make your own compost. If you have 6–12 months lead time, start composting kitchen and garden waste now. A well-managed compost bin produces 200–400 litres of compost per year — free soil that is richer than anything you can buy.
• Ask neighbours and allotment holders. Many gardeners have more compost than they can use. Allotment sites often have communal compost heaps. Free, well-rotted compost is worth its weight in gold.
• Use council green waste compost. Many UK councils sell composted green waste at £5–15 per bulk bag — far cheaper than garden centre compost. Check your local council website for collection points.

Frequently Asked Questions

Best Raised Bed Products UK 2026

Whether you are building a new raised bed or filling an existing one, these are the top-rated products available in the UK right now.

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