Compost Calculator UK

How Much Compost Do I Need?

So the new bed's built, the compost bags are stacked at the garden centre, and the label assumes you already know how many to load into the trolley.

By Gary Hodson · 15+ years UK gardening experience · Rates cross-referenced against RHS, Met Office and UK industry standards · Last reviewed 23 April 2026

Every spring in UK gardens the same scene plays out. A raised bed goes in over a weekend, and then you stand in front of a pallet of 50-litre compost bags with no idea whether you need four of them or fifteen. At £6 to £9 a bag in 2026, the wrong guess either means a second trip you didn't plan for or a stack sitting in the rain until October, slowly losing everything that made it worth buying.

The compost calculator at the top of this page removes the guesswork. Enter the bed's length and width (or radius for a round bed, or estimated square metres for an awkward border), set the depth in centimetres, and it returns the exact volume in litres and cubic metres plus the bag count for every common pack size — 40 L, 50 L, 60 L, 100 L and 1 m³ bulk. The rest of this page is the context you need to trust the number, and to avoid the compost mistakes that cost UK gardeners the most money every year.

Depth is where most people get it wrong

The single biggest source of over- and under-buying isn't area — it's depth. Two jobs use compost at very different depths. Mulching an established border to lock in moisture and feed the soil over winter wants 5 to 10 cm on top, which works out at 50 to 100 litres per square metre. Filling a new raised bed from empty wants the full growing depth — 20 to 30 cm if you're growing root veg, which is 200 to 300 litres per square metre. Three to six times more for a fill than a mulch, and it's the number most people halve in their head without realising.

The quick rule: 1 cm of compost depth over 1 m² equals 10 litres. A 2 m × 1 m raised bed at 25 cm needs 2 × 1 × 25 × 10 = 500 litres, which is ten 50-litre bags or half a bulk bag. Once the figure's over roughly 400 litres, a bulk bag (1 m³ = 1,000 litres of compost) nearly always beats stacking bags — around £80 delivered in most of England for what would cost £140 in 20 × 50 L bags, and you skip the plastic. Under 400 litres, bags are usually the better call because bulk delivery isn't worth paying for when you only need half the load.

Multi-purpose or specialist — what fits your job

Ninety percent of UK garden centres lead with multi-purpose compost, and for most jobs it's the right choice — raised beds, containers, topping up borders, potting on seedlings. It's a middle-ground product: enough nutrients for four to six weeks of growth, a reasonable peat-free fibre mix, and drainage that won't drown most plants. The catch is that "multi-purpose" is a broad label — the £2.99 value bag and the £9.99 premium bag can both wear it. The cheap ones are often lumpy, under-sieved, and stuffed with bark that hasn't fully broken down.

For specialist jobs, spend the extra. Seed compost is fine-textured and low-nutrient — seedlings need to grow roots looking for food, not be drowned in it. Ericaceous is the only option for acid-loving plants like blueberries, rhododendrons, camellias and azaleas; multi-purpose is too alkaline and the leaves yellow within a season. John Innes No. 2 or No. 3 is a loam-based mix for anything staying in a container over a year — it holds nutrients longer and doesn't slump the way peat-free multi-purpose does when it dries out. The 2026 regulatory reality: retail sales of peat-containing compost were phased out in England across 2024, so almost everything on the shelf now is peat-free. Quality varies wildly. Look for visible wood-fibre structure rather than a brown sludge — the first performs like a soil, the second compacts within a month.

Compost vs topsoil — they do different jobs

This is the confusion that costs the most money. Compost and topsoil are not interchangeable. Compost goes on top, or fills the top third. It's nutrient-rich, light, and designed to feed plants and improve soil structure over one to two growing seasons. Topsoil fills depth. It's heavier, cheaper by volume, holds structure long-term, and is what you want making up the bulk of a deep raised bed — everything below the top 15 to 20 cm. Filling a 30 cm bed entirely with multi-purpose compost isn't just expensive at roughly £140 per square metre — it also collapses over the first winter as the organic matter breaks down, and by spring your bed sits 10 cm lower than you built it.

The proper ratio for a new raised bed is 30 to 40% compost on top, 60 to 70% topsoil underneath. For a 2 m × 1 m bed at 25 cm (500 litres total), that's roughly 150 litres on top of 350 litres of screened topsoil — three 50 L bags over a bulk bag of soil. Our raised bed calculator works out the volume split properly, and the soil calculator handles the topsoil side on its own.

Common problems — and what they actually mean

Most complaints come down to three things, and none are really the fault of the bag. Too-wet compost is the commonest — you open it and it's a sodden, anaerobic mass that smells sour. That's almost always storage damage: bags stacked outdoors through a wet spring will waterlog even through intact plastic. Tip it out onto a tarp, spread it 10 cm deep, leave it a dry day or two to air. Nine times out of ten it's fine once oxygen gets back into it. If it still smells like pond bottom after airing, it's genuinely gone and the retailer should replace it.

Second is heat in a fresh heap — homemade or freshly delivered bulk still warm to the touch. That's active decomposition, which is what you want, but it's not ready to plant into. Using it hot around existing plants burns roots; using it to fill a new bed steals nitrogen from the soil as the last of the wood fibre finishes breaking down. Let it sit two to four weeks until ambient before planting. Third is the bag that's really just bark — a cheap multi-purpose where you can see fist-sized chunks of undigested chip. It won't support seedlings or most container plants, but it'll still work spread 5 cm thick around shrubs — use our mulch calculator to see how far that bag goes as a top-dressing.

Once you've got the depth right, the right product for the job, and the ratio worked out, head back to the calculator at the top of this page. Buying the right amount once beats three trips and a pile of wet bags every time.

Compost Bag Sizes & Coverage

UK garden centres sell compost in several standard bag sizes. Here's how much area each covers at different depths.

Tips for Buying Compost

Raised beds

For new raised beds, a 60/40 mix of multi-purpose compost and topsoil is ideal. It gives good drainage while retaining enough moisture and nutrients. Pure compost shrinks as it breaks down, so expect to top up after the first season.

Topping up borders

A 5cm layer of compost spread over existing borders in spring acts as a mulch and slow-release feed. Fork it lightly into the top few centimetres rather than burying it deep — worms will do the rest.

Saving money on large areas

If you need more than 500 litres, a bulk bag (1m³ = 1,000 litres) is usually 50–60% cheaper per litre than individual bags. Many garden centres and builders' merchants deliver bulk bags for free over a certain order value.

Types of Compost and When to Use Each

I get asked about this constantly, so I have put together a proper breakdown. Not all compost is the same, and using the wrong type is one of the most common mistakes I see gardeners make. There are at least 6 distinct types you will come across in any UK garden centre, and each one is formulated for a specific job. Picking the right one can save you 20-30% on wasted product and give your plants a noticeably better start.

Multi-Purpose Compost

This is the workhorse of UK gardening and accounts for roughly 70% of all compost sold. Multi-purpose compost is a blend of composted bark, green waste and sometimes coir, with added nutrients to support 4-6 weeks of plant growth. It works for containers, hanging baskets, raised beds and borders. I use it for almost everything in my garden and typically pay between 4 and 8 pounds per 40-50 litre bag. The one drawback is that it is quite light and can dry out quickly in terracotta pots, so I often mix in 10-15% perlite for containers that sit in full sun.

Ericaceous Compost

If you are growing blueberries, azaleas, rhododendrons, camellias or heathers, you need ericaceous compost. It has a pH of 4.0-5.5, which is significantly more acidic than standard multi-purpose compost at pH 6.0-6.5. Most UK garden soil is neutral to slightly alkaline (pH 6.5-7.5), so acid-loving plants will struggle without this specialist mix. Expect to pay 6-9 pounds per 50 litre bag. I recommend testing your soil pH before buying — you might find your garden is naturally acidic enough, especially if you live in areas with sandy or peaty soil like parts of Scotland, Wales or the New Forest.

Soil Improver and Composted Bark

Soil improver is coarser than multi-purpose compost and designed to be dug into existing garden soil rather than used on its own. It improves clay soil by breaking up heavy particles and helps sandy soil retain moisture. Composted bark serves a similar purpose but is even chunkier. Both cost around 3-5 pounds per 50 litre bag, making them the cheapest option by volume. I use soil improver every autumn on my vegetable beds — I spread 5cm over the surface and let the worms drag it down over winter. If your soil needs serious work, our topsoil calculator can help you work out quantities for a complete bed overhaul.

Peat-Free Compost

Since the UK government announced plans to ban peat-based compost for amateur gardeners (the retail ban took effect in 2024), peat-free options have improved enormously. Modern peat-free composts use a blend of composted bark, coir (coconut fibre), wood fibre and green waste. They cost roughly 10-20% more than peat-based equivalents — typically 5-9 pounds per 40-50 litre bag. The main thing to watch is watering: peat-free compost can be harder to re-wet once it dries out completely, so I recommend watering little and often rather than letting it go bone dry between waterings.

John Innes Composts (No. 1, No. 2 and No. 3)

John Innes is not a brand — it is a formula developed at the John Innes Horticultural Institute in the 1930s. These loam-based composts are heavier than multi-purpose (about 50% heavier per litre), which makes them ideal for larger pots that might blow over in the wind. John Innes No. 1 is for seeds and cuttings, No. 2 is for potting on young plants, and No. 3 is for mature plants, shrubs and trees. The loam content gives them excellent nutrient retention — No. 3 feeds for up to 8-10 weeks without additional fertiliser. They cost 6-10 pounds per 25-30 litre bag, making them the most expensive option per litre, but for long-term container planting they are worth every penny.

How Much Compost for Raised Beds

Raised beds are by far the most common reason people reach for this calculator, so I have put together the exact numbers for every standard UK raised bed size. The formula is simple: length (m) x width (m) x depth (m) x 1,000 = litres. But because most people build to standard timber lengths, I have done the maths for the 8 most popular sizes below.

A typical raised vegetable bed is 30cm (12 inches) deep. That is enough for salad leaves, herbs, beans, courgettes and most root vegetables. If you want to grow deep-rooted crops like parsnips or long carrots, go for 45cm (18 inches). I have personally built 14 raised beds over the years and 30cm is the sweet spot for 90% of what you will grow.

How to Fill Deep Raised Beds Cheaply

If your raised bed is deeper than 30cm, do not fill the entire thing with compost. It is a waste of money and your plants will not use the bottom 30cm anyway. I use a layered approach called Hugelkultur that cuts compost costs by 40-60% on deep beds. Start with a 10-15cm layer of cardboard at the very bottom — this suppresses weeds and breaks down over 12-18 months. On top of that, add a 15-20cm layer of rough woody material: pruned branches, wood chip, old logs. These decompose slowly and create air pockets that improve drainage.

Fill the next layer with any garden waste you have — grass clippings, dead leaves, spent annual plants. This adds another 10-15cm of volume at zero cost. Only the top 15-20cm needs to be quality compost or a compost-topsoil mix. For a 60cm deep raised bed measuring 2.4m x 1.2m, this method uses roughly 350-430 litres of compost instead of 1,728 litres — saving you between 60 and 80 pounds. The bed will settle by about 5-8cm over the first season, so add a 5cm top-up of compost each autumn. Need to work out the soil element of your mix? Use our topsoil calculator for the exact amount.

Compost Cost Guide UK 2026

Compost prices have risen roughly 15-20% since 2023, largely because of the transition to peat-free formulations. I track prices across the major UK retailers throughout the season and these are the realistic ranges you should budget for in 2026. The golden rule: the more you buy, the cheaper it gets per litre.

Bagged Compost Prices

Budget multi-purpose compost (supermarket own-brand or value ranges from B&Q and Wickes) costs 4-6 pounds per 50 litre bag. Mid-range branded compost like Westland or Miracle-Gro runs 7-10 pounds per 40-50 litre bag. Premium peat-free or specialist compost (ericaceous, John Innes) sits at 8-12 pounds per 25-50 litre bag. Watch for multi-buy deals — B&Q frequently run 3-for-2 offers on Verve compost in spring, and Amazon often discount Westland 60L bags when you buy 3 or more.

Bulk Compost Prices

Bulk bags (also called dumpy bags or builders' bags) contain 1 cubic metre — that is 1,000 litres. They typically cost 50-80 pounds delivered, which works out at 5-8p per litre. Compare that to bagged compost at 10-20p per litre and you can see why I always recommend bulk for any job over 500 litres. Local council green waste composting sites are the cheapest source of all — many sell composted green waste at 25-40 pounds per cubic metre for collection. The quality varies, but for filling raised beds and conditioning soil it is perfectly good.

Where to Buy Compost Cheaply

The cheapest compost in the UK comes from council green waste recycling centres. Many local authorities sell composted garden waste at 25-40 pounds per cubic metre — roughly half the price of retail bulk bags. Check your council website or ring them directly. Aldi and Lidl run seasonal compost deals from February to May, with 40 litre bags often under 3 pounds. B&Q's own-brand Verve compost is consistently one of the best value options on the high street at around 5 pounds per 40 litres. For bulk delivery, search for local landscape suppliers — they often undercut national garden centres by 15-25% and many offer free delivery within a 10-15 mile radius.

How to Improve Garden Soil with Compost

Adding compost is the single most effective thing you can do to improve any garden soil, and I say that after 15 years of working on everything from heavy London clay to thin chalky soil in the Chilterns. Compost adds organic matter, improves structure, feeds soil organisms and increases the water-holding capacity of light soils while opening up heavy ones. The key is knowing when to add it, how much to use and whether to dig it in or leave it on the surface.

When to Add Compost

The two best times to add compost are autumn (October-November) and early spring (February-March). In autumn, I spread a 5-8cm layer over empty vegetable beds and let winter rain wash nutrients into the soil. Earthworms and frost do the mixing for you — no digging required. In spring, add compost 2-4 weeks before planting to give it time to settle and start releasing nutrients. Avoid adding compost to waterlogged soil as it will just sit on the surface and form a crust. If your soil is still soggy in March, wait until it has dried enough that a handful crumbles when squeezed rather than forming a sticky ball.

How Much Compost Per Square Metre

The amount depends entirely on your soil condition and what you are growing. For heavy clay soil that needs serious improvement, apply a 5-10cm layer (50-100 litres per square metre) and dig it into the top 20-30cm. For established borders that just need a yearly top-up, a 3-5cm layer (30-50 litres per square metre) spread on the surface as a mulch is enough. For lawn renovation alongside fertiliser, use just 1-2cm (10-20 litres per square metre) raked into aerated turf. If you are improving a large area, our calculator at the top of this page will give you the exact volume — just enter the total area and your desired depth.

Digging In vs Top-Dressing

There are two schools of thought here, and I have tried both extensively. Digging compost into the top 20-30cm of soil gives faster results — the organic matter is immediately in contact with plant roots and soil organisms. I recommend this approach for new beds, heavily compacted soil and anywhere you are planting for the first time. Top-dressing (spreading compost on the surface and letting worms pull it down) is gentler on soil structure and better for established beds. It also acts as a mulch, suppressing weeds and reducing water loss by up to 70%. Most no-dig gardeners use this method exclusively, applying a 5cm layer every autumn.

Peat-Free Guidance for 2026

The UK retail ban on peat-based compost for amateur gardeners came into effect in 2024, and by 2026 the vast majority of compost on garden centre shelves is peat-free. The best peat-free alternatives I have used are those based on composted bark and wood fibre (like Sylvagrow) or coir-based mixes (like Coco and Coir). The main adjustment you need to make is watering: peat-free composts drain faster than peat-based ones, so water more frequently but in smaller amounts. They are also slightly lower in nutrients, so I add a slow-release fertiliser granule at planting time — about 3-4 grams per litre of compost. If you are mixing compost with topsoil for raised beds, a 50/50 blend of peat-free compost and quality topsoil gives excellent results and is cheaper than using compost alone.

Common Compost Mistakes to Avoid

I have made every single one of these mistakes at some point, so learn from my errors. Each one can set your plants back by weeks or cost you money that did not need to be spent. Here are the 5 most common compost mistakes I see UK gardeners making, along with the simple fixes.

1. Using Homemade Compost Too Fresh

Fresh garden compost that has not finished decomposing will rob nitrogen from your soil as it continues to break down, and the heat it generates can burn plant roots. Properly finished compost should be dark brown, crumbly and smell earthy — like a forest floor. If it still smells of ammonia or you can identify original materials (cabbage leaves, grass clippings), it needs another 2-4 months. I always let my homemade compost mature for a minimum of 6 months before using it on beds. If you need compost now and yours is not ready, buy bagged compost and save your homemade batch for autumn top-dressing.

2. Filling an Entire Raised Bed with Pure Compost

Pure compost is too rich for most plants and settles dramatically — a raised bed filled entirely with compost will sink by 25-30% in the first year as the organic matter continues to decompose. That 30cm bed becomes a 21cm bed by September. Worse, pure compost holds too much moisture and can become waterlogged. I recommend a 60/40 mix of multi-purpose compost and topsoil for new raised beds. The topsoil adds mineral content, improves drainage and reduces settling to around 10-15%. Use our soil calculator to work out the topsoil portion.

3. Ignoring Drainage at the Bottom of Containers

Filling a pot or raised bed with compost right to the bottom without any drainage layer is asking for waterlogged roots and root rot. All containers need drainage holes, and I add a 2-3cm layer of broken crocks, gravel or perlite at the bottom of any pot deeper than 20cm. For raised beds on soil, drainage usually takes care of itself, but if you are building on concrete, paving or heavy clay, add a 5-10cm layer of gravel or rubble before the compost goes in. This single step prevents about 80% of the root rot problems I get asked about.

4. Using the Wrong Compost for Acid-Loving Plants

Standard multi-purpose compost has a pH of 6.0-6.5, which is fine for most plants but will slowly kill acid-lovers like blueberries (which need pH 4.5-5.5), azaleas, rhododendrons and pieris. I have seen people spend 30 pounds on a beautiful blueberry bush, plant it in standard compost and wonder why the leaves turn yellow within 8 weeks — that is chlorosis caused by the wrong pH locking out iron. Always use ericaceous compost for these plants, and top-dress with it annually. If you are growing blueberries in raised beds, fill the entire bed with ericaceous compost mixed with 20% pine bark for extra acidity and drainage.

5. Storing Compost Bags in Direct Sunlight

UV light degrades the plastic bags, and the heat inside a sealed bag in direct sun can reach 50-60 degrees Celsius — enough to sterilise the beneficial microorganisms in the compost and break down the slow-release nutrients prematurely. I have measured temperatures of 55 degrees inside black compost bags left on a south-facing patio in July. Store unopened bags in a shaded spot, ideally under cover. Once opened, transfer leftover compost to a lidded bin or cover the bag with a tarpaulin. Properly stored, bagged compost stays in good condition for 12-18 months. Badly stored compost can become unusable in as little as 6-8 weeks.

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