How Much Compost Do I Need? A Glasgow Gardener's Guide to Raised Beds and Allotments

Quick answer: For a new raised bed, allow 100 litres of compost per square metre for every 10cm of depth — so a standard 2m × 1m bed filled 20cm deep needs around 400 litres, roughly half of one of our 850-litre bulk bags. For topping up an existing allotment plot or border, 3–5cm of compost per square metre is usually enough, which works out at 30–50 litres per square metre.

If you've ever stood in front of a compost order form wondering whether to buy one bulk bag or three, you're not alone. It's the single most common question we get asked at our Chapelton site before a delivery goes out — and getting it wrong in either direction is annoying: order too little and you're paying for a second delivery, order too much and you've got a soggy bulk bag sitting on the drive all winter.

This guide walks through exactly how to work out the volume you need, whether you're filling a new raised bed, topping up an allotment plot, or mulching a border — with real Glasgow-sized examples so you're not doing the maths from scratch.

How do I calculate how much compost I need?

The formula is simple: length (m) × width (m) × depth (m) = volume in cubic metres, then multiply by 1,000 to convert to litres.

So for a bed that's 2m long, 1m wide, and you want to fill it 0.2m (20cm) deep:

2 × 1 × 0.2 = 0.4m³ = 400 litres

A useful shortcut if you don't want to do the maths every time: every 1cm of depth over 1m² takes roughly 10 litres of compost. Multiply that by your area and your target depth and you've got your answer.

We'd always recommend rounding up by 10–15% on top of the raw calculation. Compost settles as it beds in, gets watered, and starts to break down further — a bed that looks perfectly filled on delivery day often needs a top-up a few weeks later if you order to the exact figure.

How deep should compost be in a raised bed?

For a new raised bed being filled from scratch, aim for 15–25cm of compost, ideally blended with topsoil rather than pure compost. For topping up soil that's already in reasonable condition, 5–10cm worked into the top layer is plenty.

Pure 100% compost isn't actually the best fill for a deep raised bed — it holds a lot of moisture, can be too rich for some vegetables, and settles more than a soil blend. Most allotment growers we supply use a 50/50 mix of our Organic Compost and Top Soil (or our ready-blended Topsoil Compost Mix) for the bulk of the bed, then a top layer of pure compost or well-rotted manure for planting.

How much compost does an allotment plot need?

A standard Scottish allotment plot (roughly 250m², sometimes called a "10-rod plot") needs somewhere between 2,500 and 5,000 litres of compost a year if you're dressing the whole plot at a working depth of 3–5cm — though most plot holders only compost the beds actually in use that season, not paths or fallow ground, which brings the realistic figure down considerably.

In practice, that's roughly 3–6 of our 850-litre bulk bags for a full plot dig-in, or fewer if you're rotating sections year to year. If you're only working a few raised beds within a larger plot, use the raised bed formula above instead — it'll be far more accurate than working off the whole plot size.

Bulk bags or tonne loads — which do I need?

Bulk bags (850 litres) suit most home gardens, single raised beds, and smaller allotment plots — anything up to roughly 3–4 bulk bags' worth. Loose tonne loads make more sense once you're ordering the equivalent of 5+ bulk bags, because the price per litre drops and HIAB delivery can tip material straight where you need it.

As a rough guide:

• 1 bulk bag (850L): one large raised bed, or several smaller containers/beds
• 2–3 bulk bags: a full allotment plot top-up, or 2–3 raised beds from scratch
• 10–20 tonne loose loads: school grounds, community gardens, farms, or landscaping jobs — call us on +44 7561 251350 and we'll talk through HIAB access for tighter sites

Compost vs. manure vs. topsoil — what's the difference, and do I need more than one?

Compost feeds soil structure and adds slow-release nutrients; manure is higher in nitrogen and best dug in well ahead of planting; topsoil is the bulk fill material that gives a bed its base depth and body. Most raised beds and allotment plots genuinely benefit from a mix of at least two of the three, not just one product on its own.

• Organic Compost — general soil improvement, moisture retention, mulching. Safe to use close to planting time.
• Manure — best for hungry crops (brassicas, squash, sweetcorn) and ideally dug in during autumn/winter so it has time to break down before spring planting.
• Top Soil — the structural base for a new bed, border, or lawn; blend with compost rather than using alone for growing.

If you're not sure which combination suits your project, that's exactly the kind of thing worth a quick call rather than guesswork — we talk growers through this most weeks.

Is Kelvin Compost peat-free and PAS 100 certified?

Yes. All our compost is produced to the BSI PAS 100 standard — the UK's recognised specification for composted materials — and matured on farmland at our site in Chapelton, South Lanarkshire, using organic materials with no synthetic chemicals or peat. That matters for raised bed and allotment growing specifically, because PAS 100 certification is your guarantee the material has been through a controlled, tested composting process rather than being unscreened green waste.

Ordering compost delivery in Glasgow

Once you've worked out your volume, ordering is straightforward:

1. Match your litre total against bulk bags (850L each) or ask us about tonne loads for larger projects
2. Choose Organic Compost, or a blend with Top Soil or Manure depending on what you're planting
3. Order online through our product pages, or call +44 7561 251350 for HIAB and multi-tonne loads

We deliver across Glasgow, East Kilbride and South Lanarkshire (£15 local delivery in East Kilbride and surrounding areas), with quotes available for the wider Central Belt. See our compost delivery page for full coverage areas.

Frequently Asked Questions

How much compost do I need for a 4x4 raised bed? A 4ft × 4ft bed (about 1.2m × 1.2m) filled 20cm deep needs roughly 290 litres — just over a third of one 850L bulk bag.

Can I use too much compost in a raised bed? Yes — pure compost that's too rich or too moisture-retentive can encourage leggy growth and root problems in some vegetables. Blending with topsoil at 50/50 for the bulk of the bed avoids this.

How often should I add compost to an allotment plot? Most plot holders dig in fresh compost or manure once a year, typically over autumn or winter, ready for spring planting.

Does compost volume shrink after delivery? Yes, compost settles as it beds in and is watered — order 10–15% more than your raw calculation to allow for this.

What's the difference between a bulk bag and a tonne of compost? An 850-litre bulk bag weighs roughly 400–500kg depending on moisture content, so it's not quite a full tonne — for anything approaching a full tonne, a loose load is usually more cost-effective than multiple bulk bags.