Stop guessing how many bags of compost to buy. Enter your raised bed dimensions and preferred soil mix — get the exact volume in litres, bags, and cubic feet. Works for single beds, multiple beds, and custom blend ratios.
From bed dimensions to a bag count in under 5 seconds. No conversions, no guessing, no second trip to the garden centre.
Measure the inside length, inside width, and intended depth of your raised bed. The inside measurements matter — not the outer frame. For depth, use the full intended fill depth if new, or only the top-up depth if the bed already has some soil in it. Enter in metres, feet, or centimetres.
Takes 2 minutes to measureChoose from five preset mixes — vegetable bed, flower and perennial, Mel's Mix, herb and salad, or peat-free standard. Or use the Custom Mix tab to enter your own ratio of topsoil, compost, and grit. The calculator breaks the total volume into each component separately.
Takes 10 secondsResults show total litres, number of 40-litre bags, and cubic feet — the three units you'll encounter when buying raised bed compost and topsoil. Each mix component is shown separately with litres and bag count, so you know exactly what to put in the trolley.
Results in under 5 secondsNot just a total volume. A complete shopping list — showing exactly how many bags of each component to buy, with no mental arithmetic at the checkout.
Every bag of topsoil, compost, or perlite sold in the UK is labelled in litres. Whether you're buying from a garden centre, a builders' merchant, or ordering online, the litre figure is what to look for on the packaging — and this calculator gives it to you directly.
The most useful output from this calculator is the component breakdown — not just "20 bags" but "12 bags topsoil, 6 bags compost, 2 bags grit." That's the actual shopping list, so there's no on-the-spot mental arithmetic in the garden centre when you're trying to remember your ratios.
Each preset is based on well-established raised bed formulas used by experienced growers. The vegetable mix (60/30/10) gives you a free-draining, nutrient-rich growing medium. Mel's Mix (⅓ each) is the classic square foot gardening recipe. The flower mix prioritises drainage for perennials and bulbs.
Already have a mix ratio from a book or a grower you trust? Use the Custom Mix tab to enter any percentages of topsoil, compost, and grit. The calculator splits the total volume according to your numbers and gives you bag counts for each component separately.
It looks like a small bed. It never is. Most first-time raised bed builders are shocked by how much soil a shallow frame actually holds.
A standard 8×4ft raised bed filled to 30cm deep holds approximately 864 litres of soil — that is 22 bags of compost at 40 litres each. Most people estimate 8–10 bags and make two or three emergency trips to the garden centre while the project stalls. Calculate first, buy once.
A vegetable bed mix is 60% topsoil, 30% compost, and 10% grit or perlite. A flower bed uses more compost and less grit. Mel's Mix uses equal thirds of compost, perlite, and vermiculite. Buying 20 bags of compost for a bed that needs mostly topsoil is both wasteful and bad for your plants.
Online topsoil and bulk compost orders typically have a minimum delivery charge of £30–£60. Ordering too little and needing a second delivery doubles that cost. One accurate calculation before placing the order pays for itself immediately.
Doubling the depth of a raised bed doubles the amount of soil needed — not the width. A 2.4m × 1.2m bed at 20cm deep needs 576 litres. The same bed at 40cm deep needs 1,152 litres. This is why accurate depth measurement is the single most important input to any raised bed soil calculation.
Fresh compost and topsoil settles 10–20% after the first watering and over the first growing season. If you fill to exactly the rim, you'll have a noticeably shallow bed by week three. The calculator includes a fill level option — 90% fill gives you 10cm headroom while accounting for natural settlement without overflow.
No app, no subscription, no account. Calculate while you're measuring the bed — before the wood is even cut. Works in any mobile browser. Enter the dimensions, choose the mix, get the bag count. That's it.
The mix you choose determines how your bed drains, how much nutrition it holds, and which plants will thrive in it. Here's exactly what each preset is designed for.
Simple volume geometry plus mix ratio applied automatically — no manual splitting of percentages required.
The core calculation is straightforward: Volume = Length × Width × Depth. The result in cubic metres is then multiplied by 1,000 to convert to litres — the unit used on every bag of growing medium sold in the UK.
For the fill level adjustment, the calculated volume is multiplied by the selected fill percentage. At 90% fill, a bed that calculates to 864 litres becomes 778 litres — accounting for the 10cm of headroom below the rim that prevents soil washing over the edge in heavy rain and allows for natural settlement.
The mix breakdown simply applies the selected percentage ratios to the total volume. A vegetable mix at 60/30/10 takes the total and splits it: 60% becomes topsoil litres, 30% becomes compost litres, 10% becomes grit litres. Each component is then divided by 40 and rounded up to give the bag count — because a partial bag is still a bag you need to buy.
L × W × D in metres = m³ × 1,000 = total litres required
90% fill = 10cm headroom + accounts for soil settlement
Total litres × each % = component litres ÷ 40 = bags (rounded up)
Half a bag of compost still costs the price of a full bag — round up every time
Use this table to estimate before measuring, or to sense-check the calculator result for your bed size.
| Bed Size | Depth | Total Litres | 40L Bags (approx) | Best For |
|---|---|---|---|---|
| 60 × 60cm (2×2ft) | 20cm | 72 L | 2 bags | Small herb or salad planter |
| 1.2 × 0.6m (4×2ft) | 25cm | 180 L | 5 bags | Compact starter veg bed |
| 1.8 × 0.9m (6×3ft) | 30cm | 486 L | 13 bags | Medium family veg bed |
| 2.4 × 1.2m (8×4ft) | 30cm | 864 L | 22 bags | Standard veg bed — most popular size |
| 2.4 × 1.2m (8×4ft) | 45cm | 1,296 L | 33 bags | Deep bed — root veg, full season crops |
| 3.6 × 1.2m (12×4ft) | 30cm | 1,296 L | 33 bags | Long production bed — allotment style |
| 4.8 × 1.2m (16×4ft) | 30cm | 1,728 L | 44 bags | Large polytunnel or market garden bed |
Getting the volume right is the easy part. These five steps ensure the soil you fill the bed with gives you the best possible growing results.
Always measure the inside dimensions of the raised bed frame — the soil fills the interior, not the full width of the timber. A 2.4m bed made from 38mm thick boards has an interior length of 2.324m. For small beds, this difference can mean a whole bag of compost over or under.
Before filling, lay overlapping sheets of cardboard on the ground inside the bed. This smothers existing weeds and grass without the need for membrane, breaks down naturally over the first season, and draws in earthworms as it decomposes — improving soil structure from the bottom up.
The whole point of a raised bed is that you never compact the soil by standing on it. Keep the bed width to 1.2m maximum so you can reach the centre from either side. Compacted raised bed soil loses the drainage and aeration that makes it better than open-ground growing in the first place.
After filling, water the bed thoroughly and leave it to settle for 48 hours before planting. Fresh compost and topsoil can contain air pockets that collapse once wetted — and the surface will drop 3–5cm after the first good watering. Top up with extra compost to the correct level before planting begins.
After each growing season, add a 5–7cm layer of fresh compost over the surface of the bed and leave it without digging in. Over winter, worms will incorporate it naturally. This annual top-dressing replaces the nutrients removed by crops and maintains the soil structure year after year without ever needing to completely refill the bed.
The questions we hear most from gardeners filling raised beds for the first time.
From a first-time grower building a single 8×4ft bed to an experienced allotment holder installing a bank of growing beds for the season.
Nothing prepares you for how much soil a freshly built raised bed actually needs. This calculator tells you the number before you're standing at the garden centre trying to remember your bed dimensions and doing mental arithmetic on bags.
The quality of the soil mix directly affects what you can grow and how well. Getting the topsoil-to-compost ratio right from the start — rather than guessing and correcting later — gives crops the growing conditions they need from day one.
Square foot gardening requires Mel's Mix — a specific ratio of compost, perlite, and vermiculite. Use the Custom Mix tab to enter the exact ⅓/⅓/⅓ ratio and get the bag count for each component for your grid size.
Quoting or ordering materials for a client's raised bed installation? Use the Multiple Beds tab to calculate the total soil volume across all beds in one go, then convert to bulk tonnes for a supplier order.
Adding one or more raised beds to an allotment plot is a significant soil investment. Calculate each bed's requirements individually, then total them for a single bulk order — typically more cost-effective than multiple bag purchases.
Community growing projects often build multiple raised beds in a single phase. The Multiple Beds calculator handles up to four beds at once and gives the combined volume needed for a group order from a local soil supplier.
Cut flower production in raised beds has grown enormously in recent years. Use the Flower & Perennial mix preset — it provides the extra drainage that most cut flowers need compared to a standard vegetable growing medium.
