Raised Bed Soil: How Much to Buy and What to Fill It With

There's a particular moment of dread that hits almost every new raised-bed gardener. You've built (or bought) a gorgeous timber bed, you've levelled the ground, you're feeling thoroughly pleased with yourself — and then it dawns on you: this thing is going to need a lot of soil. How much, exactly? And what should you actually fill it with? Topsoil? Compost? That bag of manure you've seen at the garden centre? Some mysterious "potting mix"?

I've filled more raised beds than I care to count, and I've made every mistake in the book — ordering far too little on the first run, buying overpriced bags when a bulk delivery would have cost a third of the price, and once filling an entire bed with pure compost only to watch it slump by a good few inches after the first heavy watering. So this guide is the one I wish I'd had at the start. We'll cover the volume maths (it's genuinely simple once you see it laid out), the best soil recipes for different crops, how deep your bed actually needs to be, and how to buy your soil without getting fleeced.

By the end, you'll be able to glance at any bed, do the calculation in your head, pick the right mix for what you're growing, and know whether to reach for bags or a bulk bag. Let's dig in.

The Volume Formula — Simpler Than You Think

Everything starts with volume, and the good news is that the maths is the same whether you're filling a tiny herb planter or a sprawling allotment bed. Here's the universal formula that every reputable soil calculator uses:

The only thing that trips people up is units. Most beds are measured in a mix of metres and centimetres, or feet and inches, so you need to get everything into the same system before you multiply. Here are the conversions worth memorising:

That weight figure is worth pausing on. A single cubic metre of topsoil weighs roughly 1,200 kg — that's 1.2 tonnes. If you're ordering a few cubic metres of loose soil to be tipped on your driveway, you are quite literally taking delivery of multiple tonnes of material, which has real implications for how (and where) you can move it. More on that later, but file it away now.

Let me walk you through a real example so you can see how painless this is. Say you've got a classic 2.4m × 1.2m bed and you want to fill it to a depth of 30cm. First, convert the depth: 30cm ÷ 100 = 0.30m. Now multiply everything together: 2.4 × 1.2 × 0.30 = 0.864 m³. Multiply by 1,000 and you've got 864 litres. Done. You now know exactly how much soil that bed swallows, and you can shop accordingly.

How Deep Should a Raised Bed Be?

Before you finalise your volume calculation, you need to settle on a depth — and this matters far more than most beginners realise. Depth is the single biggest lever affecting how much soil you buy. Get it wrong and you either waste money on soil you don't need, or you starve your root vegetables of the space they crave.

Here's the depth guidance broken down by what you're actually growing:

For the vast majority of gardeners, a 30cm (12-inch) fill depth is the sweet spot. It comfortably accommodates tomatoes, beans, salad leaves, brassicas and most herbs, and it doesn't ruin you financially. If you're a carrot or parsnip enthusiast, push towards 35–45cm so those long taproots have room to drive straight down — nothing's more disappointing than forked, stunted carrots that hit a barrier and gave up.

One sensible trick if you're filling a deep bed but mostly growing shallow-rooted crops: you don't necessarily need premium growing mix all the way to the bottom. The roots of most vegetables live in the top 25–30cm. We'll come back to layering strategies, but it's worth knowing you can be a little economical with the lower depths of a very deep bed.

What to Fill It With — The Best Soil Mixes

Now for the fun part — and the question I get asked more than any other. You can't just shovel in pure topsoil and hope for the best (too dense, drains poorly, low on nutrients), and you definitely can't fill a bed with pure compost (too rich, slumps badly, holds too much water). The magic is in the blend.

After years of experimenting, I've come to rely on a handful of tried-and-tested recipes. The right one for you depends on what you're growing and how much you want to fuss. Here are the five mixes worth knowing.

The 60/30/10 Rule — The Reliable All-Rounder

If you only remember one recipe, make it this one. It's the most widely recommended formula, cited consistently across multiple sources, and it's my default for general veg beds.

The beauty of 60/30/10 is balance. The topsoil gives you structure and a reservoir of minerals; the compost feeds your plants and supports soil life; the perlite (or a quality potting mix) stops the whole thing compacting into a brick. It's forgiving, it suits almost everything, and it's hard to get wrong.

The Classic Mix — Drainage First

If your garden tends towards the soggy side, or you're growing crops that hate wet feet (think Mediterranean herbs), this drainage-focused blend is a strong choice.

The 20% coarse sand really opens up the structure and lets excess water escape freely. It's best for most general gardeners who want dependable drainage without overthinking things.

Mel's Mix — The Square Foot Gardening Favourite

If you've fallen down the Square Foot Gardening rabbit hole (and many of us have), you'll know Mel's Mix. It's an equal-thirds blend that produces an incredibly light, fluffy, productive growing medium.

The vermiculite holds moisture and nutrients beautifully, the peat-free mix keeps things airy, and the compost does the feeding. It's wonderful for intensive, closely-spaced planting. The one caveat: it's the most expensive mix here because vermiculite isn't cheap, and there's no cheap-and-cheerful topsoil bulking it out. But for raised beds dedicated to high-value salad and veg, it's superb.

Vegetable Boost Mix — For Maximum Productivity

This is my go-to when I want a bed to absolutely perform — heavy feeders like courgettes, pumpkins, brassicas and tomatoes love it.

The well-rotted manure is the key ingredient here — note the "well-rotted" part. Fresh manure will scorch your plants and can introduce weed seeds and pathogens. It needs to have been aged and broken down so it's dark, crumbly and odourless. Get that right and this mix delivers genuinely impressive yields.

Comparing the Five Mixes

Here's how they stack up side by side, so you can pick the right one at a glance:

Reference Volumes for Common UK Bed Sizes

Rather than make you do the maths from scratch every time, here's a ready-reckoner for the most popular UK raised bed sizes, all calculated at the standard 30cm (12-inch) fill depth. I've included the number of 40L and 50L bags you'd need, so you can shop straight from this table.

A few observations worth flagging. First, look at the 4×4ft bed and the 8×2ft bed — both come in at 432 litres. That's because they have the same surface area (1.44 m² versus 1.44 m²), and at the same depth, area is what determines volume. Handy to know if you're deciding between bed shapes.

Second, watch how the bag counts climb. Once you're looking at the 10ft and 12ft beds, you're carting home 27 to 33 bags of soil — that's a serious amount of lugging, and as we'll see shortly, it's also the point at which bagged soil stops making financial sense.

Worked Examples — From Dimensions to Shopping List

Let me show you the full journey from "I've got a bed" to "here's what I'm buying", using a few real scenarios.

Example A is the one most readers will relate to, so let's dwell on it. That 2.4m × 1.2m bed needs 864 litres. Buy it as 35 bags of 25L topsoil at £5 each and you're looking at £175. Buy the same volume as 2 bulk bags at £65 each and it's £130. That's a £45 saving — for the exact same soil — simply by choosing the right format. And you save your back the hassle of hauling 35 individual sacks.

Bagged vs Bulk vs Tipper — How to Buy Smart

This is where the real money is won or lost. The soil itself is much the same wherever you buy it; what changes wildly is the price per litre depending on how it's packaged and delivered. Here's the landscape of options and roughly what you can expect to pay in the UK.

The pattern is clear once you do the per-litre maths. A 25L bag at £5 works out at 20p per litre. An 850L bulk bag at £65 works out at under 8p per litre. That's the difference, and it's why the rule of thumb is so stark:

So when do bags actually make sense? In my experience, bagged soil is the right call for small, one-off jobs — topping up an existing bed, filling a single small planter, or when you simply can't accept a bulk delivery (no driveway access, a flat with no kerb space, that sort of thing). Bags are clean, easy to carry one at a time, and you can buy exactly the number you need. The moment your project tips past roughly 850 litres, though, the economics swing decisively towards bulk.

Don't Forget the Settling Allowance

Here's the mistake that catches almost everyone out the first time. You carefully calculate that your bed needs 864 litres, you order exactly 864 litres, you fill the bed to the brim, you give it a good watering — and the next morning the soil has dropped by an inch or two, leaving a disappointing gap below the rim.

This is settling, and it's completely normal. Loose soil and compost contain a surprising amount of air. Watering, rain and the simple passage of time all cause the material to compact and settle into place. If you ordered the exact calculated volume, you'll now be short.

To put that in numbers: for our 864-litre bed, a 10% allowance means ordering around 950 litres, and a 15% allowance means roughly 994 litres. Given that you can't buy precise litre amounts anyway — you're rounding up to whole bags or bulk bags — building in a settling margin usually just means rounding up rather than down. With bulk bags it's effectively free, since two 850L bulk bags give you 1,700 litres, comfortably covering both your 864-litre target and any settling.

Bag Sizes Explained

Soil and compost come in a slightly bewildering array of bag sizes, and knowing what's out there helps you compare prices like for like. Here's the range you'll encounter:

The 850L bulk bag is the standard unit for builders' merchants and most large soil suppliers, and it's the format that unlocks the big savings. The smaller 20L to 100L bags are what you'll find at garden centres and DIY sheds. When comparing prices, always work out the cost per litre rather than the headline price per bag — a "cheap" small bag is often the most expensive soil you can buy once you do the maths.

How These Sizing Tools Compare

If you'd rather not do the calculations by hand, several soil calculators can do the heavy lifting. They vary in focus, so here's how the main approaches compare for filling a raised bed.

Honestly, whichever you use, they all rely on the same underlying formula we covered at the start. The main value of a calculator is that it handles the unit conversions and the bag-count division for you, and many will surface a recommended mix and a settling allowance prompt — useful nudges if you're new to this. But there's a real satisfaction in being able to do the maths yourself, and it means you're never caught out without a phone signal at the garden centre.

Quick Ratings — Bagged vs Bulk at a Glance

To summarise the buying decision, here's how I'd score the two main formats across the factors that actually matter when you're filling beds.

The takeaway: there's no single "best" format. Bulk wins decisively on cost and on large jobs; bags win on convenience and small top-ups. Match the format to the job and you'll never overspend.

Relative Cost of the Five Mixes

For those weighing up which recipe to commit to, here's a rough sense of how the mixes compare on overall cost to fill a standard bed. These are relative bars, not exact figures — the precise total always depends on your local supplier and bed size.

Mel's Mix sits at the top of the cost ladder because vermiculite is the priciest ingredient and there's no inexpensive topsoil bulking things out. The Classic and 60/30/10 mixes are the most economical for filling large volumes, because topsoil — the cheapest component — makes up the majority. If budget is tight and you're filling a big bed, lean towards those two.

Who Should Buy What

To pull it all together, here's my quick steer depending on the kind of gardener you are.

Frequently Asked Questions