How Much Soil Do You Need to Fill a Raised Bed? (Full Guide + Calculator)

Quick answer: To find how much soil a raised bed needs, multiply its length × width × depth in the same units to get the volume. A standard 1.2m × 0.8m bed filled to 30cm needs about 288 litres (0.29 m³) of soil — roughly 6 bags of 50 litres, plus about 5% extra for settling. Because compost is sold by the litre or the bag in most countries and by the cubic foot or yard in the US, the trick is converting that volume into whatever your local supplier sells.

Filling a new raised bed almost always takes more soil than people expect — and running out halfway through a bulk delivery is a genuine hassle that means a second order, a second delivery charge, and a half-finished bed. This guide walks through exactly how to work out the amount, how to buy it without overpaying, how to handle deeper beds, and the small adjustments that stop you coming up short. Use the calculator above to get your number instantly, then read on for how to buy smart.

The simple formula behind it

The volume of soil a raised bed needs is simply:

Length × Width × Depth

Keep every measurement in the same unit and you get a volume. The only real complications are which units your soil supplier uses, and the fact that fresh compost settles after its first few waterings. The calculator above handles both — this section explains what’s going on underneath so you can trust the number and sanity-check it yourself.

For a rectangular bed measured in metres, the result is in cubic metres, and one cubic metre equals exactly 1,000 litres. So a bed that is 1.2m long, 0.8m wide, and 0.3m deep works out to 1.2 × 0.8 × 0.3 = 0.288 m³, which is 288 litres. If you prefer imperial, a 4ft × 3ft bed at 12 inches deep is 4 × 3 × 1 = 12 cubic feet, and since one cubic foot is about 28.3 litres, that is roughly 340 litres.

Step 1: Measure the inside of the bed

Measure the internal dimensions, not the outer frame. A bed built from 2-inch timber can be a couple of centimetres narrower inside than out, and on a large bed that difference adds up to a bag or more of wasted compost. Measure the length and width along the inside faces of the boards.

You also need the depth of soil you actually intend to fill, which isn’t always the full height of the bed. Many gardeners leave a few centimetres of “freeboard” at the top so that soil and mulch don’t spill over the edge when the bed is watered or when heavy rain falls. If your bed is 30cm tall, you might fill it to 27cm and leave 3cm clear. Use the depth you’ll actually fill, not the height of the frame.

Step 2: Decide how deep to fill

Depth matters more than beginners expect, because it multiplies directly into the total volume — and therefore into the cost. Doubling the depth doubles the soil you need and doubles the bill.

As a rough guide: salad leaves, herbs, and other shallow-rooted crops manage in 15–20cm (6–8in) of soil; most vegetables — beans, courgettes, brassicas, tomatoes — do best in around 30cm (12in); and root vegetables like carrots and parsnips want 45cm or more so they can grow long and straight. If your bed sits on open soil rather than a hard surface, roots can grow beyond the frame into the ground below, so you can sometimes fill slightly shallower. On a patio, deck, or concrete base, fill to the full working depth because roots have nowhere else to go.

Step 3: Convert to the units your supplier sells

This is where most online calculators fall down — nearly all of them assume you’re buying by the cubic foot or cubic yard, which is a US convention. Elsewhere, compost and topsoil are sold in litre bags (commonly 40, 50, or 60 litres in the UK and Europe, and 50 litres as standard across the UAE and much of the Gulf) or in bulk bags.

For reference, one cubic metre of soil is 1,000 litres. So our example bed of 288 litres works out, at 50 litres per bag, to six bags — plus a little extra for settling (more on that below). At 40 litres per bag it’s about seven or eight bags. The calculator above lets you switch between regions so it shows you the bag sizes your local garden centre actually stocks, whether you’re in the UK, US, Australia, Europe, or the Gulf.

Step 4: Add a settling allowance

Fresh compost is fluffy and full of air. After the first few good waterings it compacts, and the level drops — sometimes noticeably. If you fill exactly to the brim on planting day, you may find the bed looking under-filled a fortnight later, with the soil sitting a few centimetres below where you left it.

Adding roughly 5% extra covers normal settling. For very loose, peat-free composts that hold a lot of air, 10% is safer. On our 288-litre bed, 5% is about 14 litres — under a third of a bag — so it rounds up neatly. The calculator includes this as an adjustable option so your final number reflects what the bed will look like once it’s settled, not just on planting day.

Should you fill the whole bed with compost?

For deeper beds, filling the entire volume with bought compost gets expensive fast. A common and sensible approach is to part-fill the bottom third with cheaper bulk material — garden soil, well-rotted manure, or even the “hugelkultur” method of logs and woody debris that break down over time — and reserve the good compost for the top two-thirds where roots do most of their work.

If you plan to do this, only calculate the volume for the layer you’re buying in bags, and set the depth in the calculator to that layer’s depth rather than the full bed. A 45cm-deep bed filled with 30cm of bought compost over 15cm of free bulk material can cut your compost bill by a third or more, with no effect on how your plants grow.

A worked example

Say you’re filling two identical beds, each 1.8m long, 0.9m wide, filled to 30cm deep:

• Volume per bed: 1.8 × 0.9 × 0.3 = 0.486 m³ = 486 litres
• Two beds: 972 litres
• Plus 5% settling: about 1,020 litres
• In 60-litre bags: 17 bags

At that scale, a single bulk bag (around 1,000 litres) delivered loose is usually cheaper per litre than buying 17 individual bags — which is exactly the kind of trade-off worth checking before you order. As a rule of thumb, once you need more than about 400–500 litres, a bulk-bag delivery tends to win on price; below that, individual bags are more convenient and don’t carry a delivery charge.

Common mistakes to avoid

The most frequent error is measuring the outside of the bed and over-ordering. The second is forgetting the settling allowance and coming up short. The third is filling a deep bed entirely with expensive compost when the bottom third could be cheaper bulk material. And the fourth, on balconies and roof terraces, is not accounting for weight — moist soil weighs roughly 0.5–0.6 kg per litre, so a 300-litre bed holds around 165kg once watered, which matters for the structure beneath it. Getting each of these right saves money and effort.