Building a raised bed is one of the easiest ways to start growing vegetables, but filling it is where many gardeners get caught out. Beds swallow far more soil than they look like they will, and bagged compost is sold in litres or cubic feet while bulk soil is sold by the cubic yard or tonne. A few minutes of arithmetic before you buy saves an extra trip to the garden centre or a heap of leftover soil on the drive.
Step 1: Measure the inside of the bed
Measure the internal length, width and depth, not the outside of the frame. Timber boards are often 1.5 to 2 inches thick, and on a small bed that difference adds up.
- Length: the long side, inside edge to inside edge
- Width: the short side, inside edge to inside edge
- Depth: the height you actually plan to fill, which is usually an inch or two below the top of the boards
Step 2: Work out the volume in cubic feet
For a rectangular bed:
Volume (cu ft) = length (ft) × width (ft) × depth (ft)
If you measured in inches, divide each measurement by 12 first, or multiply all three in inches and divide the result by 1,728 (the number of cubic inches in a cubic foot).
Example: a bed 8 ft long, 4 ft wide and 11 inches deep.
- Depth in feet: 11 ÷ 12 ≈ 0.917 ft
- Volume: 8 × 4 × 0.917 ≈ 29.3 cubic feet
Step 3: Allow for settling
Fresh soil and compost settle as they're watered and as organic matter breaks down. Most gardeners add 10–20% extra to allow for this. For our example:
29.3 × 1.15 ≈ 33.7 cubic feet
You can top the bed up again the following season, so there's no need to overdo it.
Step 4: Convert to what you're buying
Bagged soil or compost
Bags are commonly sold in cubic feet (in the US) or litres (in the UK and Europe).
- 1 cubic foot ≈ 28.3 litres
- A 1.5 cu ft bag holds about 42 litres
- A 50-litre bag holds about 1.77 cu ft
For 33.7 cu ft:
- Using 1.5 cu ft bags: 33.7 ÷ 1.5 ≈ 22.5, so 23 bags
- Using 50-litre bags: 33.7 × 28.3 ≈ 954 litres, ÷ 50 ≈ 19.1, so 20 bags
Bulk soil by the cubic yard
One cubic yard is 3 ft × 3 ft × 3 ft = 27 cubic feet.
33.7 ÷ 27 ≈ 1.25 cubic yards
Bulk delivery is usually cheaper per cubic foot than bags once you need more than about a cubic yard, though delivery charges vary.
Other bed shapes
- Round beds: volume = π × radius² × depth. A 4 ft diameter bed (radius 2 ft) filled 1 ft deep holds about 3.14 × 4 × 1 ≈ 12.6 cu ft.
- L-shaped beds: split into two rectangles, calculate each, then add.
- Sloping ground: measure the depth at several points and use the average.
If you'd rather not do the conversions by hand, a dedicated tool such as https://www.cubicfeetcalculator.org/cubic-feet-calculator-raised-bed/ handles inches, feet and bag sizes in one go.
Saving soil in deep beds
Tall beds, especially those over 18 inches, can be expensive to fill entirely with soil. Common approaches:
- Hügelkultur-style filling: logs, branches and woody material in the bottom third, which break down slowly over years.
- Lasagne layering: alternating layers of cardboard, grass clippings, leaves and compost.
- Coarse material at the base for drainage in beds sitting on hard surfaces.
Most vegetables root in the top 12 inches, so what matters most is the quality of the upper layer. Expect beds filled with organic material to sink more over time.
Choosing a mix
A common home-garden recipe is roughly 60% topsoil, 30% compost and 10% something to improve drainage, such as grit or perlite. Pre-mixed "raised bed soil" is convenient but check what's in it. Avoid filling beds with pure compost, which can be too rich and slumps as it decomposes.
Quick checklist
- Measure inside dimensions and planned fill depth.
- Multiply to get cubic feet.
- Add 10–20% for settling.
- Convert to bags (cu ft or litres) or cubic yards.
- Round up, and keep the receipt for any unopened bags.
With the numbers worked out in advance, you can compare bags against bulk delivery properly and fill the bed in one go.