As a raised bed soil calculator, the sum is simple: to work out how much soil a bed needs, multiply its inside length by its width by the fill depth, all in metres, then multiply by 1,000 to get litres. A 1.2 × 2.4 m bed (about 4 × 8 ft) filled 30 cm deep needs 0.864 m³, or 864 litres: 18 bags of 50 litres.
This page works as a calculator you can do on paper: the formula, a ready-made table for common bed sizes and depths, bag counts, the RHS mix split into compost, sand and topsoil, and how much to allow for topping up.
The raised bed soil formula
- Measure inside the frame in metres: length, width and the depth you will fill to.
- Convert centimetres to metres by dividing by 100 (30 cm = 0.3 m).
- Multiply: length × width × depth = cubic metres (m³).
- Convert to litres: m³ × 1,000 = litres.
- Count bags: litres ÷ litres per bag, rounded up.
Example: a 1.2 × 1.8 m bed filled 30 cm deep is 1.2 × 1.8 × 0.3 = 0.648 m³ = 648 litres. In 50-litre bags, 648 ÷ 50 = 12.96, so 13 bags.
If your supplier sells by weight, Rolawn, a UK topsoil supplier, gives roughly 0.67 m³ per tonne of standard topsoil, though it says density varies by product. On that basis, 0.864 m³ is about 1.3 tonnes.
How much soil in a raised bed: ready-made table
Litres and cubic metres for a completely empty bed filled to each depth.
| Bed (inside) | 15 cm deep | 20 cm deep | 30 cm deep | 45 cm deep | 60 cm deep |
|---|---|---|---|---|---|
| 0.6 × 1.2 m (small or balcony) | 108 L | 144 L | 216 L | 324 L | 432 L |
| 1.2 × 1.2 m (about 4 × 4 ft) | 216 L | 288 L | 432 L | 648 L | 864 L |
| 0.9 × 1.8 m (about 3 × 6 ft) | 243 L | 324 L | 486 L | 729 L | 972 L |
| 1.0 × 2.0 m | 300 L | 400 L | 600 L | 900 L | 1,200 L |
| 1.2 × 1.8 m (about 4 × 6 ft) | 324 L | 432 L | 648 L | 972 L | 1,296 L |
| 1.2 × 2.4 m (about 4 × 8 ft) | 432 L | 576 L | 864 L | 1,296 L | 1,728 L |
| 1.2 × 3.0 m | 540 L | 720 L | 1,080 L | 1,620 L | 2,160 L |
Divide any figure by 1,000 for cubic metres (864 L = 0.864 m³). Not sure what depth to fill to? See how deep a raised bed should be.
4x8 raised garden bed: how much soil?
The metric sizes above are rounded. If your bed is built in feet, here are the exact figures (1 ft = 30.48 cm):
| Bed | Fill depth | Volume | 50-litre bags | 40-litre bags |
|---|---|---|---|---|
| 4 × 8 ft | 6 in | 453 L | 10 | 12 |
| 4 × 8 ft | 1 ft | 906 L | 19 | 23 |
| 4 × 8 ft | 18 in | 1,359 L | 28 | 34 |
| 4 × 4 ft | 1 ft | 453 L | 10 | 12 |
| 3 × 6 ft | 1 ft | 510 L | 11 | 13 |
How many bags of soil or compost?
Bag counts for the metric sizes, rounded up to whole bags. Check the litres printed on your bags; if they differ, divide the litres from the table above by your bag size.
| Bed (inside) | 30 cm deep: 50 L bags | 30 cm deep: 40 L bags | 45 cm deep: 50 L bags | 45 cm deep: 40 L bags |
|---|---|---|---|---|
| 0.6 × 1.2 m | 5 | 6 | 7 | 9 |
| 1.2 × 1.2 m | 9 | 11 | 13 | 17 |
| 0.9 × 1.8 m | 10 | 13 | 15 | 19 |
| 1.0 × 2.0 m | 12 | 15 | 18 | 23 |
| 1.2 × 1.8 m | 13 | 17 | 20 | 25 |
| 1.2 × 2.4 m | 18 | 22 | 26 | 33 |
| 1.2 × 3.0 m | 22 | 27 | 33 | 41 |
Bulk bags are sold in different sizes by different suppliers, so compare the volume each one states rather than assuming a standard size.
How much compost do I need for a raised bed?
Do not fill a raised bed with compost alone; the RHS recommends mostly soil, because compost breaks down and slumps. Its standard mix is about 3 parts organic matter, 2 parts horticultural sharp sand and 7 parts topsoil, so compost or manure is a quarter of the volume. Split for a 30 cm fill (rounded to the nearest litre):
| Bed (inside) | Total at 30 cm | Organic matter (3) | Sharp sand (2) | Topsoil (7) |
|---|---|---|---|---|
| 0.6 × 1.2 m | 216 L | 54 L | 36 L | 126 L |
| 1.2 × 1.2 m | 432 L | 108 L | 72 L | 252 L |
| 0.9 × 1.8 m | 486 L | 122 L | 81 L | 284 L |
| 1.0 × 2.0 m | 600 L | 150 L | 100 L | 350 L |
| 1.2 × 1.8 m | 648 L | 162 L | 108 L | 378 L |
| 1.2 × 2.4 m | 864 L | 216 L | 144 L | 504 L |
| 1.2 × 3.0 m | 1,080 L | 270 L | 180 L | 630 L |
For a bed on concrete or paving, the RHS uses 3 parts organic matter, 4 parts sand and 7 parts topsoil. For a 1.2 × 2.4 m bed filled 30 cm deep, that is about 185 litres of organic matter, 247 litres of sand and 432 litres of topsoil.
For any other size, divide the total litres by 12 (or by 14 for the hard-surface mix) and multiply by each part. Our guide to the best soil for raised beds explains what to buy for each part.
Settling and topping up
The soil in a new bed settles, but no authoritative source gives a percentage to add for it, so we have not built one into these tables. Instead, follow the RHS method: fill to about 5 cm below the rim, firm it, let it settle for a couple of weeks and top up before planting. (BBC Gardeners' World fills to the brim and also waits two weeks.) The steps are in how to fill a raised bed.
After that, the RHS adds about 5 cm of organic matter to no-dig beds each year, and notes that a 5 cm layer works out at about 50 litres per square metre. Here is what a yearly top-up comes to:
| Bed (inside) | 5 cm layer | 7.5 cm layer |
|---|---|---|
| 0.6 × 1.2 m | 36 L | 54 L |
| 1.2 × 1.2 m | 72 L | 108 L |
| 0.9 × 1.8 m | 81 L | 122 L |
| 1.0 × 2.0 m | 100 L | 150 L |
| 1.2 × 1.8 m | 108 L | 162 L |
| 1.2 × 2.4 m | 144 L | 216 L |
| 1.2 × 3.0 m | 180 L | 270 L |
The 7.5 cm column matches the RHS's ideal depth for biodegradable mulches. The 5 cm column is also how much less you need on day one if you fill to 5 cm below the rim.
Once you know how much soil to order, the next job is deciding what goes in it. Sowstory takes your bed's inside size, lays out the rows to scale and tells you how many plants of each crop fit, with a month-by-month plan for England. It is coming soon to the App Store, and your first bed is free.
To count plants by hand, use the plant spacing calculator; to choose a bed size first, see raised bed sizes.