How much soil does a raised bed need?
A 4 × 8 ft bed needs about 2.7 cu ft of soil per inch of depth, so 26.7 cu ft at 10 inches.
cubic feet = length × width × (soil depth in inches ÷ 12)
A 4 × 8 ft bed filled 10 inches deep holds 4 × 8 × 0.833 = 26.7 cu ft. Two of them, the default above, hold 53.3 cu ft, or 1.98 cubic yards. In 1.5 cu ft bags that is 36 bags.
Use the depth of soil you will actually put in, which is usually an inch or two below the top of the frame, not the height of the boards.
Garden bed calculator: soil for common bed sizes
A 4 × 4 ft bed holds 13.3 cu ft at 10 inches and a 4 × 8 ft bed holds 26.7 cu ft.
| Bed (ft) | 6 in | 8 in | 10 in | 12 in | 18 in |
|---|---|---|---|---|---|
| 2 × 4 | 4 | 5.3 | 6.7 | 8 | 12 |
| 3 × 6 | 9 | 12 | 15 | 18 | 27 |
| 4 × 4 | 8 | 10.7 | 13.3 | 16 | 24 |
| 4 × 8 | 16 | 21.3 | 26.7 | 32 | 48 |
| 4 × 12 | 24 | 32 | 40 | 48 | 72 |
Divide by 27 for cubic yards (the cubic feet to cubic yards chart has the common values), or by your bag size for the number of bags; bags per cubic yard lists every common size. Bagged garden soil is sold in sizes such as 0.75, 1, 1.5 and 2 cu ft.[2]
How deep should the soil be?
University of Maryland Extension gives minimum soil depths for raised beds built on hard surfaces: at least 8 inches for leafy greens, beans and cucumbers, and 12–24 inches for peppers, tomatoes and squash.[1] Those minimums are for beds on paving or other hard surfaces, where roots cannot reach the ground below. In-ground beds and lawns are covered in the topsoil depth guide.
Topsoil and compost: splitting the order
The same guide suggests a mix of compost and topsoil at about 1:2 or 1:1, aiming for 25–50% organic matter by volume.[1] For the 53.3 cu ft example:
| Mix | Compost | Topsoil |
|---|---|---|
| 1 part compost : 2 parts topsoil | 17.8 cu ft | 35.6 cu ft |
| 1 : 1 | 26.7 cu ft | 26.7 cu ft |
For an existing bed, Colorado State suggests working in only about ¼ inch of compost a year,[3] so top-ups are small: a 4 × 8 ft bed needs about 0.7 cu ft. For large areas of open ground, the topsoil calculator works in cubic yards and tons.
Metric bedsSwitch to meters for liter bags. A 1.2 × 2.4 m bed filled 25 cm deep holds 0.72 m³, or 720 liters, which is 15 bags of 50 L. One major UK brand packs its compost in 10, 25, 50 and 60 L bags.[4]
Frequently asked questions
How much soil do I need for a raised garden bed?
Length × width × soil depth, all in feet, gives cubic feet. A 4 × 8 ft raised garden bed filled 10 inches deep holds 26.7 cu ft, about 1 cubic yard.
How many bags of soil for a 4 × 8 raised bed?
At 10 inches deep the bed holds 26.7 cu ft: 18 bags of 1.5 cu ft, 27 bags of 1 cu ft or 36 bags of 0.75 cu ft.
How many cubic yards of soil for a raised bed?
Cubic feet ÷ 27. A 4 × 8 ft bed at 12 inches is 32 cu ft, or 1.19 cubic yards.
How deep should a raised bed be?
On hard surfaces, at least 8 inches of soil for leafy greens, beans and cucumbers, and 12–24 inches for tomatoes, peppers and squash.[1]
What should I fill a raised bed with?
University of Maryland Extension suggests compost and topsoil in roughly a 1:2 or 1:1 ratio.[1]