How do I calculate soil for a raised bed?

Multiply the inside length and width in feet by the soil depth in inches divided by 12. That gives cubic feet. Divide by 27 for cubic yards. A 4 × 8 ft bed filled 12 inches deep holds 4 × 8 × 1 = 32 cubic feet, about 1.19 cubic yards.

How much soil does a round galvanized raised bed need?

Use 3.14 × radius × radius for the area, then multiply by the depth in feet. A 4 ft round bed filled 17 inches deep holds 3.14 × 2 × 2 × 1.42 = about 17.8 cubic feet.

How many bags of soil fill a 4x8 raised bed?

At 12 inches deep a 4 × 8 bed holds 32 cubic feet: 32 bags of 1 cubic foot, 22 bags of 1.5 cubic feet or 16 bags of 2 cubic feet, before any extra for settling.

Why does my raised bed soil sink?

Loose soil settles as it is watered and as the organic matter in it breaks down. Expect the level to drop over the first season and plan to top up with an inch or two of compost each year.

Should I measure the outside or inside of the bed?

The inside. Boards are about 1.5 inches thick, so a bed that is 4 × 8 ft outside is about 3.75 × 7.75 ft inside, which needs roughly 9 percent less soil.

Raised Bed Soil Calculator: Cubic Feet, Yards and Bags

Enter the inside size of your beds, how deep the soil will be and how many beds you have. The planner card shows how much raised bed soil to buy, in bags or by the yard.

Linocut print of two wooden raised garden beds, one being filled with dark soil, the other planted with tomatoes

How the raised bed calculator works

A raised bed is a box, so the soil it needs is length × width × depth. The calculator does the same thing for each bed and multiplies by the number of beds.

rectangular bed: cubic feet = length (ft) × width (ft) × soil depth (in) ÷ 12 round bed: cubic feet = 3.14 × (diameter ÷ 2)² × soil depth (in) ÷ 12 cubic yards = cubic feet ÷ 27

Worked example: two 4 × 8 ft beds, 12 inches of soil.

One bed: 4 × 8 × (12 ÷ 12) = 32 cubic feet
Two beds: 32 × 2 = 64 cubic feet = 2.37 cubic yards
With 10% extra for settling: 70.4 cubic feet = 2.61 cubic yards
In bags: 71 bags of 1 cubic foot, 47 bags of 1.5 cubic feet or 36 bags of 2 cubic feet

Worked example: one round galvanized bed, 4 ft across, 17 inches of soil.

Area: 3.14 × 2 × 2 = 12.57 square feet
Volume: 12.57 × (17 ÷ 12) = 17.8 cubic feet
With 10% extra: 19.6 cubic feet = 0.73 cubic yards, or 14 bags of 1.5 cubic feet

Tick Round ring (galvanized) in the calculator for the second kind. Many metal beds are oval rather than round; for those, treat the straight middle as a rectangle and the two rounded ends as one circle, and add them.

Raised bed soil chart: rectangular beds

Soil volume in cubic feet, with no extra added. Divide by 27 for cubic yards.

Bed size (inside)	6 in deep	12 in deep	18 in deep	24 in deep
2 × 4 ft	4	8	12	16
2 × 8 ft	8	16	24	32
3 × 6 ft	9	18	27	36
4 × 4 ft	8	16	24	32
3 × 8 ft	12	24	36	48
4 × 6 ft	12	24	36	48
4 × 8 ft	16	32	48	64
4 × 10 ft	20	40	60	80
4 × 12 ft	24	48	72	96

A useful landmark: a 3 × 6 ft bed 18 inches deep holds exactly 27 cubic feet, one cubic yard.

Raised bed soil chart: round beds

Diameter (inside)	12 in deep	17 in deep	24 in deep	32 in deep
2 ft	3.1 cu ft	4.5 cu ft	6.3 cu ft	8.4 cu ft
3 ft	7.1 cu ft	10.0 cu ft	14.1 cu ft	18.8 cu ft
4 ft	12.6 cu ft	17.8 cu ft	25.1 cu ft	33.5 cu ft
5 ft	19.6 cu ft	27.8 cu ft	39.3 cu ft	52.4 cu ft
6 ft	28.3 cu ft	40.1 cu ft	56.5 cu ft	75.4 cu ft

Doubling the diameter of a round bed multiplies the soil by four, so check the bigger rings carefully before you buy.

Measure the soil depth, not the board height

Lumber is sold by its nominal size, but the real board is narrower. For standard US dimensional lumber:

Board	Actual width	Two boards stacked
2×6	5.5 in	11 in
2×8	7.25 in	14.5 in
2×10	9.25 in	18.5 in
2×12	11.25 in	22.5 in

Most people fill to an inch or two below the top so water and mulch don’t wash over the edge. So a bed made of two stacked 2×6 boards usually holds about 9 to 10 inches of soil, not 12. Enter the depth you will really fill.

Buying raised bed soil

Bags make sense for one or two small beds, for balconies, and anywhere a truck can’t reach. Bags are usually 1 to 2 cubic feet.
Bulk is usually cheaper once you need more than a cubic yard or so, which is about two 4 × 8 beds 6 inches deep. Compare the price per cubic foot both ways; the bags of soil calculator shows how.
What to fill with. Plain topsoil on its own can pack down in a deep bed. Most gardeners use a blend of topsoil and compost, or a bagged or bulk “raised bed mix”. The raised bed soil mix calculator splits your volume into a recipe.

For a size-by-size chart with bag counts, see how much soil for a raised bed. To decide how deep to build, read how deep should a raised bed be.

The tons figure for raised bed mix is a rough middle value (about 0.7 to 1 ton per cubic yard). Blends vary a lot, so ask your supplier for theirs.

Questions gardeners ask

How do I calculate soil for a raised bed?: Multiply the inside length and width in feet by the soil depth in inches divided by 12. That gives cubic feet. Divide by 27 for cubic yards. A 4 × 8 ft bed filled 12 inches deep holds 4 × 8 × 1 = 32 cubic feet, about 1.19 cubic yards.
How much soil does a round galvanized raised bed need?: Use 3.14 × radius × radius for the area, then multiply by the depth in feet. A 4 ft round bed filled 17 inches deep holds 3.14 × 2 × 2 × 1.42 = about 17.8 cubic feet.
How many bags of soil fill a 4x8 raised bed?: At 12 inches deep a 4 × 8 bed holds 32 cubic feet: 32 bags of 1 cubic foot, 22 bags of 1.5 cubic feet or 16 bags of 2 cubic feet, before any extra for settling.
Why does my raised bed soil sink?: Loose soil settles as it is watered and as the organic matter in it breaks down. Expect the level to drop over the first season and plan to top up with an inch or two of compost each year.
Should I measure the outside or inside of the bed?: The inside. Boards are about 1.5 inches thick, so a bed that is 4 × 8 ft outside is about 3.75 × 7.75 ft inside, which needs roughly 9 percent less soil.

Raised bed soil calculator