Calculate how much soil you need for a raised bed in cubic feet, cubic yards and bags, with example mix amounts, or find the fill depth from soil you have.
Raised Bed Soil Formula
CY = V / 27
- V is the soil volume in cubic feet (cu ft)
- L is the inside length of the bed in feet (ft)
- W is the inside width of the bed in feet (ft)
- D is the soil depth in inches (in)
- CY is the soil volume in cubic yards (cu yd)
For a round bed, the volume uses the circle area instead: V = 3.1416 * (d/2)2 * (D/12), where d is the inside diameter in feet. The calculator multiplies by the number of identical beds. An optional 10 or 15 percent extra-ordering allowance increases the calculated quantity; it does not predict a universal settling rate or guarantee the final depth.
If a bottom-fill layer is entered, its assumed uniform depth is subtracted before estimating the growing mix above it. This is a geometric adjustment, not a recommendation for a particular fill material or crop-rooting depth. Gaps between irregular pieces can change the actual soil needed. The reverse mode solves for depth instead of volume:
This tells you how deep the soil you already own will fill the bed, which is useful when you have leftover bags and want to know if they are worth spreading or if you need to buy more.
Soil Volumes, Bag Counts, and Bottom Fill Savings
The first table shows geometric volume at a 12-inch fill depth. Its bag counts use an illustrative 1.5 cubic foot bag; use the volume printed on the product you plan to buy.
| Bed size | Soil at 12 in deep | Cubic yards | 1.5 cu ft bags |
|---|---|---|---|
| 2 ft x 4 ft | 8 cu ft | 0.30 | 6 |
| 4 ft x 4 ft | 16 cu ft | 0.59 | 11 |
| 4 ft x 8 ft | 32 cu ft | 1.19 | 22 |
| 4 ft x 12 ft | 48 cu ft | 1.78 | 32 |
| 4 ft round | 12.57 cu ft | 0.47 | 9 |
Some tall, hugel-style beds use woody material below the growing medium to reduce purchased fill. Suitability depends on the material, crop-rooting space, drainage, and bed design; decomposition can cause settling and temporary nitrogen shortages. The second table illustrates the geometric reduction in growing-mix volume for a uniform bottom layer in a 4 ft x 8 ft bed, before any extra-ordering allowance.
| Total fill depth | Bottom fill layer | Soil mix needed | Soil saved |
|---|---|---|---|
| 18 in | 6 in of branches | 32 cu ft | 16 cu ft (33%) |
| 24 in | 10 in of logs | 37.3 cu ft | 26.7 cu ft (42%) |
| 30 in | 16 in of logs | 37.3 cu ft | 42.7 cu ft (53%) |
Choose the growing-medium depth for the crops and site, including whether roots can reach suitable soil below the bed. Woody fill may settle as it decomposes. Replenish the growing medium as needed and base compost or fertilizer additions on soil-test results.
Example Problems
Example 1: How much soil for a 4 ft x 8 ft bed?
You want to fill a 4 ft x 8 ft raised bed 12 inches deep. First, convert the depth to feet: 12 / 12 = 1 ft. Then multiply: V = 8 * 4 * 1 = 32 cubic feet. Divide by 27 to get cubic yards: 32 / 27 = 1.19 cu yd. In 1.5 cubic foot bags, that is 32 / 1.5 = 21.3, rounded up to 22 bags. For the selected example 50/30/20 recipe by volume, the breakdown is 16 cu ft of garden soil, 9.6 cu ft of compost, and 6.4 cu ft of a suitable aeration component. These proportions are an example, not a universal recommendation.
Example 2: How deep will 20 bags fill the same bed?
You already have 20 bags of 1.5 cubic foot soil, which is 20 * 1.5 = 30 cubic feet. The bed area is 8 * 4 = 32 square feet. Depth = (30 / 32) * 12 = 11.25, or about 11.3 inches. Whether this depth is suitable depends on the crop, drainage, and access to usable soil beneath the bed; the depth calculation alone cannot determine whether more growing medium is needed.
FAQ
How many bags of soil do I need for a raised bed?
Divide the required volume by the labelled volume per bag and round up. A 40 lb weight does not determine a fixed soil volume; product composition, moisture, and bulk density matter. One cubic yard equals 27 one-cubic-foot bags or 18 bags of 1.5 cubic feet. For an optional mass estimate, use a bulk density appropriate to the actual material and moisture condition. Compare local bag and bulk quotes including delivery fees, minimum orders, and material quality; there is no universal two-cubic-yard price threshold.
What is the best soil mix for a raised bed?
The calculator offers example volume recipes: 50/30/20 soil, compost, and an aeration component; 60/30/10 topsoil, compost, and an aeration component; or equal thirds of compost, peat/coir, and vermiculite. These presets are not proven best for every bed. Suitable materials and proportions depend on crop needs, drainage, existing soil, and whether the bed is open to the ground or on a hard surface. Perlite, sand, and coir have different properties and are not automatically interchangeable. Use local extension and soil-test guidance to choose a mix.
Should I account for soil settling?
Loose materials can settle, and decomposing organic fill can lose volume, but the amount and timing vary. The calculator’s optional allowance adds the selected percentage to the geometric quantity for ordering; it does not predict shrinkage or settled depth. Recheck the fill after installation and replenish as needed, choosing amendments from the material and soil-test results rather than automatically adding compost.
