The formula
Volume is length times width times depth, all in the same unit. Beds are measured in feet and depth in inches, so divide the depth by 12 first.
Then convert as needed: divide cubic feet by 27 for cubic yards, which is how bulk soil is sold, or multiply by 28.32 for liters. For a 4 by 8 foot bed filled 12 inches deep: 4 × 8 × 12 ÷ 12 = 32.0 cubic feet, 1.19 cubic yards, or 906 liters. Measure the inside of the frame: with boards 1.5 inches thick, a bed 4 feet wide outside is 45 inches wide inside, which trims the volume by 6 percent. The volume calculator does the same math for any box, and the guide to cubic yards explains the bulk unit.
Soil for common bed sizes
Cubic feet for each bed at four fill depths, with cubic yards in parentheses:
| Bed (ft) | 6 in | 8 in | 12 in | 18 in |
|---|---|---|---|---|
| 4 × 4 | 8.0 (0.30) | 10.7 (0.40) | 16.0 (0.59) | 24.0 (0.89) |
| 4 × 8 | 16.0 (0.59) | 21.3 (0.79) | 32.0 (1.19) | 48.0 (1.78) |
| 3 × 6 | 9.0 (0.33) | 12.0 (0.44) | 18.0 (0.67) | 27.0 (1.00) |
| 2 × 8 | 8.0 (0.30) | 10.7 (0.40) | 16.0 (0.59) | 24.0 (0.89) |
| 4 × 12 | 24.0 (0.89) | 32.0 (1.19) | 48.0 (1.78) | 72.0 (2.67) |
Doubling the depth doubles the soil. Going from 8 to 12 inches in a 4 by 8 bed adds 10.7 cubic feet, the same as a whole extra 4 by 4 bed at 8 inches.
How many bags
Bagged soil, compost and potting mix state their volume on the bag. Divide the bed volume by the bag size and round up, because a part bag still has to be bought. The bag sizes below are examples; use the number printed on the bag you buy.
| Bed, depth | Cubic feet | 1 cu ft bags | 1.5 cu ft bags | 2 cu ft bags |
|---|---|---|---|---|
| 4 × 4 ft, 12 in | 16.0 | 16 | 11 | 8 |
| 4 × 8 ft, 12 in | 32.0 | 32 | 22 | 16 |
| 3 × 6 ft, 12 in | 18.0 | 18 | 12 | 9 |
| 2 × 8 ft, 12 in | 16.0 | 16 | 11 | 8 |
| 4 × 12 ft, 12 in | 48.0 | 48 | 32 | 24 |
If you move soil by bucket, a 5-gallon bucket holds 5 × 231 ÷ 1,728 = 0.668 cubic feet, so that same bed would take 48 buckets, which is why a bulk delivery starts to make sense around a cubic yard or more.
Splitting the volume into a mix
Extension services recommend a light, well-drained mix rather than plain garden soil. Iowa State University suggests equal parts topsoil, organic matter (well-rotted manure, compost or peat) and coarse sand, or about 70 percent topsoil and 30 percent compost. University of Maryland Extension suggests compost and purchased topsoil at 1:2 or 1:1, or compost and a soilless growing mix at 1:1, and says beds made only of compost or soilless mix dry out faster than beds that contain mineral soil.
The same 32.0 cubic feet for a 4 by 8 bed, 12 inches deep, split by each recipe:
| Mix | Cubic feet of each |
|---|---|
| Compost and topsoil, 1:1 (Maryland) | Compost 16.0, Topsoil 16.0 |
| Compost and topsoil, 1:2 (Maryland) | Compost 10.7, Topsoil 21.3 |
| Compost and soilless mix, 1:1 (Maryland) | Compost 16.0, Soilless mix 16.0 |
| Topsoil, organic matter, coarse sand, equal parts (Iowa State) | Topsoil 10.7, Organic matter 10.7, Coarse sand 10.7 |
| About 70% topsoil, 30% compost (Iowa State) | Topsoil 22.4, Compost 9.6 |
Maryland adds one limit for deep beds on hardscape: topsoil can be added up to 20 percent by volume for beds at least 16 inches deep. In an 18 inch deep 4 by 8 bed that is at most 9.6 of the 48.0 cubic feet. Peat and coconut coir media should be thoroughly moistened before they go in, according to the same page.
Round beds, L shapes and topping up
The same idea works for any shape: find the area in square feet, then multiply by the depth in feet.
- Round bed or stock tank: area is π × radius². A 6 foot round bed filled 12 inches deep holds π × 3² × 1 = 28.3 cubic feet, or 1.05 cubic yards.
- L shape or U shape: split it into rectangles that do not overlap, work out each one and add them.
- Topping up an existing bed: measure how far the soil sits below the rim and fill only that layer. Adding 2 inches to a 4 by 8 bed takes 4 × 8 × 2 ÷ 12 = 5.3 cubic feet, about 4 bags of 1.5 cubic feet.
Maryland's page includes a useful check on this arithmetic: it says 3 cubic yards of compost cover 1,000 square feet with a 1 inch layer. The formula gives 1,000 × 1 ÷ 12 ÷ 27 = 3.09 cubic yards, which rounds to the same figure.
Building on top of lawn
For a bed placed over grass, University of Maryland Extension describes two routes. You can smother the lawn first by covering the area with cardboard, newspaper or weed barrier fabric, which it says can take 6 to 8 weeks. Or, if the bed will be at least 6 inches deep, you can cut the grass and weeds at ground level and cover them with the growing medium, letting the vegetation break down in place. Either way, the volume you buy is the full bed volume from the tables above, since the old lawn adds nothing measurable.
On ordinary garden ground without grass, Maryland suggests spreading 1 to 2 inches of compost on the area before forming beds that have no frame. For a 4 by 8 foot area that is 2.7 to 5.3 cubic feet of compost.
How deep to fill
Depth depends on what is under the bed. Maryland's guidance for a bed on hardscape or heavily compacted soil is at least 8 inches for leafy greens, beans and cucumber and 12 to 24 inches for pepper, tomato and squash. On loosened garden soil the bed is only the top layer, and Maryland describes mixing 4 to 6 inches of compost into the top 4 inches of soil inside a frame. Iowa State suggests adding a few inches of the mix at a time and working it into the existing soil, repeating until the bed is full, so the layers blend instead of forming a sharp boundary.
For mulch on top, the mulch calculator uses the same area times depth math. The guide to planting by frost date covers when to fill the bed with plants.
Frequently asked questions
How much soil do I need for a 4 by 8 raised bed?
Multiply length by width by depth in feet. A 4 by 8 foot bed filled 12 inches deep holds 4 × 8 × 1 = 32 cubic feet, which is about 1.19 cubic yards or 906 liters. At 8 inches deep it holds about 21.3 cubic feet, and at 6 inches 16 cubic feet.
How many bags of soil fill a raised bed?
Divide the volume in cubic feet by the bag size printed on the bag and round up. A 32 cubic foot bed takes 32 one-cubic-foot bags, 22 bags of 1.5 cubic feet or 16 bags of 2 cubic feet. Bags are sold by volume, so check the cubic feet or quarts on the label rather than the weight.
How deep should a raised bed be?
University of Maryland Extension suggests at least 8 inches for leafy greens, beans and cucumber and 12 to 24 inches for pepper, tomato and squash when the bed sits on hardscape or heavily compacted soil. Those figures are for beds on hard or compacted ground; over loosened garden soil, Maryland describes mixing compost into the soil below the frame instead.
What should I fill a raised bed with?
Iowa State University suggests equal parts topsoil, organic matter and coarse sand, or about 70 percent topsoil and 30 percent compost. University of Maryland Extension suggests compost and topsoil at 1:2 or 1:1, or compost and soilless mix at 1:1, and notes that beds of only compost or soilless mix dry out faster than beds with mineral soil.
When is buying soil by the cubic yard cheaper than bags?
Bulk soil is sold by the cubic yard, which is 27 cubic feet, so one yard replaces 27 one-cubic-foot bags. For a single small bed bags are easier to handle; for several large beds compare the bulk price plus delivery with the bag count from the table.
Sources
- University of Maryland Extension: Filling your raised bed (at least 8 inches deep for leafy greens, beans and cucumber, 12 to 24 inches for pepper, tomato and squash on hardscape or compacted soil; compost and topsoil 1:2 or 1:1; compost and soilless mix 1:1; topsoil up to 20% by volume in beds at least 16 inches deep; all-compost beds dry out faster)
- Iowa State University Extension: What would be a good soil mix for a raised bed? (light, well-drained soil; equal parts topsoil, organic matter and coarse sand, or about 70% topsoil and 30% compost; add a few inches and mix into the existing soil)