How Much Soil Does a Raised Bed Need? Cubic Feet, Cubic Yards and Bag Counts
A new raised bed looks small until you start filling it. A single 4 by 8 foot bed a foot deep holds 32 cubic feet of soil, which is thirty-two 1 cubic foot bags or more than a cubic yard. Knowing that number before you shop is the difference between one trip and four.
Every figure in this guide comes from the GardenStuffs Soil Volume Calculator. Choose a rectangular or circular bed, enter the length and width in feet (or the diameter), enter the fill depth in inches, and it returns the volume in cubic feet and cubic yards plus the number of 1, 1.5 and 2 cubic foot bags, each rounded up to a whole bag.
The formula
Volume in cubic feet is length times width times depth, with the depth converted from inches to feet by dividing by 12. Divide cubic feet by 27 to get cubic yards, the unit bulk soil is sold in. For a round bed, the area is pi times the radius squared. The calculator does all of this for you, but it helps to know there is nothing hidden in it.
Common bed sizes
| Bed | Fill depth | Cubic feet | Cubic yards | 1 cu ft bags | 1.5 cu ft bags | 2 cu ft bags |
|---|---|---|---|---|---|---|
| 4 x 8 ft | 6 in | 16 | 0.593 | 16 | 11 | 8 |
| 4 x 8 ft | 10 in | 26.67 | 0.988 | 27 | 18 | 14 |
| 4 x 8 ft | 12 in | 32 | 1.185 | 32 | 22 | 16 |
| 4 x 8 ft | 18 in | 48 | 1.778 | 48 | 32 | 24 |
| 4 x 4 ft | 12 in | 16 | 0.593 | 16 | 11 | 8 |
| 3 x 6 ft | 10 in | 15 | 0.556 | 15 | 10 | 8 |
Use the depth you will actually fill, not the height of the boards. A bed built from two 6-inch boards is about 11 inches tall inside once it sits on the ground, and most people leave an inch or two of rim so water and mulch stay in. A 10-inch fill in a 12-inch bed is a realistic figure.
How deep does the fill need to be?
Depth is the input that moves the number most, so decide it on purpose. A bed built on top of ordinary garden soil that you have loosened with a fork does not need to hold every inch of root itself: roots grow on down into the ground below, and 6 to 10 inches of good fill is often enough for salad crops, herbs and most vegetables. A bed sitting on compacted ground, gravel, a patio or a rooftop has no soil below it to borrow, so it needs the full depth for its crops, and deep-rooted plants such as tomatoes, squash and parsnips do better with 12 inches or more. Going from 12 to 18 inches on a 4 x 8 bed adds 16 cubic feet, half as much soil again, so only build that deep where the site needs it.
Several beds at once
If you are building three 4 x 8 beds a foot deep, the total is the same as one 12 by 8 foot area: 96 cubic feet, or 3.556 cubic yards, or 48 bags of 2 cubic feet. At that size, bulk delivery is usually worth pricing. Ask suppliers for a raised bed or garden mix rather than plain fill, ask what it contains, and remember that a cubic yard of moist soil is very heavy, far more than most car trailers and every car trunk should carry. Have it delivered onto a tarp close to the beds.
Splitting the volume between topsoil and compost
The calculator gives the total volume; the mix is your choice. A blend of screened topsoil with a generous share of compost is a common starting point for vegetable beds. As an example only, a 60 to 40 split of a 4 x 8 bed filled 12 inches deep is:
- Topsoil: 19.2 cubic feet, which is 13 bags of 1.5 cubic feet or 10 bags of 2 cubic feet.
- Compost: 12.8 cubic feet, which is 9 bags of 1.5 cubic feet or 7 bags of 2 cubic feet.
Do not fill a bed with bagged potting mix alone unless it is a container or a very shallow bed; potting mix is designed for pots and is costly at this volume. Do not fill it with compost alone either, which shrinks a great deal as it breaks down. Mix the two materials on a tarp or in layers that you fork together, then water well and let the bed settle for a few days before planting.
Settling and topping up
Fresh soil settles after the first few soakings, often by an inch or more in a deep bed. Rather than overfilling at the start, plan to top up. A 2-inch layer of compost on a 4 x 8 bed is 5.33 cubic feet, or 3 bags of 2 cubic feet. Adding that each spring keeps the level up and feeds the soil at the same time; see how much compost to add to a garden bed.
Round beds and stock tanks
For a round planter or a stock tank, choose the circular shape. A tank 3 feet across filled 24 inches deep takes 14.14 cubic feet, or 8 bags of 2 cubic feet. Deep containers like this drain best when the whole depth is a free-draining mix, so do not assume the bottom can be filled with garden soil.
What the calculator does not cover
- Weight. It reports volume only. Soil weight varies with the material and how wet it is, so check what your wheelbarrow, vehicle and back can safely move.
- Settling and compaction. The result is the volume of the space. Loose material settles, so expect to top up.
- Irregular shapes. L-shaped or keyhole beds can be split into rectangles and circles and added together.
- Soil quality. Volume says nothing about whether a delivery is good growing soil. Ask for a description of the mix, look at it before it is tipped if you can, and consider a soil test before you plant.
- What goes in afterward. For how many plants the bed will hold and which to keep apart, see planning a 4 x 8 raised bed and the Plant Spacing Calculator.
Safety for food beds
- Old soil and old sites. Soil near older houses, painted structures and busy roads can contain lead. If you plan to reuse soil from around the property for food beds, have it tested first, or buy new soil for vegetable beds.
- Old treated timber. Lumber pressure-treated before the mid-2000s was often treated with CCA, which contains arsenic, and old painted wood may carry lead paint. Do not reuse either for food beds. Newer treated lumber uses different preservatives; if you are unsure, line the inside of the bed or choose untreated rot-resistant wood, stone or block.
- Digging. If you are digging out the base or setting posts, call your utility-locate service before you dig (811 in the United States) so buried cables and pipes are marked.
- Lifting. Bags of soil are heavy. Lift with your knees, keep the bag close, use a wheelbarrow or hand truck, and split large loads rather than straining.
- Gloves and hygiene. Wear gloves, keep cuts covered, wash your hands after handling soil and compost, and keep your tetanus vaccination up to date.
Measure each bed, decide how deep you will fill it, run the numbers, and you will buy the right amount the first time.