How Much Soil for a Raised Bed: Depth by Crop, Mix and Bags (Plus Gravel for a Patio Base)
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A 4 × 8-foot raised bed holds 21.3 cubic feet of soil at 8 inches deep and 32 cubic feet at 12 inches, before any settling. The volume is simple geometry: inside length × inside width × fill depth. What goes into that volume, and how deep it should be, depends on what you plan to grow, and university extension services give specific ranges for both.
This page covers the volume math, depth by crop, what the soil labels mean, how to buy by the bag or the yard, and, because the same volume-to-order steps apply, how much gravel a patio base takes.
How deep a raised bed should be
University of Maryland Extension gives minimum depths for raised beds placed on a hard surface such as a driveway, where roots cannot reach the ground below:
| Crops | Minimum depth on a hard surface |
|---|---|
| Leafy greens, beans, cucumbers | 8 inches |
| Peppers, tomatoes, squash | 12–24 inches |
Beds on open ground can be shallower because roots grow into the soil underneath, especially if that soil drains well and is loosened before filling.
Soil volume for a 4 × 8 bed
| Fill depth | Calculation | Cubic feet | Cubic yards | 1.5-cu-ft bags | 2-cu-ft bags |
|---|---|---|---|---|---|
| 6 in | 32 × 6 ÷ 12 | 16.0 | 0.59 | 11 | 8 |
| 8 in | 32 × 8 ÷ 12 | 21.3 | 0.79 | 15 | 11 |
| 10 in | 32 × 10 ÷ 12 | 26.7 | 0.99 | 18 | 14 |
| 12 in | 32 × 12 ÷ 12 | 32.0 | 1.19 | 22 | 16 |
| 18 in | 32 × 18 ÷ 12 | 48.0 | 1.78 | 32 | 24 |
| 24 in | 32 × 24 ÷ 12 | 64.0 | 2.37 | 43 | 32 |
A cubic yard is exactly 27 cubic feet, so a 12-inch 4 × 8 bed needs 1.19 yards; one yard would leave it 5 cubic feet short. Measure the inside of the frame and the intended fill line. Thick boards make the interior smaller than the nominal size, and most beds are filled to a little below the top.
For any rectangle: cubic feet = length × width × depth in inches ÷ 12, and bags = cubic feet ÷ bag volume, rounded up. Read the bag size on the label; bagged soil comes in 1, 1.5 and 2 cubic foot sizes.
What to fill it with
“Topsoil,” “garden soil” and “raised-bed mix” are retail names, not standard recipes. Topsoil is mineral soil whose texture and organic content vary by source. Compost is decomposed organic matter whose feedstock, maturity and salt content vary. Garden and raised-bed soils are blends whose proportions the name does not reveal. Read the ingredient statement and any analysis on the back of the bag.
The extension services give these benchmarks:
- Organic matter share. University of Maryland Extension says a raised bed containing garden soil should have 25 to 50 percent organic matter by volume, or 5 to 15 percent by weight as measured by a soil lab. It allows topsoil up to 20 percent by volume in beds at least 16 inches deep.
- Not compost alone. Oregon State University Extension cautions against growing in compost by itself; it is an amendment mixed into soil.
- Too much organic matter is possible. Rutgers notes that 3 to 5 percent organic matter by weight is considered high for native soils in New Jersey and that more than 10 percent is not recommended for general garden use.
- Improving existing ground. Rutgers suggests 1 to 2 inches of compost mixed into the top 6 inches of soil; Maryland suggests 2 to 4 inches of compost mixed into the top 4 inches where the existing soil is in good condition.
If you blend your own, multiply the total volume by each component’s share. A 32-cubic-foot bed at a chosen one-third compost is about 10.7 cubic feet of compost and 21.3 cubic feet of the other material. A soil test tells you what your existing soil needs.
Mixes settle after watering, by an amount that depends on the material. Ask the supplier about the product, and remeasure before buying a top-up rather than overfilling at the start.
Bags or bulk
Compare orders on the quantity the supplier actually sells. For the 32-cubic-foot bed:
| Sale unit | Order that covers 32 cu ft | Delivered volume |
|---|---|---|
| 1-cu-ft bags | 32 bags | 32 cu ft |
| 1.5-cu-ft bags | 22 bags | 33 cu ft |
| 2-cu-ft bags | 16 bags | 32 cu ft |
| Bulk in quarter-yard steps | 1.25 cu yd | 33.75 cu ft |
| Bulk in half-yard steps | 1.5 cu yd | 40.5 cu ft |
Multiply each by its price, then add delivery, minimum-order charges and tax. Oregon State’s rule of thumb for compost works the same way: one 1-cubic-foot bag covers 4 square feet at 3 inches, so 100 square feet at 3 inches takes 25 bags. Some suppliers sell by weight; ask for that product’s tons-per-yard figure, because moisture and composition change it.
Gravel for a patio base: volume, then loose volume, then tons
A gravel base involves three numbers: the compacted volume you design, the larger loose volume that gets delivered, and the weight the supplier bills.
- Compacted design volume. For a 10 × 20-foot patio and a specified compacted base of 4 inches: 10 × 20 × 4 ÷ 12 = 66.7 cubic feet, or 2.47 cubic yards. The base depth is a design decision that depends on soil, drainage, frost and use; paver manufacturers such as Belgard give their own guidance, and 4 inches here is only an example.
- Loose volume. Gravel compacts, so more loose material is needed. Get the loose-to-compacted factor for the exact material from the supplier or designer. At a factor of 1.20, 2.47 compacted yards becomes 2.96 loose yards; at 1.10 it is 2.72 and at 1.25 it is 3.09.
- Tons. Aggregate is usually sold by weight: tons = loose cubic yards × the supplier’s tons per cubic yard. At 1.50 tons per yard, 2.96 yards is 4.44 tons; at 1.35 it is 4.00 and at 1.65 it is 4.89.
Because moisture, rock type and gradation change density, a delivered weight ticket can differ from a volume estimate without either being wrong. Confirm the material, factor, density, delivery increments and minimum order with the supplier before ordering. This arithmetic does not design the base: excavation, drainage slope, geotextile, lift thickness and compaction come from the paver’s installation instructions.
Sources
Extension pages were read on October 4, 2026.
- University of Maryland Extension: Soil to fill raised beds
- Oregon State University Extension: How to use compost in gardens and landscapes (EM 9308)
- Rutgers Cooperative Extension: Raised bed gardening (FS1328)
- Belgard: Installation guidance
Enter the inside dimensions, depth, bag size and local prices in the mulch and soil calculator to compare bags and bulk. These are general-information estimates, not gardening or construction advice.