To find how much soil a raised bed needs, use the raised bed soil calculator below or multiply its inside length and width in feet by the depth in inches, divide by 12, and add about 10 percent for settling. A 4 × 8 ft bed filled 12 inches deep holds 32 cubic feet, so plan on about 35 cu ft: roughly 1.3 cubic yards, 36 bags of 1 cu ft, or 24 bags of 1.5 cu ft.
If you are filling several beds of the same size, enter the number of beds and the calculator multiplies for you.
How to measure a raised bed
- Measure the inside, not the outside. Frame boards take up room. A 2x board is actually 1.5 inches thick, so a bed that measures 8 × 4 ft on the outside is about 7.75 × 3.75 ft inside. For a bed 12 inches deep, that is about 3 cu ft less soil than the outside size suggests, and the gap multiplies with every bed.
- Measure the fill depth in inches. Measure from the ground (or the bottom of the bed) to where you want the soil to stop. If you like to leave an inch or two below the rim for mulch and watering, measure to that line instead of the top edge.
- Convert odd sizes to feet. A bed 45 inches wide is 45 ÷ 12 = 3.75 ft. Enter decimals; the calculator accepts them.
- Count identical beds. If your beds are different sizes, run each one separately and add the results.
- Check what is already in the bed. If you are topping up an old bed, measure only the empty space at the top, not the full height.
The raised bed soil formula
The calculator uses this math:
- Volume (cu ft) = length (ft) × width (ft) × depth (in) ÷ 12 × number of beds × 1.10
- Cubic yards = cubic feet ÷ 27
- Bags = cubic feet ÷ bag size, rounded up
The × 1.10 adds 10 percent for settling. Fresh fill has little soil structure, so it sinks over time, and regular digging and planting add to it. University of Illinois Extension calls settling "simply a fact of raised bed gardening," so it makes sense to buy a little more than the empty box holds.
Worked example (the calculator's default values)
One bed, 8 ft long, 4 ft wide, filled 12 inches deep:
- Raw volume: 8 × 4 × 12 ÷ 12 = 32 cu ft
- Add 10 percent for settling: 32 × 1.10 = 35.2 cu ft
- Cubic yards: 35.2 ÷ 27 = 1.30 cubic yards
- 1 cu ft bags: 35.2, rounded up to 36 bags
- 1.5 cu ft bags: 35.2 ÷ 1.5 = 23.5, rounded up to 24 bags
The calculator returns: "You need about 35.2 cu ft of soil (1.30 cubic yards). That is 36 bags of 1 cu ft or 24 bags of 1.5 cu ft, including 10% for settling."
Soil needed for common raised bed sizes
All volumes below include the calculator's 10 percent settling allowance and assume you fill to the full depth listed.
| Bed size (L × W × depth) | Soil (cu ft) | Cubic yards | 1 cu ft bags | 1.5 cu ft bags |
|---|---|---|---|---|
| 4 × 4 ft × 6 in | 8.8 | 0.33 | 9 | 6 |
| 4 × 4 ft × 12 in | 17.6 | 0.65 | 18 | 12 |
| 4 × 8 ft × 6 in | 17.6 | 0.65 | 18 | 12 |
| 2 × 8 ft × 12 in | 17.6 | 0.65 | 18 | 12 |
| 3 × 6 ft × 12 in | 19.8 | 0.73 | 20 | 14 |
| 4 × 8 ft × 12 in | 35.2 | 1.30 | 36 | 24 |
| 4 × 8 ft × 18 in | 52.8 | 1.96 | 53 | 36 |
| 4 × 12 ft × 12 in | 52.8 | 1.96 | 53 | 36 |
| 4 × 8 ft × 24 in | 70.4 | 2.61 | 71 | 47 |
Notice how fast volume grows with depth. Going from 12 to 24 inches doubles the soil, which is why deep beds are often filled with bulk soil by the cubic yard rather than bags.
How deep should the soil be?
It depends on what you grow and what the bed sits on. University of Maryland Extension gives these minimum depths for beds built on hard surfaces such as patios, where roots can't reach the ground below:
| Crops | Soil depth |
|---|---|
| Leafy greens, beans, cucumbers | At least 8 in |
| Peppers, tomatoes, squash | 12 to 24 in |
| Beds on pavement, if topsoil is part of the mix | At least 16 in |
If the bed sits on open ground and you loosen the soil underneath first, as Iowa State Extension suggests, roots can keep growing below the frame. On a patio or driveway, the frame is all the root room there is.
What to fill a raised bed with
The calculator tells you how much. Extension services agree on the what: a blend of mineral soil and compost, mixed well.
- University of Minnesota Extension: 1/2 to 2/3 topsoil with 1/2 to 1/3 plant-based compost. It advises against potting soil, which dries out too quickly in a bed.
- Iowa State University Extension: about 70 percent topsoil and 30 percent compost, or equal parts topsoil, organic matter, and coarse sand.
- University of Maryland Extension: for beds on a hard surface, compost and a soilless growing mix in a 1:1 ratio, with topsoil added up to 20 percent by volume only if the bed is at least 16 inches deep.
To split your total, multiply by each fraction. For the 35.2 cu ft example with a two-thirds topsoil blend, that is about 23.5 cu ft of topsoil and 11.7 cu ft of compost. With Iowa State's 70/30 blend, it is about 24.6 cu ft of topsoil and 10.6 cu ft of compost.
Iowa State also recommends adding the mix a few inches at a time and working it into the layer below. Distinct layers slow water movement and discourage root growth.
Bagged vs. bulk soil
Bagged raised bed soil is commonly sold in 1 and 1.5 cu ft bags. Bags are easy to move with a car and a hand truck, which suits one or two beds. For several deep beds, get a quote on bulk soil sold by the cubic yard; it saves hauling dozens of bags, though you will need a place to dump it and a wheelbarrow.
Read the bag label, too. Some manufacturers' fill guidance calls for more product than the raw volume of the bed. The label for Miracle-Gro Organic Raised Bed and Garden Soil, for example, calls for two 1.5 cu ft bags (3 cu ft) to fill a 2 × 2 ft bed 6 inches deep, a space of 2 cu ft. If your chosen product has label guidance like that, follow it; it may call for more bags than the calculator's 10 percent allowance.
Also check what is actually in the bag. University of Maryland Extension notes that bagged "organic soil" products typically contain no true topsoil. They are blends of materials such as wood waste, bark fines, compost, and peat moss.
Common mistakes to avoid
- Using outside dimensions. Board thickness shrinks the box. Measure inside.
- Filling with potting mix alone. It is made for containers and dries too quickly in a garden bed.
- Skipping the settling allowance. A bed filled exactly to the rim will sit noticeably lower after a few waterings.
- Dumping in layers. Mix as you fill so water and roots move evenly.
- Building beds too wide. University of Minnesota Extension suggests no more than about 5 ft wide if you can reach from both sides, and 2.5 ft if you can only reach from one.
Frequently asked questions
How many bags of soil do I need for a 4 × 8 raised bed?
For 12 inches deep, about 36 bags of 1 cu ft or 24 bags of 1.5 cu ft, including 10 percent for settling. For 6 inches deep, about 18 bags of 1 cu ft or 12 bags of 1.5 cu ft.
How much soil is in a cubic yard?
One cubic yard is 27 cubic feet. That fills a 4 × 8 ft bed about 10 inches deep before settling, or roughly 27 bags of 1 cu ft.
How deep should a raised bed be?
For beds on pavement or another hard surface, University of Maryland Extension calls for at least 8 inches for greens, beans, and cucumbers, and 12 to 24 inches for tomatoes, peppers, and squash. On open ground, loosening the soil underneath gives roots more room. On hot pavement, University of Minnesota Extension suggests raising beds 1 to 2 feet.
Can I use topsoil alone to fill a raised bed?
Extension guidance recommends blending topsoil with compost rather than using it alone. A common starting point is about two-thirds topsoil and one-third compost, mixed thoroughly.
Do I need to replace raised bed soil every year?
No. University of Illinois Extension recommends an annual top dressing of a few inches of compost or good topsoil to make up for settling. Use the calculator with that depth to size your top-up.
Does the calculator include settling?
Yes. It adds 10 percent to the raw volume. If your bag label calls for more product per cubic foot of space, go by the label.
Sources
- University of Maryland Extension: Soil to Fill Raised Beds
- University of Minnesota Extension: Raised Bed Gardens
- Iowa State University Extension: What would be a good soil mix for a raised bed?
- University of Illinois Extension: Refreshing raised bed soil generates exceptional results
- Miracle-Gro Organic Raised Bed and Garden Soil, 1.5 cu ft (product label details)
- Southern Pine Inspection Bureau: Nominal vs. Actual Lumber Sizes