You need 21 to 22 bags of standard potting soil for an 8-by-4 raised bed filled to 10 inches deep
An 8-by-4 raised bed holds roughly 267 cubic feet of soil when filled 10 inches deep, which is the standard depth for most vegetables. A standard bag of potting soil contains 2 cubic feet, so you will need about 21 to 22 bags. If you fill it deeper — say 12 inches for root vegetables like carrots or potatoes — you will need 28 to 30 bags instead.
The exact number depends on three things: how deep you fill the bed, whether you use pure potting soil or a mix of soil and compost, and the brand of soil you buy. Some bags compress more than others, so you may need one or two extra bags to reach the top.
Key Takeaways
- A standard 8-by-4 raised bed filled 10 inches deep requires 21 to 22 bags of 2-cubic-foot potting soil.
- Filling to 12 inches deep — better for root crops — requires 28 to 30 bags instead.
- Mixing potting soil with compost or garden soil can reduce the total bags needed and lower your cost.
- Soil settles over time, so fill the bed slightly higher than your target depth to account for settling.
How to calculate soil for your specific bed depth
The math is straightforward: multiply length by width by depth in feet, then divide by 2 (the cubic feet per bag). For an 8-by-4 bed, that is 8 × 4 × depth ÷ 2.
At 8 inches deep, you need 128 cubic feet, or about 16 bags. At 10 inches, you need 160 cubic feet, or about 21 bags. At 12 inches, you need 192 cubic feet, or about 29 bags. If your bed is not a standard 2-cubic-foot bag size — some bags hold 1.5 cubic feet or 3 cubic feet — adjust the division accordingly.
Mixing soil to stretch your budget
You do not have to fill the entire bed with potting soil. Many gardeners use a layered approach: 4 to 6 inches of compost or aged manure on the bottom, then 4 to 6 inches of potting soil on top. This cuts the potting soil needed roughly in half while still giving plants the drainage and nutrients they need.
Garden soil from your yard can also work for the bottom layers, though it compacts more than potting soil and may contain weed seeds. Reserve the potting soil for the top 6 inches where plant roots spend most of their time. If you use this method for an 8-by-4 bed, you might need only 10 to 12 bags of potting soil instead of 21.
Why depth matters for different crops
Shallow-rooted plants like lettuce, spinach, and herbs do fine in 6 to 8 inches of soil. Medium-rooted crops like tomatoes, peppers, and beans need 10 to 12 inches. Deep-rooted crops like carrots, parsnips, and potatoes need 12 to 14 inches to reach full size without hitting the bottom of the bed.
If you plan to grow a mix of crops, aim for 10 to 12 inches as a middle ground. You can always add more soil later if you find you need it, though it is harder to remove soil once the bed is planted.
Accounting for soil settling and compaction
Soil settles over the first few weeks as water moves through it and air pockets collapse. Fill your bed about 1 to 2 inches higher than your target depth to account for this settling. If you want the final soil level to be 10 inches, fill to 11 or 12 inches initially.
After the first season, you will lose some soil to settling and to plants being harvested with roots attached. Plan to add 2 to 4 inches of fresh compost or potting soil each spring to top it back up.
Comparing soil types and their coverage
| Soil Type | Typical Bag Size | Bags Needed for 10 Inches Deep | Cost Range Per Bag |
|---|---|---|---|
| Potting soil (pure) | 2 cubic feet | 21 to 22 | $3 to $8 |
| Potting mix (with perlite) | 2 cubic feet | 21 to 22 | $4 to $10 |
| Garden soil | 2 cubic feet | 21 to 22 | $2 to $5 |
| Compost | 2 cubic feet | 10 to 12 (for layering) | $3 to $7 |
| Topsoil | 2 cubic feet | 21 to 22 | $2 to $4 |
Potting soil drains well and holds moisture evenly, making it the best choice for raised beds, but it costs more than garden soil or topsoil. If budget is tight, use potting soil for the top 6 inches and fill the bottom with cheaper topsoil or garden soil. Compost is nutrient-rich but compacts quickly, so use it as a layer rather than the entire fill.
The choice between soil types also depends on what you are growing. Vegetables that need consistent moisture — like tomatoes and peppers — benefit from potting soil's water retention. Herbs and root crops tolerate garden soil or topsoil mixed with compost better.
Where to buy soil in bulk
For 21 bags, buying at a garden center or big-box store works if you have a truck or can arrange delivery. Home Depot, Lowe's, and local nurseries all stock 2-cubic-foot bags. Some garden centers offer bulk soil by the cubic yard, which is cheaper per cubic foot if you can haul it yourself — 10 cubic yards costs less than 50 bags but requires a trailer.
Check whether the store offers free delivery on large orders. Some do if you spend over a certain amount. Bagged soil is heavier than it looks — 21 bags weighs roughly 500 to 600 pounds — so factor in delivery or pickup logistics before you order.
Frequently Asked Questions
Can I use less soil if I add drainage material at the bottom?
Adding gravel or landscape fabric at the bottom does not reduce the soil you need — it just changes what goes underneath. The soil depth is what matters for root growth. If you want to use less soil, layer it with compost or garden soil instead.
How much soil do I need if my raised bed is 6 inches deep instead of 10?
A 6-inch-deep 8-by-4 bed needs about 13 bags of 2-cubic-foot soil. This works for shallow-rooted crops like lettuce and herbs but is too shallow for tomatoes or peppers.
Should I buy potting soil or garden soil?
Potting soil drains better and is lighter, making it ideal for raised beds. Garden soil is heavier and compacts more but costs less. Many gardeners use potting soil for the top 6 inches and garden soil or compost below.
Will one bag of soil go further if I break it up more?
No. A bag contains a fixed volume of soil. Breaking it up or mixing it does not create more volume — it just changes how it is distributed. You still need 21 to 22 bags to fill an 8-by-4 bed to 10 inches.
Can I reuse soil from last year?
Yes. Soil from last year can be reused, though it will have settled and lost some nutrients. Mix in 2 to 4 inches of fresh compost or potting soil to refresh it before planting again.