Calculate soil volume by bed dimensions
The amount of soil you need depends on three measurements: the length, width, and depth of your raised bed. Multiply those three numbers together to get the volume in cubic inches, then divide by 1,728 to convert to cubic feet. Most garden soil is sold by the cubic foot or cubic yard, so working in cubic feet makes shopping straightforward.
For example, a bed that is 4 feet long, 2 feet wide, and 12 inches deep needs 8 cubic feet of soil. Here's the math: 4 × 2 × 1 (converting 12 inches to 1 foot) = 8 cubic feet. A bed that is 8 feet long, 4 feet wide, and 10 inches deep needs about 26.7 cubic feet—round up to 27 when you order.
If your bed has an unusual shape—an L-shape, a trapezoid, or a circle—break it into rectangles or use the formula for that shape, calculate each section separately, then add them together. A circular bed 4 feet across and 12 inches deep needs about 12.6 cubic feet.
Key Takeaways
- Multiply length × width × depth (in feet) to find how many cubic feet of soil your bed needs.
- Most raised beds are 10 to 12 inches deep, but vegetables with deep roots like tomatoes and carrots need 12 to 18 inches.
- One cubic yard of soil fills about 27 cubic feet and typically costs between $30 and $60 depending on your region and soil quality.
- Soil settles over time, so fill your bed slightly higher than the rim and expect it to drop 1 to 2 inches in the first few weeks.
- Mixing topsoil, compost, and other amendments is cheaper and often better for plants than buying bagged potting mix for large beds.
Standard depths for different plants
The depth you choose affects how much soil you buy and what you can grow. Shallow-rooted plants—lettuce, spinach, radishes, and herbs—need only 6 to 8 inches. If you're growing mostly salad greens, a 6-inch bed is workable and uses less soil.
Medium-rooted plants—beans, beets, chard, and peppers—do well in 10 to 12 inches. This is the most common depth for raised beds because it balances cost, ease of building, and what most home gardeners want to grow.
Deep-rooted plants—tomatoes, carrots, parsnips, and root crops—need 12 to 18 inches to develop properly. Tomatoes especially benefit from deeper soil because their roots can reach down and find moisture during dry spells. If you plan to grow a mix, go with 12 inches as a practical middle ground.
Soil blends and where to buy
You have two main routes: buy bagged potting mix or order bulk soil and amend it yourself. Bagged potting mix (brands like Miracle-Gro, Espoma, or store brands) is convenient for small beds but becomes expensive fast. A 2-cubic-foot bag costs $5 to $15, so filling a 4 × 8 × 1-foot bed (32 cubic feet) would cost $80 to $240 in bags alone.
Bulk soil from a landscape supply yard or garden center is cheaper per cubic foot—usually $1.50 to $3 per cubic foot for topsoil, less if you buy a full cubic yard. Many suppliers will deliver for a fee ($50 to $150 depending on distance). You can also mix your own: combine topsoil, compost, and peat moss or coconut coir in a ratio like 60% topsoil, 30% compost, and 10% peat moss. This blend drains well, holds nutrients, and costs less than premium potting mix.
Check whether your supplier's soil is screened (free of rocks and debris) and whether it includes compost or amendments. Screened topsoil is worth the extra cost because you won't spend hours picking out stones.
Account for settling and future additions
Soil compacts and settles after watering and over the growing season. Expect your bed to drop 1 to 2 inches in the first few weeks, and another inch or so over the first year. Fill your bed slightly higher than the rim—about 1 to 2 inches above—so it looks full after settling.
Each year, add 1 to 2 inches of compost or aged manure to the top. This replaces organic matter that breaks down and keeps the soil fertile. Over time, you'll need to top up, so budget for a few bags of compost or a small bulk delivery each spring.
If you're building a very large bed or multiple beds, consider ordering slightly more soil than your calculation suggests. It's easier to use extra soil to top up in spring than to place a second order.
Common mistakes that waste soil or money
The biggest mistake is underestimating depth. A bed that looks deep enough when empty often feels shallow once soil settles. Measure twice before you order, and remember that 10 inches is the practical minimum for most vegetables.
Another common error is buying only potting mix for a large bed. A 4 × 8 × 1-foot bed in potting mix alone can cost $150 to $300. The same bed in bulk topsoil and compost costs $40 to $80. Potting mix is designed for containers where drainage matters most; raised beds benefit from a heavier, cheaper blend.
Don't assume all "garden soil" or "topsoil" is the same. Some is mostly clay or sand with little organic matter. Ask your supplier what's in it or look for soil labeled as a "garden mix" or "raised bed mix," which usually includes compost already blended in.
Quick reference table for common bed sizes
| Bed Size (L × W × D) | Cubic Feet Needed | Cubic Yards Needed | Approximate Bulk Cost* |
|---|---|---|---|
| 4 × 2 × 1 ft | 8 cu ft | 0.3 cu yd | $12–$24 |
| 4 × 4 × 1 ft | 16 cu ft | 0.6 cu yd | $24–$48 |
| 4 × 8 × 1 ft | 32 cu ft | 1.2 cu yd | $48–$96 |
| 4 × 8 × 1.5 ft | 48 cu ft | 1.8 cu yd | $72–$144 |
| 6 × 6 × 1 ft | 36 cu ft | 1.3 cu yd | $54–$108 |
*Costs vary by region and supplier. Prices shown are for bulk topsoil or garden mix; potting mix costs roughly 3 to 5 times more per cubic foot.
Frequently Asked Questions
Can I use less soil if I put cardboard or newspaper underneath?
Cardboard or newspaper under the bed prevents weeds from growing up through the bottom, but it doesn't reduce the soil you need. The soil depth is determined by what your plants need to root into. You can use cardboard as a base layer, but fill the bed to the full depth your vegetables require.
What if I order too much soil?
Extra soil is rarely wasted. You can use it to top up the bed in spring, fill containers, amend garden beds elsewhere on your property, or offer it to a neighbor. Soil keeps for years if stored dry, so it's better to have a little left over than to run short and need a second delivery.
Do I need to mix in compost, or can I use straight topsoil?
Straight topsoil works, but plants grow better in a blend. Topsoil alone can compact and drain poorly. Mixing in 20 to 30 percent compost improves drainage, adds nutrients, and creates a fluffier texture roots prefer. The extra cost is small compared to the improvement in plant growth.
How do I know if my soil is good quality?
Good soil is dark brown or black, crumbly, and smells earthy. It should hold together slightly when squeezed but break apart easily. Avoid soil that is pale, compacted, or smells sour or chemical. Ask your supplier whether the soil has been tested or screened, and whether it contains any weed seeds or debris.
Should I fill the bed all the way to the top?
Fill it to about 1 to 2 inches below the rim so water doesn't spill over when you water. The soil will settle, so starting slightly high (1 to 2 inches above the rim) is fine—it will drop to the right level within a few weeks.