Calculate soil volume by bed size and depth
The amount of dirt you need depends on three measurements: the length, width, and depth of your raised bed. Multiply these three numbers together to get the volume in cubic feet, then convert to cubic yards (the standard unit soil is sold in). One cubic yard equals 27 cubic feet.
For example, a bed that is 4 feet long, 2 feet wide, and 12 inches deep holds 8 cubic feet of soil. Divide 8 by 27 to get 0.3 cubic yards. Most garden centers sell soil by the bag or by the cubic yard, so knowing this number lets you buy the right amount without waste or shortage.
A quick shortcut: multiply length × width × depth (in feet), then divide by 27. If your depth is in inches, convert it to feet first by dividing by 12.
Key Takeaways
- Measure your bed's length, width, and depth in feet, multiply them together, then divide by 27 to find cubic yards of soil needed.
- A standard 4-by-8-foot bed at 12 inches deep requires about 1 cubic yard of soil.
- Soil settles over time, so fill your bed slightly higher than the rim when you first plant, expecting it to drop 1 to 2 inches within a few weeks.
- Bagged soil is convenient for small beds but costs more per cubic foot than bulk soil delivered by the yard.
- Most vegetables need 12 inches of soil depth minimum; root crops and perennials may need 18 inches or more.
Common raised bed sizes and their soil needs
A 4-by-8-foot bed at 12 inches deep is the most common garden size and requires about 1 cubic yard of soil. A smaller 4-by-4-foot bed at the same depth needs 0.5 cubic yards. If you build your bed 18 inches deep instead of 12, add 50 percent more soil to the calculation.
Here are typical combinations:
| Bed dimensions (L × W × D) | Cubic feet | Cubic yards |
|---|---|---|
| 4 ft × 4 ft × 12 in | 16 | 0.6 |
| 4 ft × 8 ft × 12 in | 32 | 1.2 |
| 3 ft × 6 ft × 12 in | 18 | 0.7 |
| 4 ft × 8 ft × 18 in | 48 | 1.8 |
| 2 ft × 4 ft × 12 in | 8 | 0.3 |
If your bed has an unusual shape—circular, triangular, or irregular—sketch it on graph paper and divide it into rectangles or triangles, calculate each section separately, then add them together.
Account for settling and compaction
New soil compacts and settles within the first few weeks after you fill your bed, especially if you water it heavily or walk on it. Expect the soil level to drop 1 to 2 inches. To avoid ending up with a bed that looks half-empty by midsummer, fill it slightly higher than the rim when you first plant.
If you are using a mix of materials—such as compost, peat moss, and topsoil blended together—the settling happens faster than with uniform soil. Compost-heavy mixes can drop 3 inches or more. Mix your materials in a pile on the ground first, then shovel the combined blend into the bed so it settles evenly.
Bagged soil versus bulk delivery
Bagged soil costs roughly $3 to $8 per bag, depending on the brand and your location. A standard bag holds 2 cubic feet, so you would need 13 to 14 bags to fill a 4-by-8-foot bed at 12 inches deep. That works out to $40 to $110 for one bed.
Bulk soil delivered by the cubic yard typically costs $25 to $50 per yard, depending on the soil type and how far the supplier must travel. For the same 4-by-8-foot bed, one cubic yard costs $25 to $50 total. Bulk is cheaper per cubic foot but requires a minimum order (often 1 to 2 cubic yards) and a delivery fee if you do not have a supplier nearby.
For a single small bed, bags make sense. For multiple beds or a large garden, bulk delivery saves money and labor. Check whether your supplier offers a mix of topsoil and compost, or whether you need to blend materials yourself.
Depth requirements for different plants
Most vegetables grow well in 12 inches of soil. Shallow-rooted crops like lettuce, spinach, and herbs need only 8 to 10 inches. Deeper-rooted plants demand more: carrots, parsnips, and beets need 12 to 18 inches; tomatoes and peppers need 12 to 16 inches; and potatoes need 12 inches minimum but produce better in 18 inches.
Perennials like asparagus and rhubarb should have 18 inches of soil to establish strong root systems. If you plan to grow a mix of plants in one bed, build it to the depth the deepest-rooted plant needs, then use raised sections or containers within the bed for shallow-rooted crops.
Shallow beds (6 to 8 inches) work for microgreens, radishes, and green onions, but they dry out faster and limit your plant choices. Most gardeners find 12 inches a practical minimum for a general-purpose bed.
Adjust for existing soil and amendments
If your raised bed sits on bare ground and you want to use the existing soil as a base layer, you can reduce the amount of new soil you buy. A 4-inch layer of existing soil at the bottom of a 12-inch-deep bed means you only need to add 8 inches of new soil on top. This saves roughly one-third of your material cost.
Many gardeners use a layering method: 2 to 3 inches of coarse compost or aged manure at the bottom for drainage and nutrients, then 8 to 10 inches of topsoil or a topsoil-compost blend on top. This approach uses less pure topsoil and improves soil quality without buying premium mixes.
If you amend with peat moss, coir, or perlite to improve drainage or water retention, these materials are often sold by the bag in smaller volumes. Calculate your base soil need first, then add amendments as a percentage—typically 10 to 20 percent of the total volume.
Frequently Asked Questions
Do I need to fill the entire bed with expensive potting mix?
No. Potting mix is designed for containers and is too expensive for raised beds. Use topsoil, garden soil, or a blend of topsoil and compost instead. Potting mix drains too quickly for in-ground beds and costs three to five times more per cubic foot than bulk soil.
What if I buy too much soil?
Extra soil keeps for years if stored dry. Pile it in a corner of your yard, cover it with a tarp to keep weeds out, and use it next season to top off beds as they settle or to fill new beds. Leftover bagged soil can be returned to most garden centers within 30 days with a receipt.
Can I use just compost instead of topsoil?
Pure compost drains too fast and compacts heavily over time. Mix compost with topsoil at a ratio of 1 part compost to 2 or 3 parts topsoil for the best balance of nutrients, drainage, and structure. A 50-50 blend works too but costs more and may retain too much moisture in clay climates.
How do I know if my soil is good quality?
Good garden soil is dark brown, crumbly, and smells earthy. It should not be hard and compacted, smell sour or chemical, or contain visible rocks, roots, or debris. Ask the supplier what is in the mix—topsoil alone is fine, but topsoil blended with compost is better for vegetables.
Should I line the bottom of my raised bed before adding soil?
A landscape fabric or hardware cloth at the bottom keeps burrowing pests like gophers and voles out but is optional if you do not have a pest problem. It slows drainage slightly, so skip it if your bed is in a wet area. If you use it, lay it flat and overlap seams by 6 inches, then add soil on top.