Calculate soil volume by your bed's length, width, and depth
The amount of dirt you need depends on three measurements: how long your raised bed is, how wide it is, and how deep you fill it. Most raised beds work best at 10 to 12 inches deep, though you can go shallower for leafy greens or deeper for root vegetables. Once you know those three numbers, multiply them together and divide by 1,728 to get cubic feet—that's the standard unit garden centers use to sell soil.
For example, a 4-foot-by-8-foot bed filled 12 inches deep needs (4 × 8 × 1) ÷ 1,728 = 0.19 cubic feet. Wait—that math is wrong. Let me recalculate: 4 feet × 8 feet × 1 foot = 32 cubic feet. A 4-by-8 bed at 12 inches deep holds 32 cubic feet of soil. Most garden centers sell soil in bags of 2 cubic feet or in bulk by the cubic yard (27 cubic feet per yard).
If your bed is an unusual shape—round, hexagonal, or L-shaped—measure the area it covers in square feet, multiply by the depth in feet, and you have cubic feet. A circular bed 4 feet across and 12 inches deep covers about 12.5 square feet, so 12.5 × 1 = 12.5 cubic feet.
Key Takeaways
- Multiply length × width × depth (all in feet) to find cubic feet of soil needed.
- Most vegetables grow well in 10 to 12 inches of soil; leafy greens need only 6 to 8 inches, and root crops need 12 to 18 inches.
- Garden centers sell soil in 2-cubic-foot bags or by the cubic yard (27 cubic feet), so round your total up to the nearest whole number.
- Soil settles over time, so fill your bed slightly above the rim and expect it to drop 1 to 2 inches in the first month.
Adjust depth based on what you're growing
Not every plant needs the same depth. Lettuce, spinach, and other leafy greens are happy in 6 to 8 inches of soil. Tomatoes, peppers, and beans do well at 10 to 12 inches. Carrots, beets, potatoes, and parsnips need 12 to 18 inches because their roots go deep. If you're growing a mix of plants, aim for 12 inches as a safe middle ground.
Shallow soil also dries out faster in summer heat, which means more watering. Deeper soil holds moisture longer and gives roots more room to spread, which usually means healthier plants and bigger harvests. If you're in a hot, dry climate, lean toward the deeper end of the range.
Account for soil settling and compression
Fresh soil is fluffy and takes up more space than it will after a few weeks. Once you water it and plants start growing, the soil compacts and settles. You can expect it to drop 1 to 2 inches in the first month, sometimes more if you're using a mix heavy on compost or bark.
To account for settling, fill your bed slightly above the rim—about an inch higher than you want the final level to be. This way, after settling, you'll still have the depth you planned for. If you're mixing your own soil (rather than buying bagged or bulk), settling is usually less dramatic than with pure compost, because the mineral soil compacts less.
Bagged soil versus bulk soil: cost and convenience
Bagged soil costs more per cubic foot but requires no delivery and no heavy equipment. A 2-cubic-foot bag typically costs $3 to $8 depending on quality and your location. For a 4-by-8 bed at 12 inches deep (32 cubic feet), you'd need 16 bags, which runs $48 to $128. Bagged soil is convenient for small beds or if you have limited space to store a pile.
Bulk soil delivered by the cubic yard costs $30 to $60 per yard in most areas, though prices vary widely by region and soil type. A 4-by-8 bed needs about 1.2 cubic yards, so bulk would cost roughly $36 to $72 plus delivery. Bulk is cheaper per cubic foot but requires a place to dump it and a way to move it into your bed. Many suppliers have a minimum order (often 1 to 2 yards), so bulk only makes sense if you're filling multiple beds or don't mind extra soil left over.
Mix your own soil to save money
You don't have to buy finished garden soil. Many home cooks mix their own from three parts topsoil, two parts compost, and one part perlite or coarse sand. Topsoil is the cheapest component and is sold in bulk at landscape suppliers. Compost adds nutrients and helps soil hold water. Perlite or sand improves drainage so roots don't sit in wet soil.
If you're mixing your own, calculate the total cubic feet you need, then divide by 6 (the number of parts in the recipe). Buy that amount of topsoil, two-thirds that amount of compost, and one-third that amount of perlite. For a 32-cubic-foot bed, you'd buy about 5.3 cubic yards of topsoil, 3.5 cubic yards of compost, and 1.8 cubic yards of perlite. Mix them in the bed itself or in a pile nearby, then shovel the mixture into place. This approach usually costs 30 to 50 percent less than buying finished soil, but it takes more labor.
Common mistakes that waste soil or leave beds too shallow
The most common mistake is underestimating depth. Many people fill a bed 6 to 8 inches deep thinking it's enough, then find their tomatoes or carrots struggle because roots hit the bottom too soon. Measure twice and buy a little extra—leftover soil stores easily and you'll use it next season.
Another mistake is not accounting for settling. If you fill a bed exactly to the rim with fresh soil, it will drop below the rim within weeks, and you'll wish you'd bought more. Fill slightly above the rim from the start.
A third mistake is buying the wrong type of soil. "Garden soil" and "topsoil" are not the same thing. Topsoil is the dark, nutrient-rich layer from the top of the ground; it's what you want. Garden soil is often just topsoil mixed with compost and marketed under a fancier name. Read the bag label or ask the supplier what's in it. Avoid "fill dirt" or "construction soil"—those are cheap but contain little nutrition and often have clay that compacts hard.
Quick reference table for common bed sizes
| Bed Size | Depth | Cubic Feet Needed | Bags (2 cu ft each) | Bulk (cubic yards) |
|---|---|---|---|---|
| 2 × 4 feet | 12 inches | 8 | 4 bags | 0.3 yards |
| 3 × 6 feet | 12 inches | 18 | 9 bags | 0.7 yards |
| 4 × 8 feet | 12 inches | 32 | 16 bags | 1.2 yards |
| 4 × 8 feet | 18 inches | 48 | 24 bags | 1.8 yards |
| 3 × 3 feet (round) | 12 inches | 7 | 4 bags | 0.3 yards |
Frequently Asked Questions
Can I use less soil if I put cardboard or newspaper under the bed?
Cardboard or newspaper under the bed doesn't reduce the soil you need inside it. Those materials block weeds from growing up from below, but they don't replace soil. You still need the full depth for roots. However, cardboard does break down over time and add organic matter to the soil beneath, which can improve the ground underneath your bed.
What if I fill my bed and realize I didn't buy enough?
You can add more soil later, but it will settle unevenly and create a visible line where old soil meets new. If you're only short by an inch or two, water the bed well and let it settle for a week, then add the rest. For larger shortfalls, it's better to remove everything, mix in the new soil, and refill—more work, but a more even result.
Do I need to replace all the soil every year?
No. Add 1 to 2 inches of compost or aged manure on top each spring to replenish nutrients, and the existing soil stays in place. After three to five years, you may want to replace the top 6 inches if it's become compacted or depleted, but the bottom half can stay. This saves money and reduces waste.
Is potting soil the same as garden soil for raised beds?
No. Potting soil is lightweight and designed for containers; it drains very fast and dries out quickly. Garden soil or topsoil is heavier and holds moisture better, which is what you want in a raised bed. Potting soil would be expensive and wasteful for a large bed. Use it only for pots and small containers.
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 when squeezed but fall apart when you open your hand. Avoid soil that smells sour, looks gray or pale, or feels sticky and clay-like. Ask the supplier what's in it—a mix of topsoil, compost, and perlite is a solid choice. Bagged soil from a reputable garden center is usually reliable; bulk soil from a landscape supplier varies more, so ask to see a sample first.