How Much Soil Does Your Garden Bed Need?

Depth depends on what you plant

Shallow-rooted annuals and salad greens are happy in a few inches of good soil, while tomatoes, squash, and most perennials appreciate a foot or more of loose, fertile ground. If you are improving an existing bed instead of building from scratch, you only need enough new soil to bring the surface up to the level you want; the native soil underneath still counts.

• Lettuce, herbs, and annual flowers: about 6–8 inches
• Most vegetables: about 8–12 inches
• Root crops and large perennials: 12 inches or more

Measure the space you will actually fill

Measure inside the bed’s edging, not outside it. For beds with uneven ground, push a stake in at several points and measure how far down the finished surface will sit; averaging those readings gives a fair working depth.

Odd shapes are easier than they look. Break them into rectangles and circles and add the pieces together — the Garden Area Calculator does this for you.

From cubic feet to something you can buy

Multiply length by width by depth, keeping every number in feet, to get cubic feet. Garden centers sell bags labeled in cubic feet or liters, while landscape yards sell by the cubic yard, which is 27 cubic feet. A bed that needs 40 cubic feet is about 1.5 cubic yards, or 27 bags of 1.5 cubic feet.

Leave room for settling

Fresh soil is fluffy. After a few waterings it compacts noticeably, so a bed filled exactly to the top in spring often sits lower by midsummer. Adding a 10–20% allowance, or keeping a few bags in reserve to top up, saves an extra trip.

Topsoil, garden soil, or a blend?

Bulk “topsoil” is usually native soil stripped from a site and screened. It is economical for filling large areas and leveling, but its quality varies a lot from one supplier to the next. “Garden soil” and “raised bed mix” are typically topsoil blended with compost or other organic matter, which makes them better for planting but more expensive per yard.

For an in-ground vegetable or flower bed, many gardeners use screened topsoil and work compost into the top several inches. For a bed you’re building up from scratch, a pre-blended garden soil saves the mixing step. Whatever you choose, ask to see or feel a sample: good soil crumbles in your hand rather than forming a sticky ball or running through your fingers like sand.

When bulk delivery makes sense

Bulk soil is sold by the cubic yard and delivered by truck, usually with a fee. It tends to win on price once you need more than a yard or two, and it saves dozens of trips to the car. The trade-offs are that it arrives in one pile, it needs a place to be dumped where the truck won’t damage the lawn, and you’ll be moving it by wheelbarrow.

Bags make sense for small beds, for backyards with no truck access, and when you’ll fill beds over several weekends. Enter both prices in the Soil Calculator to see where the break-even point falls for your project.

Worked example: a 10 × 4 ft vegetable bed, 10 inches deep

Helpful tips

Frequently asked questions

The bottom line

Measure the inside of the bed, pick a depth that suits your plants, convert to cubic feet, and add a settling allowance. With that one number you can compare bags against bulk and buy once.

Further reading

• USDA Natural Resources Conservation Service
• Cooperative Extension (extension.org)