Calculate soil volume by bed dimensions

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 inches, then convert to cubic feet (divide by 1,728) or cubic yards (divide by 46,656). Most garden soil is sold by the cubic foot or cubic yard, so this conversion tells you exactly what to order.

For example, a bed that is 4 feet long, 2 feet wide, and 12 inches deep needs 96 cubic feet of soil. A bed that is 8 feet long, 4 feet wide, and 10 inches deep needs about 27 cubic feet. The depth matters as much as the length and width—a shallow bed needs far less soil than a deep one, even if the footprint is identical.

Key Takeaways

  • Multiply length × width × depth in inches, then divide by 1,728 to get cubic feet, which is how soil is usually sold.
  • Most vegetable gardens work well with 10 to 12 inches of soil depth, though root vegetables and perennials may need 12 to 18 inches.
  • A standard 4-by-8-foot bed at 12 inches deep requires about 32 cubic feet of soil, or roughly one cubic yard.
  • Soil settles over time, so filling to the very top of your frame means you will have less usable depth after the first season.
  • Buying soil in bags costs more per cubic foot than bulk delivery, but works for small beds or when you cannot access a truck.

Standard bed sizes and soil amounts

A 4-by-8-foot bed filled to 12 inches deep holds about 32 cubic feet of soil—roughly one cubic yard. A 4-by-4-foot bed at the same depth holds 16 cubic feet, or half a cubic yard. A 3-by-6-foot bed holds about 18 cubic feet. These are the most common dimensions, and knowing them saves you from calculating every time.

If your bed is shallower or deeper, adjust from there. A 4-by-8-foot bed at 10 inches deep needs about 27 cubic feet. The same bed at 18 inches deep needs 48 cubic feet. Depth changes the total faster than length or width does, because you are multiplying it by the entire footprint.

Write down your bed's exact dimensions before you order. Measure the inside of the frame from corner to corner, not the outside. If your frame is made of boards, measure the space where the soil will actually sit.

Why depth matters for different plants

Shallow-rooted plants like lettuce, spinach, and herbs can grow in 6 to 8 inches of soil. Most vegetables—tomatoes, peppers, beans, squash—need 10 to 12 inches. Root vegetables like carrots, beets, and parsnips need at least 12 inches, and ideally 14 to 18 inches so the roots can extend fully without hitting the bottom.

Perennials and shrubs planted in raised beds need even more depth—16 to 24 inches is better. If you plan to grow a mix of plants, aim for 12 inches as a middle ground. You can always add more soil later if you find you need it, but removing soil is messy and wasteful.

Account for soil settling and compaction

New soil settles over time, especially during the first season when you water it regularly. Expect to lose 1 to 2 inches of depth in the first year as the soil compacts and organic matter breaks down. If you fill your bed to the very top of the frame, you may end up with only 10 inches of usable soil by midsummer, even though you started with 12.

To account for this, fill slightly higher than your target depth—about 1 to 2 inches above where you want the final soil line to be. After a few weeks and several waterings, the soil will settle to your intended depth. This is especially important if you are using a mix of topsoil and compost, which settle faster than aged, dense soil.

Choosing between bulk delivery and bagged soil

Bulk soil delivered by truck costs less per cubic foot than bagged soil, usually between $25 and $50 per cubic yard depending on your region and soil type. A cubic yard covers about 32 cubic feet, so one truckload fills a standard 4-by-8-foot bed at 12 inches deep. Bulk delivery works best if you have a driveway or yard where a truck can back up, and if you need at least one full cubic yard.

Bagged soil costs more—typically $3 to $8 per cubic foot—but is easier to handle and does not require truck access. A 2-cubic-foot bag is the standard size at garden centers. For a 4-by-8-foot bed at 12 inches deep, you would need 16 bags. Bagged soil is practical for small beds, balcony containers, or when you cannot fit a delivery truck in your yard.

Some gardeners split the difference by ordering bulk soil for the base layer and adding bagged compost or topsoil on top. This saves money while keeping the heavy lifting manageable.

Soil composition and quality

The type of soil matters as much as the amount. Garden soil, topsoil, and potting mix are not the same thing. Topsoil is the upper layer of native soil, often sold for filling and grading. Garden soil is topsoil mixed with compost and organic matter, designed for planting. Potting mix is a lightweight blend meant for containers, not raised beds.

For a raised bed, order garden soil or a mix of topsoil and compost. Many gardeners use a ratio of 60 percent topsoil to 40 percent compost, which gives good drainage and nutrients without being too expensive. Ask the supplier what is in their product—some "garden soil" is mostly clay with little organic matter, while others are rich and dark.

If you are filling a very deep bed, you can use cheaper topsoil for the bottom 6 to 8 inches and better-quality garden soil or compost for the top layer where roots actually grow. This saves money without sacrificing plant health.

Frequently Asked Questions

Can I use less soil if I add compost on top?

Compost breaks down and compacts faster than soil, so it is not a substitute for depth. You can layer compost on top of soil to add nutrients, but you still need the full depth of soil underneath for roots to grow into. A 2-inch layer of compost over 10 inches of soil works, but 10 inches of compost alone will compact to 6 or 7 inches by next season.

What if I order too much soil?

Extra soil keeps for years if stored dry. You can use it to top off the bed next season after settling, fill containers, amend garden areas outside the raised bed, or give it to a neighbor. Soil does not spoil, so ordering slightly more than you calculate is safer than running short and having to make a second order.

Do I need to put anything under the soil in the raised bed?

No. Soil can sit directly on grass or bare ground. If you want to block weeds from growing up into the bed, lay cardboard or landscape fabric on the ground first, then add soil on top. This step is optional but reduces weeding work in the first year.

How do I know if my soil is good quality?

Good garden 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 the supplier if the soil has been tested for contaminants, especially if you are buying from an unknown source.

Can I reuse soil from year to year?

Yes. At the end of each season, mix in 1 to 2 inches of fresh compost to replenish nutrients and organic matter. After three to five years, the soil will be mostly decomposed organic matter and may need partial replacement. Refreshing the top layer is cheaper and easier than starting over completely.