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 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 easier.

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 12 inches deep needs 32 cubic feet.

If your bed has an unusual shape—L-shaped, trapezoidal, or circular—break it into rectangles or use the formula for that shape, then add the volumes together. A circular bed 4 feet across and 12 inches deep needs about 12.6 cubic feet (using the formula for cylinder volume: π × radius² × height).

Key Takeaways

  • Multiply length × width × depth (in feet) to find how many cubic feet of soil your bed needs.
  • Most raised beds are 12 inches deep, but shallower beds for herbs or lettuce need less soil, and deeper beds for root vegetables need more.
  • Bagged soil is convenient for small beds but expensive for beds larger than 20 cubic feet; bulk delivery is cheaper for larger volumes.
  • Soil settles after watering, so you may need to add 10 to 15 percent extra in the first season to keep the bed full.

Standard depths and what they support

The depth you choose affects how much soil you buy and what you can grow. A 12-inch depth is the most common and works for most vegetables, herbs, and flowers. It gives roots enough room to spread and holds moisture without being wasteful.

Shallower beds—6 to 8 inches—work for lettuce, spinach, radishes, and herbs like basil and parsley. These crops have shallow root systems and don't need the extra depth. A 4-foot by 2-foot bed at 8 inches deep needs only about 5 cubic feet of soil.

Deeper beds—18 to 24 inches—are necessary for root vegetables like carrots, parsnips, and beets, and for tomatoes and peppers that develop long root systems. A 4-foot by 2-foot bed at 18 inches deep needs 12 cubic feet. Going deeper than 24 inches rarely adds benefit and wastes money and material.

Bagged soil versus bulk delivery

Bagged soil from a garden center or hardware store comes in standard sizes: 1 cubic foot, 1.5 cubic feet, and 2 cubic feet per bag. The math is straightforward—if you need 8 cubic feet, buy eight 1-cubic-foot bags or four 2-cubic-foot bags. Bagged soil costs more per cubic foot but requires no delivery and no equipment to spread.

Bulk soil delivered by truck is cheaper per cubic foot and is the practical choice for beds larger than 20 cubic feet. You order by the cubic yard (27 cubic feet). A 4-foot by 8-foot bed at 12 inches deep needs about 32 cubic feet, which is just over 1 cubic yard. Bulk delivery usually has a minimum order (often 1 cubic yard) and a delivery fee, so call local landscape suppliers or soil yards for pricing in your area.

Some garden centers will deliver bagged soil if you buy enough, and some will load bulk soil into your truck if you bring one. Ask what options are available before you decide which route to take.

Account for settling and compaction

New soil settles over time, especially after the first watering. You may lose 10 to 15 percent of the volume in the first season as the soil compacts and moisture redistributes. To avoid ending up with a half-empty bed by midsummer, buy slightly more than your calculation suggests—add about 10 percent to your total.

If you are mixing soil with compost or other amendments, the settling effect is less dramatic because compost breaks down more slowly. A bed filled with 50 percent soil and 50 percent compost will settle less than one filled with soil alone.

After the first season, you will need to top off the bed each spring. Most gardeners add 2 to 4 inches of fresh compost or soil annually to replace what has decomposed and settled. This is much cheaper than refilling the entire bed.

Soil mixes and amendments

You do not have to fill a raised bed with pure topsoil. Many gardeners use a mix: topsoil as the base layer, compost in the middle, and finished compost or aged manure on top. This layering approach is cheaper than filling entirely with premium soil and often produces better results because it combines drainage, nutrients, and organic matter.

A common formula is 40 percent topsoil, 40 percent compost, and 20 percent aged manure or peat moss. For an 8-cubic-foot bed, that would be about 3.2 cubic feet of topsoil, 3.2 cubic feet of compost, and 1.6 cubic feet of manure. You can adjust these ratios based on what you are growing and what is available locally.

If you are filling a large bed, buying these components separately and mixing them yourself is significantly cheaper than buying pre-mixed "garden soil" or "raised bed mix" from a retailer.

Measuring and ordering tips

Before you order, measure your bed carefully. Use a tape measure for length and width, and measure the interior depth (not the height of the boards, which may be taller). Write down all three numbers and double-check them—ordering too little means a second trip, and ordering too much means wasted money and leftover soil.

When you call a soil supplier, tell them the cubic feet or cubic yards you need and ask what they have in stock. Topsoil, garden soil, and potting mix are different products with different prices and compositions. Ask what the soil is made from (whether it includes compost, what kind of compost, whether it has been screened) so you know what you are getting.

If you are buying bagged soil, check the bag weight before you buy in bulk. A 2-cubic-foot bag of moist soil can weigh 40 to 50 pounds, which adds up quickly. Make sure you have a way to move and spread that weight—a wheelbarrow and a helper make the job much faster.

Frequently Asked Questions

Can I use garden soil from my yard instead of buying it?

You can, but it is usually not ideal. Yard soil often compacts heavily in a raised bed and may contain clay, rocks, or weed seeds. If you do use it, mix it 50-50 with compost or purchased topsoil to improve drainage and add nutrients. Sifting out rocks and large debris first saves frustration later.

What if I fill my bed and it settles too much?

Top it off with compost or fresh soil in the spring. Most gardeners plan to add 2 to 4 inches annually anyway. If settling is severe (more than 20 percent), the soil may have been too wet when you filled it, or the bed may not have been level. Level the bed and let it dry slightly before adding more soil.

Do I need to line the bottom of the bed before adding soil?

A landscape fabric or hardware cloth at the bottom keeps burrowing rodents out and is worth doing if you live in an area with gophers or moles. It does not affect how much soil you need. If you use a solid liner (plastic or wood), make sure water can still drain—drill holes or leave gaps so soil does not become waterlogged.

How much does a cubic yard of soil weigh?

Topsoil weighs roughly 1,300 to 1,500 pounds per cubic yard when moist, and compost weighs about 1,000 pounds per cubic yard. A single cubic yard is heavy enough to require a truck or trailer to move. If you are having it delivered, the supplier handles the weight, but if you are loading it yourself, plan for multiple trips or rent equipment.

Can I use potting mix instead of garden soil?

Potting mix is designed for containers and is much more expensive than garden soil or topsoil. It drains very quickly and does not hold nutrients as well in a large bed. Save potting mix for pots and use topsoil or a topsoil-compost blend for raised beds.