How much soil do I need for a raised bed?

The volume math

A raised bed is a box, and the soil to fill it is a volume: length x width x depth. The only trap is units. Length and width are usually measured in feet, but depth is thought of in inches, so divide the depth by 12 before you multiply.

The Garden Soil Calculator does this and then reports cubic feet, cubic yards, and a bag count rounded up to a whole bag. It defaults to a 6 inch depth and a 1.5 cubic foot bag. In metric mode you enter meters, centimeters, and liters, and it converts them to feet and inches, runs the same math, and converts the answer back to liters and cubic meters.

Use the inside dimensions of the frame, not the outside. A bed that measures 4 ft x 8 ft outside is a little smaller inside once you subtract the thickness of the boards, and over a full bed the difference can be a bag or two.

Worked examples

The tool's own example is a 6 ft x 10 ft bed filled 8 inches deep. The area is 60 sq ft, the volume is 60 x 8 / 12 = 40 cubic feet, and 40 / 27 = 1.48 cubic yards. With 1.5 cubic foot bags, 40 / 1.5 = 26.7, rounded up to 27 bags.

For the very common 4 ft x 8 ft bed filled 12 inches deep, the volume is 4 x 8 x 1 = 32 cubic feet. That is 32 / 27 = 1.19 cubic yards, 32 / 1.5 = 21.3, so 22 bags of 1.5 cubic feet, or 16 bags if the bags are 2 cubic feet.

In metric, a 2 m x 3 m bed filled 20 cm deep is 2 x 3 x 0.2 = 1.2 cubic meters, which is 1,200 L. With 40 L bags that is exactly 30 bags. Note that a 1.5 cubic foot bag is about 42.5 L (1 cubic foot is 28.3168 L), so a 40 L bag is slightly smaller, and bag counts will not match one-to-one between the two modes.

How deep should the soil be?

Depth drives the volume more than any other input, so it is worth a decision before you shop. Extension sources give practical anchors rather than one magic number. Oregon State University Extension says, in its instructions for mounded beds, that a soil and organic mix about 8 inches deep is sufficient for adequate rooting of most vegetable plants. The Inyo and Mono Counties Cooperative Extension guide says a bed should be at least 7 inches tall and that about 11 inches is probably better, and adds that too high just wastes soil.

The calculator's own guidance sits in the same range: 6 to 12 inches is typical, with about 10 to 12 inches for vegetable beds and less for shallow-rooted flowers. The deeper the soil, the more you buy: for a 4 x 8 ft bed, 7 inches is 18.7 cubic feet, 11 inches is 29.3, and 12 inches is 32.

Your site changes the answer. If the bed sits on loosened native soil, the roots can use that too. Oregon State suggests digging or tilling the ground under the frame at least 6 inches and mixing it into the fill as you go. Over compacted or poor ground, a taller bed matters more. Keep widths to about 4 feet so you can reach the middle without stepping in, which is also the maximum Oregon State recommends for adult gardeners.

The soil and compost mix

Do not fill a raised bed with straight potting soil or straight compost. University of Minnesota Extension recommends a bulk mix of about two-thirds to one-half topsoil and one-half to one-third compost, notes that potting soil dries out too quickly for raised beds, and warns that too much compost causes water-holding problems. If you buy bagged material, look for products labeled for raised beds.

The Inyo and Mono guide suggests about half topsoil and half organic material, at least half of which should be compost, and notes that taller beds are more forgiving: one-third compost with two-thirds sandy soil works fine in an 11 inch bed. Either way, treat the ratios as a starting point and adjust for your native soil and your local supplier's blends.

Turn the ratio into quantities from the calculator's total. For the 32 cubic foot bed, a two-thirds to one-third split is about 21.3 cubic feet of topsoil and 10.7 cubic feet of compost (0.79 and 0.40 cubic yards). A half-and-half split is 16 and 16 cubic feet. Round each component up separately when you buy, and mix on a tarp or in the bed rather than layering, so water moves evenly.

Settling and top-ups

The calculator returns the volume to fill the bed once at your chosen depth. Loose soil compacts when it is watered in and as organic matter breaks down. The tool's note is that soil typically settles 10% to 20% once watered, which is a planning allowance rather than a measured figure for your soil. Oregon State puts it simply: organic matter decomposes and disappears, so add more of it often.

To see what that looks like, take the 32 cubic foot bed. An allowance of 10% is 35.2 cubic feet and 24 bags of 1.5 cubic feet. At 20% it is 38.4 cubic feet and 26 bags. You do not have to buy that much on day one; many gardeners fill to the brim, water thoroughly, and top off a few weeks later, which spreads the cost and lets you see the real settling.

Plan an annual top-up with compost as well, because a bed that was full this spring will sit lower next year.

Bulk versus bagged

One cubic yard is 27 cubic feet, which equals 18 bags of 1.5 cubic feet, 13.5 bags of 2 cubic feet, or 36 bags of 0.75 cubic feet. A single 4 x 8 ft bed at 12 inches deep is already 1.19 cubic yards, so it is close to 22 bags. Three such beds are 96 cubic feet, or 3.56 cubic yards, about 64 bags.

University of Minnesota Extension notes that bulk topsoil by the cubic yard is generally much cheaper than individual bags. The tradeoff is logistics: delivery fees and minimum orders vary, you need a spot to dump it, and you must move it by wheelbarrow. For one small bed, bags are simpler and let you choose a specific raised bed mix; for several beds or a large one, price a bulk delivery of the topsoil and compost separately and compare it with bags using your local prices.

Oregon State also points out that landscape suppliers offer separate mixes for different uses, so tell them it is for a vegetable bed, because a coarse mix sold for lawns may not suit one.

Odd shapes, tall beds, and where the formula bends

The calculator takes a length and a width, so it treats every bed as a rectangle. For a round bed, work out the area first. A 6 ft diameter bed has an area of pi x 3 x 3 = 28.27 sq ft. Filled 10 inches deep that is 28.27 x 10 / 12 = 23.56 cubic feet, or 15.7 bags of 1.5 cubic feet, so 16 bags. To use the calculator, enter 5.32 ft for both length and width, since 5.32 x 5.32 is 28.3 sq ft.

Very tall beds are a special case. University of Minnesota Extension notes that in beds taller than 2 feet, gardeners look for cheaper alternatives to topsoil, such as layering sticks, leaves, and compost. A 4 x 8 ft bed at 24 inches is 64 cubic feet or 2.37 cubic yards, which is twice the 32 cubic feet of a 12 inch bed. Any coarse material you put in the bottom will also break down and settle, so expect to top up more.

Tall or long frames also need structure: Oregon State recommends reinforcing beds longer than 6 feet or taller than about 18 inches, because the weight of soil and irrigation water pushes the sides outward.

Common mistakes

Most short or over-ordered fills trace back to a few slips.

• Entering depth in feet instead of inches, or the reverse.
• Using the outside dimensions of the frame instead of the inside.
• Forgetting that bag sizes are printed in different units; compare cubic feet with cubic feet, or convert 1 cubic foot to 28.3 L.
• Buying exactly the calculated volume and nothing for settling.
• Filling with potting soil alone, which dries quickly in a raised bed.
• Using too much compost, or none, instead of a balanced topsoil and compost blend.
• Pricing bags for a big job without comparing a bulk delivery.