Raised Bed Soil Calculator

Mel's Mix Math (1/3-1/3-1/3)

Mel's Mix is the soil recipe from Square Foot Gardening, made of equal parts by volume: one-third compost, one-third peat moss, and one-third coarse vermiculite. The blend drains well, holds moisture, and feeds plants without heavy native soil. To fill a bed, calculate the total soil volume, then divide by three for each component. A 4 x 8 ft bed at 12 inches deep needs 32 cubic feet of soil, which is 10.67 cubic feet each of compost, peat moss, and vermiculite. The calculator shows the per-part volume so you can buy the right number of bags of each ingredient.

Mulch Layer on Top

A raised bed needs a mulch cap on top of the soil. Raised beds sit above ground and dry out from all four sides, so a 2 inch layer of straw, leaf, or compost mulch cuts watering by 25 to 50 percent. The mulch also blocks the weeds that blow into open soil. Calculate the mulch separately from the soil since it sits at a shallower depth. A 4 x 8 ft bed needs about 5.3 cubic feet of mulch at 2 inches, or 3 bags of 2 cubic feet. Keep the mulch 2 inches off plant stems.

Standard Bed Sizes

Most raised bed kits come in a few standard footprints, and the calculator preloads them. A 4 x 8 ft bed is the most popular size because it gives plenty of growing room while staying reachable from both sides. A 4 x 4 ft bed suits small spaces and square-foot gardening grids. Fill tall beds partway with coarse material like branches and leaves to cut soil cost, a method that also feeds the bed as it breaks down. Always add 10 percent extra soil for settling, since fresh mix compacts after the first watering and drops below the rim.

What soil depth does a raised bed need?

Raised beds need 10 to 12 inches of soil for most vegetables. Shallow-rooted lettuce and herbs grow in 6 inches. Deep-rooted carrots, tomatoes, and potatoes need 12 to 18 inches. A bed on open ground can be shallower because roots reach into the soil below, while a bed on a patio needs the full depth inside the box.

Can I use bagged soil for a raised bed?

Bagged soil works for small raised beds but gets expensive past one cubic yard. A 4 x 8 ft bed at 12 inches needs about 21 bags of 1.5 cubic feet, which costs more than a bulk yard delivered. For one or two small beds, bags are convenient. For several beds or a large garden, order bulk soil or mix your own Mel's Mix from bulk compost, peat, and vermiculite.

Filling a Tall Raised Bed Cheaply

Tall raised beds use a lot of soil, so fill the bottom with cheaper material. Line the base with logs, branches, and leaves, a method called hugelkultur, then top with the soil mix. The wood breaks down over years, feeding the bed and holding moisture. This cuts the premium soil you need by a third or more on a deep bed. For a 17 inch tall metal kit, fill the bottom 6 inches with coarse organic matter and the top 11 inches with Mel's Mix.

Layering also improves drainage in deep beds. The coarse bottom layer lets excess water escape so roots do not sit in soggy soil. Cover the wood with a few inches of compost before the final soil mix so seeds have a fine surface to germinate in. Top the finished bed with 2 inches of mulch to hold moisture. As the wood base decomposes, the soil level drops, so top up with compost each spring to keep the bed full.

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