Raised Bed Soil Calculator

Work out how much soil fills your raised beds in cubic feet, cubic yards and bags, then split the total into ingredients for a 60/30/10, Mel's Mix or 50/50 recipe.

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Results will appear here once you enter your values.

How raised bed volume is calculated

A raised bed is a box, so its soil volume is length x width x depth. A standard 4 by 8 foot bed filled 12 inches deep holds 4 x 8 x 1 = 32 cubic feet. Dividing by 27 gives 1.185 cubic yards. Three of those beds need 96 cubic feet, or about 3.56 yards, which is the point where a bulk delivery usually beats buying bags.

If you measure in inches, the calculator converts to feet first: 48 x 96 x 12 inches is 55,296 cubic inches, and dividing by 1,728 cubic inches per cubic foot gives the same 32 cubic feet. Use the inside dimensions of the frame, since 2 inch thick lumber trims several cubic feet from a large bed.

Comparing the three soil mixes

Each recipe splits the same total volume differently. For one 32 cubic foot bed:

60/30/1019.2 cu ft topsoil, 9.6 cu ft compost, 3.2 cu ft perlite, bark fines or aged manure
Mel's Mix10.7 cu ft each of peat moss or coco coir, coarse vermiculite and blended compost
50/5016 cu ft topsoil and 16 cu ft compost

Topsoil-based mixes are the cheapest per cubic foot and hold nutrients well in deep beds. Mel's Mix, from Square Foot Gardening, is light and drains fast, which suits shallow 6 inch beds, but vermiculite and peat make it the most expensive option. The 50/50 blend is rich enough for heavy feeders such as tomatoes and squash.

How deep to fill a raised bed

Depth decides what you can grow. Lettuce, spinach, radishes and most herbs root in 6 inches of soil. Tomatoes, peppers, carrots and potatoes do best with 10 to 12 inches, and beds over hard or contaminated ground should be at least 12 inches deep with no contact with the native soil.

  • 6 in: salad greens and herbs
  • 8 to 12 in: most vegetables, especially over loosened native soil
  • 18 to 24 in: accessible waist-high beds, often with coarse fill in the bottom third

Plan for settling and topping up

A freshly filled bed sinks as compost decomposes and air pockets collapse. Expect 10 to 20 percent settling in the first season, more in compost-heavy blends like 50/50. Set the extra allowance to 10 or 15 percent, or overfill the bed by 1 to 2 inches and water it in thoroughly before planting.

Each spring, add 1 to 2 inches of compost to replace what breaks down. For a 4 by 8 foot bed that is 2.7 to 5.3 cubic feet a year, or roughly 3 to 6 bags of 1 cu ft compost. Compressed peat bales and coco coir bricks expand well beyond their packed size, so check the expanded volume on the label before buying.

Frequently Asked Questions

Common questions about the Raised Bed Soil Calculator

A 4 by 8 foot bed filled 12 inches deep needs 32 cubic feet, or 1.185 cubic yards. At 6 inches deep it needs 16 cubic feet.

Sources & References

Raised bed gardens

Raised bed sizing, depth and soil guidance for home vegetable gardens.
→Source: University of Minnesota Extension

Raised Bed Gardening

Construction, fill depth and soil mix recommendations for raised beds.
→Source: Utah State University Extension

Refreshing raised bed soil generates exceptional results

Why raised bed soil settles and how to replenish it with compost each season.
→Source: University of Illinois Extension

Improving Lawn and Landscape Soils (G6955)

Using topsoil and compost to build productive planting beds.
→Source: University of Missouri Extension

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