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Raised Bed Soil Calculator
Enter your bed size and depth to get the soil volume in cubic feet, cubic yards and litres - plus a Mel's Mix shopping list (compost, peat or coir, and vermiculite in thirds) with bag counts.
Calculator
How the math works
Soil volume is just length x width x depth. Measure the inside of the frame in feet, convert the depth from inches to feet by dividing by 12, and multiply. Measure the depth you will actually fill - usually an inch or so below the top board - not the height of the lumber.
- Cubic feet = length (ft) x width (ft) x depth (in) / 12
- Cubic yards = cubic feet / 27 (bulk soil is sold by the yard)
- Litres = cubic feet x 28.32 (1 ft is exactly 0.3048 m)
- Bags = cubic feet / bag size, rounded up
- Mel's Mix = the total split three ways, one part per ingredient
Worked example
A 4 x 8 ft bed, 12 in deep
- 4 x 8 x (12 / 12) = 32 cu ft
- 32 / 27 = 1.19 cu yd
- 32 x 28.32 = 906 L
- Mel's Mix: 32 / 3 = 10.7 cu ft of each ingredient
- Plus 10% for settling: buy about 35.2 cu ft (11.7 cu ft of each)
Soil for common raised bed sizes
Cubic feet of soil for popular bed footprints at four fill depths, before any settling allowance. Divide by 27 for cubic yards; for Mel's Mix, divide by 3 for each ingredient.
| Bed size | 6 in deep | 8 in deep | 12 in deep | 18 in deep |
|---|---|---|---|---|
| 2 x 4 ft | 4 cu ft | 5.3 cu ft | 8 cu ft | 12 cu ft |
| 2 x 8 ft | 8 cu ft | 10.7 cu ft | 16 cu ft | 24 cu ft |
| 3 x 3 ft | 4.5 cu ft | 6 cu ft | 9 cu ft | 13.5 cu ft |
| 3 x 6 ft | 9 cu ft | 12 cu ft | 18 cu ft | 27 cu ft |
| 4 x 4 ft | 8 cu ft | 10.7 cu ft | 16 cu ft | 24 cu ft |
| 4 x 6 ft | 12 cu ft | 16 cu ft | 24 cu ft | 36 cu ft |
| 4 x 8 ft | 16 cu ft | 21.3 cu ft | 32 cu ft | 48 cu ft |
| 4 x 10 ft | 20 cu ft | 26.7 cu ft | 40 cu ft | 60 cu ft |
| 4 x 12 ft | 24 cu ft | 32 cu ft | 48 cu ft | 72 cu ft |
| 5 x 5 ft | 12.5 cu ft | 16.7 cu ft | 25 cu ft | 37.5 cu ft |
Mel's Mix explained
Mel's Mix is the soil recipe from the Square Foot Gardening method: one-third blended compost, one-third peat moss and one-third coarse vermiculite, measured by volume (buckets or bags, not weight). The Square Foot Gardening Foundation describes it as three ingredients in equal parts, and Washington State University Extension gives the same 1/3 - 1/3 - 1/3 blend as the basis of the method. The calculator simply divides your total by three.
- Blended compost is the food. Use compost made from several sources (the Foundation suggests at least three) rather than one bagged product, so the nutrient mix is broad enough that the bed needs no fertilizer in its first season.
- Peat moss holds water and keeps the mix light. Coconut coir is an accepted swap and performs the same in the bed; it is the more sustainable choice because coir is a renewable by-product while peat is harvested from bogs that regrow extremely slowly. Peat is also sold compressed - see the packaging note below.
- Vermiculite must be the coarse grade (often labelled A-3), not the fine grade sold for seed starting. The large flakes are what keep the mix airy and free-draining.
How the shopping list handles packaging
- Compost and plain mix use the bag size you choose (1, 1.5, 2 or 3 cu ft) and round up.
- Coarse vermiculite is priced per 4 cu ft bag, the usual retail size.
- Peat moss is sold compressed. Manufacturers rate a 3 cu ft bale at roughly6 cu ft once fluffed - Sun Gro (Black Gold) says a 3.0 cu ft bale expands at about a 2:1 ratio to 6 cu ft, and Premier rates its 3.8 cu ft bale at 6 cu ft. The calculator assumes about 6 loose cu ft per bale.Treat that as an estimate: how much a bale fluffs depends on how dry it is and how well you break it up, so check the yield printed on the bale you buy.
- 5-gallon buckets are shown because that is how most people mix Mel's Mix on a tarp. One cubic foot is about1.5 level buckets.
For the full recipe, mixing method and cheaper substitutions, readthe raised bed soil mix guide.
How deep should a raised bed be?
Depth is the biggest lever on your soil bill - a 12 in bed needs exactly twice the soil of a 6 in bed. Here is what university extension services recommend, so you can fill only as deep as your crops need.
Salad greens, herbs, radishes, bush beans, onions and short carrot varieties. Washington State University Extension describes the classic method as mixing the three ingredients "to create six inches of soil for each bed", and the SFG Foundation adds a 12 x 12 in "top hat" box on top of one square for long carrots.
University of Maryland Extension: beds on patios, driveways or other hard surfaces are fine if they are at least 8 inches deep for leafy greens, beans and cucumbers.
Utah State University Extension recommends a box at least 6 to 12 inches high to accommodate the rooting depth of most vegetables, and says beds shallower than 12 inches should have no bottom so roots can reach the soil below. Maryland recommends 12 to 24 inches for peppers, tomatoes and squash on hard surfaces.
New Mexico State University Extension: carrots and parsnips "need closer to 18 or 24 inches of soil depth, respectively". South Dakota State University Extension says parsnip soil should be worked to at least 18 to 24 inches and that a raised bed can supply that depth.
If your bed sits on open ground, roots can continue into the soil below, so a shallower frame works. On concrete, a deck or a lined bottom, the frame depth is all the root room there is - use the deeper end of each range.
Next
Plan what goes in the bed
Drop vegetables onto a grid and get counts, a shopping list and sow dates.
Reference
Plant spacing chart
How many of each crop fit in one square foot: 1, 4, 9 or 16.
Guide
The raised bed soil mix
The Mel's Mix recipe, how to mix it on a tarp, and cheaper swaps.
Raised bed soil questions
How much soil do I need for a 4x8 raised bed?+
A 4 x 8 ft bed filled 12 inches deep holds 32 cubic feet (1.19 cubic yards, about 906 litres) of soil. At 6 inches deep it holds 16 cubic feet. Loose mix settles, so add about 10% and buy roughly 35.2 cubic feet for the 12-inch-deep bed.
How many bags of soil do I need for a 4x4 raised bed?+
A 4 x 4 ft bed 12 inches deep holds 16 cubic feet, which is 11 bags of 1.5 cu ft or 8 bags of 2 cu ft. At 6 inches deep it holds 8 cubic feet: 6 bags of 1.5 cu ft or 4 bags of 2 cu ft. Always round up, and add a bag or two so you can top the bed up after it settles.
How do I calculate cubic yards of soil for a raised bed?+
Multiply length x width x depth, all in feet, to get cubic feet, then divide by 27 (there are 27 cubic feet in a cubic yard). A 4 x 8 ft bed 12 inches (1 foot) deep is 4 x 8 x 1 = 32 cubic feet, and 32 / 27 = 1.19 cubic yards. Bulk soil is sold by the cubic yard; bagged soil by the cubic foot.
What is Mel's Mix and how much of each ingredient do I need?+
Mel's Mix is the square-foot-gardening soil: one-third blended compost, one-third peat moss (or coconut coir) and one-third coarse vermiculite, measured by volume. Divide the bed volume by three. A 4 x 8 ft bed 12 inches deep holds 32 cubic feet, so you need about 10.7 cubic feet of each ingredient (11.7 each with a 10% settling allowance).
How deep should a raised bed be?+
Six inches is the square-foot-gardening standard and grows salad greens, herbs, radishes, bush beans and short carrots. University extension services recommend a box at least 6 to 12 inches high for most vegetables, at least 8 inches for greens and beans when the bed sits on a hard surface, 12 to 24 inches for tomatoes, peppers and squash on a hard surface, and closer to 18 inches for long carrots and parsnips.
How many 5-gallon buckets of soil make one cubic foot?+
One cubic foot is about 7.48 US gallons, so it takes about 1.5 five-gallon buckets (a level 5-gallon bucket holds roughly 0.67 cubic feet). The 8 cubic feet of Mel's Mix for a 4 x 4 ft bed 6 inches deep is about 12 buckets - 4 buckets of each ingredient, the same figure the SFG Foundation uses.
Sources
- Washington State University Extension - Square Foot Gardening (PDF): the 1/3 vermiculite, 1/3 peat moss, 1/3 compost mix "to create six inches of soil for each bed"; coconut coir as a renewable alternative to peat
- Utah State University Extension - Raised Bed Gardening: a box "at least 6 to 12 inches high to accommodate the rooting depth of most vegetables"; beds shallower than 12 inches should have no bottom
- University of Maryland Extension - Growing Vegetables in Raised Beds: on hard surfaces, at least 8 inches deep for leafy greens, beans and cucumbers; 12 to 24 inches for peppers, tomatoes and squash
- New Mexico State University Extension (Marisa Y. Thompson, PhD) - carrots and parsnips "need closer to 18 or 24 inches of soil depth, respectively"
- South Dakota State University Extension - Parsnips: soil worked to at least 18 to 24 inches; a raised bed can provide that depth
- Square Foot Gardening Foundation - The Magic of Mel's Mix: equal parts blended compost, peat moss and coarse vermiculite; coconut coir and PittMoss are acceptable substitutes
- Square Foot Gardening Foundation - Mel's Mix resources: a 4 x 4 ft bed 6 inches deep needs 8 cubic feet; four 5-gallon buckets of each ingredient
- Square Foot Gardening Foundation - Growing carrots: six inches is the standard bed depth; longer varieties get a 12 x 12 in "top hat"
- Sun Gro Horticulture (Black Gold) - How do I calculate how much peat to add: a 3.0 cu ft compressed bale expands at about a 2:1 ratio, to roughly 6 cubic feet
- Premier Sphagnum Peat Moss listing: the 3.8 cu ft compressed bale "expands to 6 cubic feet once fluffed"
- A.M. Leonard - Vermiculite, A-3 coarse grade, 4 cu ft bag (the usual retail size)
Unit conversions are exact by definition: 1 ft = 0.3048 m, so 1 cu ft = 28.32 L; 1 US gallon = 231 cu in, so 1 cu ft = 7.48 gal. Depth and packaging figures are rounded guidance, not rules - check your seed packet and your bale label.