How the calculation works
A raised bed is a box, so the soil it holds is length × width × depth (a round bed is π × radius² × depth). The calculator adds up every bed you list, then splits the total by the ratio of the mix you pick.
Cubic feet = length (ft) × width (ft) × depth (in) ÷ 12
Divide cubic feet by 27 for cubic yards, the unit bulk soil is sold in. Bags are rounded up for each ingredient, so a two part mix can need one bag more than a single product.
How much soil for a 4x8 raised bed
An 8 by 4 foot bed filled 12 inches deep holds 8 × 4 × 1 = 32 cubic feet, about 1.19 cubic yards: 16 bags of 2 cu ft or 22 bags of 1.5 cu ft. A 2 : 1 mix of topsoil and compost splits into about 21.3 cubic feet of topsoil and 10.7 of compost.
| Bed size | 6 in deep | 8 in deep | 10 in deep | 12 in deep |
|---|---|---|---|---|
| 4 × 8 ft | 16 cu ft (0.59 cu yd) | 21.3 cu ft (0.79 cu yd) | 26.7 cu ft (0.99 cu yd) | 32 cu ft (1.19 cu yd) |
| 4 × 4 ft | 8 cu ft (0.3 cu yd) | 10.7 cu ft (0.4 cu yd) | 13.3 cu ft (0.49 cu yd) | 16 cu ft (0.59 cu yd) |
| 3 × 6 ft | 9 cu ft (0.33 cu yd) | 12 cu ft (0.44 cu yd) | 15 cu ft (0.56 cu yd) | 18 cu ft (0.67 cu yd) |
| 2 × 8 ft | 8 cu ft (0.3 cu yd) | 10.7 cu ft (0.4 cu yd) | 13.3 cu ft (0.49 cu yd) | 16 cu ft (0.59 cu yd) |
| 4 × 12 ft | 24 cu ft (0.89 cu yd) | 32 cu ft (1.19 cu yd) | 40 cu ft (1.48 cu yd) | 48 cu ft (1.78 cu yd) |
These use the bed's outside size. Walls take a little room: a 4x8 frame of 2x6 boards holds about 29.1 cubic feet at 12 inches, which leaves a margin for settling.
Alabama Cooperative Extension calls 6 to 12 inches of soil desirable for most raised beds. Maryland Extension recommends at least 8 inches for leafy greens, beans and cucumbers, and 12 to 24 inches for peppers, tomatoes and squash.
Choosing a soil mix
University of Maryland Extension suggests compost and purchased topsoil at 1 : 2 or 1 : 1, or compost and a soilless growing mix at 1 : 1. It warns that bagged "organic soil" often contains no mineral topsoil at all, and that good topsoil is dark and crumbly with an earthy smell.
Mel's Mix, the square foot gardening recipe, is 1/3 peat, 1/3 vermiculite and 1/3 blended compost according to the Square Foot Gardening Foundation, with coir allowed in place of peat. Their 4 × 4 ft bed, 6 inches deep, takes 8 cubic feet: 2.67 cubic feet of each part.
The "60/30/10" mix quoted online (topsoil, compost, other) appears in no extension or university source we could find, so it is not offered here.
Filling a tall bed cheaply
Hugelkultur fills the bottom of a deep bed with wood. Clemson Extension describes cardboard first, then logs, branches and twigs from largest to smallest, gaps filled with clippings and leaves, and topsoil and compost on top. The wood rots and the bed sinks; Washington State University gives hugelkultur mounds about 5 to 6 years, and Clemson advises adding compost each season.
A 4 × 8 ft bed 24 inches deep with 8 inches of soil on top needs 21.3 cubic feet of mix instead of 64. The other 42.7 cubic feet is wood and leaves.
Bags, bulk and cost
Bulk soil is sold and delivered by the cubic yard. Enter a price per bag or per cubic yard to see the total, and try both to compare. Maryland Extension suggests asking a topsoil supplier for soil test results and where the soil came from.
Lumber and blocks
Long boards run the full length of the bed and the ends fit between them, so a 4x8 bed uses uncut 8 ft boards on the long sides. Actual widths (ProWood) are 5 1/2 in for a 2x6, 7 1/4 in for a 2x8, 9 1/4 in for a 2x10 and 11 1/4 in for a 2x12. The calculator stacks enough courses to reach the soil depth and lays the pieces onto the board length you pick (the USDA Wood Handbook lists 8 to 18 ft in 2 ft steps), allowing 1/8 inch per saw cut.
For the 4x8 bed at 12 inches in 2x6: 3 courses, a 16 1/2 in wall and 9 boards of 8 ft, 6 used whole and 3 cut into two 3 ft 9 in ends. A 4x4 post sits in each corner plus one wherever a wall runs past 4 ft, the spacing in Washington State University's Skagit County bed plans: 6 posts from two 8 ft 4x4s.
Block counts use a standard block's actual 15 5/8 × 7 5/8 × 7 5/8 in. The same bed takes 20 blocks a course, 40 blocks for 2 courses, and only about 18.4 cubic feet of soil fits inside.
Treated or untreated? Oregon State University Extension grew vegetables for four seasons in treated Douglas-fir beds and found no increase in copper in the vegetables; soil copper rose only within an inch of the wood. It recommends residential, ground contact lumber with treated cut ends; untreated boxes had decayed in the lower half after six years. University of Maine Extension calls the risk low but prefers untreated hemlock or cedar.
Water and top-ups
N.C. Cooperative Extension suggests about 1 inch of water a week; University of Maine Extension gives 1.25 to 1.5 inches. One inch over a square foot is 0.623 gallons, so a 32 square foot bed needs about 20 gallons a week, or 25 to 30 at the Maine rate.
Alabama Extension notes you may need to add 1 to 2 inches of mix each year as the organic part breaks down: 2.67 to 5.33 cubic feet for a 4x8 bed.
For pots use the potting soil calculator, for in-ground beds the garden soil calculator, and to fill the bed with plants the plant spacing calculator.