Raised Garden Bed Calculator

How the raised bed calculator works

A standard 8 × 4-ft raised bed 12 inches tall takes 1.13 cubic yards of soil — about 31 one-cubic-foot bags — plus 6 eight-foot 2×8 boards and roughly 56 deck screws. The inside of the frame measures 7'9" × 3'9", which is 29.1 cubic feet of soil, or 30.5 with a 5% settling allowance. Two courses of 2×8 get you to 14½ inches of frame height, a little over the 12 inches of fill.

Soil: internal volume = (length − 2 × board thickness) × (width − 2 × board thickness) × bed height. Then convert to cubic yards (or m³). Add 5% waste for settling and gaps. The calc assumes 1½" / 3.8 cm board thickness for standard 2× lumber.

Lumber: rows of boards = bed height ÷ board face width, rounded up. Boards per row = (2 × length + 2 × width) ÷ board length, rounded up. Corner brackets or 4×4 stakes at each corner; 3" deck screws every 12" along seams.

Soil mix recipes

The calculator gives you a total volume. What goes into that volume is the decision that determines whether the bed grows anything. Three components do three different jobs: mineral soil supplies structure, mineral nutrients, and water-holding capacity; compost supplies organic matter, biology, and slow-release fertility; an aeration amendment keeps the mix from compacting into a brick. Ratios are by volume.

That last row is the correction worth making. "Just fill it with compost" is common advice online and it is a reliable way to lose a season. Pure compost is often high in soluble salts, mineralises quickly and then slumps to half its volume, and swings between nitrogen excess and deficiency as it finishes decomposing. University extension horticulture programs — Cornell, Penn State, and UMN Extension all publish raised-bed guidance along these lines — consistently recommend a blend built on mineral soil with compost as an amendment, typically in the range of a quarter to a third of the volume, rather than compost as the growing medium.

Two more things extension guidance is consistent about, and both are worth doing before you order soil. First, get a soil test — most state extension services run one cheaply, and it tells you pH and nutrient levels rather than making you guess. Second, if the bed sits on ground near an old painted structure, a former orchard, or a roadside, test for lead before you grow root crops in it; a raised bed with a barrier layer is one of the standard remediations for contaminated urban soil, but only if you know what you are working around.

Whatever ratio you pick, buy about 10–15% more than the bed's nominal volume. The calculator's 5% waste allowance covers settling during filling; it does not cover the further drop you will see over the first season as organic matter breaks down. Plan to top a new bed up once at the end of year one — that is normal, not a mistake.

Lumber choice

The pressure-treated question comes up constantly and deserves a straight answer. The lumber that earned PT its bad reputation in vegetable gardens was CCA-treated — chromated copper arsenate — which was phased out of residential use in the United States at the end of 2003. Everything sold at a home centre since then uses copper-based preservatives without arsenic, and extension guidance generally treats it as acceptable for raised beds. If you want belt and braces, line the inside faces of the boards with heavy plastic sheeting, leaving the bottom open for drainage. Where you should be genuinely careful is with salvaged lumber: old decking, reclaimed ties, and mystery timber of unknown vintage. If you cannot date it, do not grow food in it.

Bed depth by crop

Depth is the input that moves your soil cost most, because volume scales linearly with it. The right answer is set by root depth, and by whether the bed sits on open ground — a 12-inch bed over loosened native soil effectively gives roots far more room than 12 inches over a patio.

Width matters too, and it is not a soil-volume question. Keep beds 4 feet or less across if you can reach both sides, or 2–2.5 feet against a wall or fence — the point of a raised bed is that you never step in it, and soil you never compact is soil you never have to till. Length is limited only by lumber; leave at least 2 feet between beds for a person, or 3 feet if a wheelbarrow needs to pass.

Worked example: a compact 4 × 4-ft herb bed, 18 inches tall

Enter 4 ft × 4 ft, 18 inches tall, 2×6 lumber, 8-ft boards, 5% waste. The frame is 1½ inches thick on each side, so the inside measures 3'9″ × 3'9″; 3.75 × 3.75 × 1.5 ft = 21.1 cubic feet, or 22.1 with waste — 0.82 cubic yards, 23 one-cubic-foot bags. On the lumber side, 18 inches ÷ 5.5-inch board face rounds up to 4 courses; the 16-foot perimeter takes 2 eight-foot boards per course, so 8 boards and 112 deck screws.

Notice that four courses of 2×6 gives a 22-inch frame for an 18-inch fill — the calculator always rounds courses up, so you end up with freeboard rather than a short wall. That is usually what you want (soil settles, and a lip keeps mulch in), but if the extra 4 inches bothers you, one course of 2×12 plus one of 2×6 gets you to 16¾ inches with far fewer seams. Also note the screw count: 112 screws for a 4-foot bed is a lot of driving, which is the honest cost of stacking narrow boards.

Worked example: a long, shallow 12 × 3-ft bed against a fence

Enter 12 ft × 3 ft, 10 inches tall, 2×12 lumber, and set board length to 12 ft in Advanced options. Inside dimensions are 11'9″ × 2'9″; 11.75 × 2.75 × 0.833 ft = 26.9 cubic feet, or 28.3 with waste — 1.05 cubic yards, 29 bags. Because 10 inches fits inside a single 11¼-inch 2×12 face, this is 1 course. The 30-foot perimeter divided by 12-foot boards rounds up to 3 boards, with 24 screws.

Read the board count carefully, because this is where the calculator's simplification shows. Three boards is the right total length — 36 feet of lumber for a 30-foot perimeter — but you cannot cut two 12-foot sides and two 3-foot ends out of it without one board yielding both short ends, which it does comfortably. On a bed with a less convenient perimeter the length-based count can come up a board short once real cuts are assigned, so sketch the cut list before you go to the yard. Also: a 12-foot span of 2×12 will bow outward under wet soil. Add a cross-tie or an outside stake at the midpoint — a few dollars that keeps the bed square for a decade.

Filling deep beds without buying a mountain of soil

A 24-inch bed holds twice the soil of a 12-inch one, and almost no annual vegetable roots into the bottom foot. For deep or accessible beds, fill the bottom third with coarse organic material — logs, branches, wood chips, autumn leaves, straw — and the top 12–18 inches with your real soil mix. The technique is often sold under the name hügelkultur; the practical version is simply that the lower layer decomposes slowly, holds water, and costs nothing. Expect it to settle noticeably in the first two years and plan to top up. Do not use this trick in a bed shallower than 18 inches, where you would be stealing root zone to save a few bags.

Frequently asked questions

For the soil itself, see the topsoil calculator. For mulch around or on top of beds, use the mulch calculator.