How Much Soil for a Raised Garden Bed? Bed-Size Table & Mix Ratios

What this guide is not: our topsoil-for-sod guide covers lawn grading and laying sod over fresh topsoil, and topsoil vs compost vs garden soil compares amendments for existing ground. This page is bed math and mixes only — volumes by bed size, bag counts, what goes in a raised bed, and when bulk soil beats bags.

The bed-volume formula

A raised bed is a rectangular box, so the math is the simplest in this whole series:

• Cubic feet = length × width × (depth in inches ÷ 12)
• Cubic yards = cubic feet ÷ 27
• Bags = round up(cubic feet ÷ bag cubic feet)

The standard example, worked out: a 4×8 ft bed, 12 inches deep → 4 × 8 × 1 ft = 32 ft³; 32 ÷ 27 = 1.19 yd³. In 1.5 ft³ raised-bed mix bags: 32 ÷ 1.5 = 21.33 → 22 bags (always round up — a bed that is one bag short is a bed you drive back for).

Bed-size tables

The three most common bed footprints, at the four standard depths. Yards are the bulk-order unit; bags assume 1.5 ft³ raised-bed mix bags:

A practical note on the 4-ft width: it is not arbitrary. Four feet is the widest a bed should be if you can reach it from both sides — a 6-ft bed like the third table needs access paths on both long sides, and nobody enjoys leaning two extra feet into compost every morning.

Bag counts and the bulk breakpoint

The tables use 1.5 ft³ raised-bed mix bags, the most common size. If your store stocks 2 ft³ bags instead, divide the cubic-feet column by 2 and round up (the 4×8×12-in bed takes 16 of those).

The economics flip at about 1 cubic yard: a yard of bulk topsoil typically costs less than the ~22 bags that fill the same volume, and bulk arrives with zero plastic to haul to the curb. Below ~10–15 bags, bags win on convenience — no delivery minimum, no shoveling off a driveway pile. The 4×8×12-in bed at 1.19 yd³ sits right on the breakpoint, which is why you see both strategies recommended: one bed → bags; two or more beds → call the landscape yard. Our topsoil calculator converts either way and flags the weight: 1.19 yd³ of topsoil at 2,400 lbs/yd³ is ~2,856 lbs — plan delivery, because that is not a trunk-load.

Soil-mix ratios

Knowing the volume is half the job; knowing what goes in the volume is the other half. The widely used home-gardener mix is:

~60% topsoil / ~30% compost / ~10% aeration

Topsoil provides the mineral base and structure; compost feeds the soil biology and holds moisture; aeration (perlite, coarse vermiculite, or rice hulls) keeps the mix from compacting into a brick. Call this what it is: common guidance, not a standard — there is no governing body for raised-bed recipes, and experienced growers tweak freely.

Worked for the 4×8×12-in bed (32 ft³): ~19.2 ft³ topsoil (0.71 yd³), ~9.6 ft³ compost (0.36 yd³), ~3.2 ft³ aeration (0.12 yd³). In bags: that is roughly 13 × 1.5 ft³ topsoil bags, 7 × 1.5 ft³ compost bags, and 3 × 1.5 ft³ perlite bags — 23 bags, slightly more than the all-raised-bed-mix 22 because bagged topsoil and compost come in smaller standard bags.

Not Mel's Mix. The famous Square Foot Gardening recipe — 1/3 peat moss (or coco coir), 1/3 coarse vermiculite, 1/3 compost — is a soilless mix: it contains no topsoil at all. It drains beautifully and is expensive to scale past one small bed. Say the choice plainly: the 60/30/10 topsoil mix is the cheap bulk route; Mel's Mix is the premium small-bed route. Do not blend the two halfway and expect either one's results.

Hügelkultur for deep beds

An 18–24 in bed eats shocking amounts of soil — 4 yd³ for a single 6×12×18-in bed, and premium mix at bag prices is painful. Hügelkultur (German for “mound culture”) is the standard workaround: fill the bottom third to half of a deep bed with logs, branches, and wood chips, then top with 8–12 inches of real growing mix.

• Use only untreated wood. Pressure-treated lumber, painted wood, and plywood off-gas chemicals you do not want near food.
• Expect settling. The wood layer decomposes and the bed sinks 6–12 inches over the first year — fill proud, water it in, and top off before planting.
• The top layer still needs to be mix. Roots live in the top 8–12 inches; wood is fill, not soil. A 24-in bed is ~10 in of wood and ~14 in of mix, not 24 in of wood.
• Nitrogen tie-up is real but temporary. Decomposing wood borrows nitrogen from surrounding soil for the first season — the compost fraction of your top mix buffers it.

How deep should beds be?

Depth decides what you can grow and what the bed costs, so pick it before you run the tables:

• 8 in — herbs, lettuce, radishes, microgreens. Shallowest worth building; dries out fast.
• 12 in — the sweet spot: salad greens, peppers, tomatoes, beans, most flowers. Fits almost everything with shallow-to-medium roots.
• 18 in — carrots, parsnips, potatoes, and anywhere you want extra moisture reserve. Consider Hügelkultur fill.
• 24 in — accessibility beds at wheelchair height, or deep-rooted perennials. Definitely Hügelkultur the bottom half.

Where to buy: bags vs bulk vs the pickup truck

There are three ways to get soil to a raised bed, and the right one depends on how far past the ~1-yard breakpoint you are:

• Home-center bags (under ~15 bags). 1.5 ft³ raised-bed mix bags are the path of least resistance for a single 4×8 bed. The premium is convenience, and at this scale it is worth it — no delivery fee, no pile on the driveway.
• Landscape-yard bulk (1–5 yards). Order topsoil and compost separately and mix on-site or in the bed — this is the cheapest way to build the 60/30/10 mix, and most yards will deliver 1-yard minimums. Ask for screened topsoil; unscreened is cheaper and full of clods.
• Super-sacks / tote bags (5+ yards). One-cubic-yard tote bags lifted off a flatbed — the middle ground between hauling 96 individual bags and taking a full dump-truck load. Worth asking about at the landscape yard.

Watch the delivery math: a “$30 delivery fee” on 1.19 yards of $35/yard topsoil is a 70% surcharge on the material. If you are near the breakpoint, buying bags for the first bed and going bulk when you add beds two and three is often the rational move. And whatever you buy, buy it before the final frost — spring prices and stock both get worse in April.

The filling order: how to layer the bed

Volume in hand, the order of operations matters — mix everything at once and the heavy topsoil sinks while the perlite floats:

1. Cardboard base (optional). A layer of plain cardboard on bare ground smothers grass and weeds; it rots away within a season. Skip it on concrete or patios — drainage matters there.
2. Wood fill (Hügelkultur, deep beds). Logs and branches first, packed tight, gaps stuffed with wood chips.
3. Coarse fill. A rough inch or two of wood chips or straw over the wood to keep mix from sifting down.
4. The mix, in thirds. Add the topsoil, compost, and aeration in alternating shovelfuls or three thin layers, then turn the whole bed with a fork. Mixing in the bed beats mixing in a wheelbarrow when the volume exceeds ~1 yard.
5. Water it in. Soak the bed thoroughly and let it drain overnight — this pre-settles 5–10% of the volume. Top off to the rim before planting.

Common mistakes

Filling the whole bed with bagged raised-bed mix at 24 in. That is 96 bags for a 6×12 — Hügelkultur the bottom and save real money. Using pure compost. Compost alone shrinks dramatically and holds too much water; it is 30% of the mix, not 100%. Forgetting the settle tax. Fresh mix drops 10–20% in the first season; fill to the rim and top off. Building wider than you can reach. 4 ft max from one side, or walkable access on both sides. Mixing up units. The formula needs depth in inches ÷ 12; a 12-in bed is 1 ft, but a 10-in bed is not. Skipping the weight math. A yard of wet topsoil approaches 3,000 lbs — one pickup load at a time, and mind your truck's payload rating.

Size your exact beds — dimensions, depth, yards, bags, and weight — in the topsoil calculator. Planning the lawn around the beds? See how much topsoil for sod.