Article — Raised Bed Soil Calculator
A standard 4 by 8 foot raised bed at 12-inch depth holds 32 cubic feet of soil — about 1.2 cubic yards, roughly 35 cubic feet once you allow for settling. That works out to 18 bags of two-cubic-foot soil mix or, for any project this size or larger, one short delivery from a landscape supplier. The calculator above does the geometry for rectangular, circular, and L-shaped beds, and breaks the total into compost, topsoil, and amendments so you know exactly what to order.
The rest of this article covers depth requirements for different crops, the famous Mel's Mix recipe, when bagged loses to bulk, and the settling allowance most gardeners forget until their bed sinks two inches in the first month.
Raised-bed soil volume basics
Soil volume is a pure geometry problem: length times width times depth for a rectangle, π·r²·h for a circle. The only place the math goes sideways is mixing units — depth almost always comes in inches while length and width are in feet, so you divide the depth by 12 before multiplying. A bed 4 feet by 8 feet by 12 inches deep is 4 × 8 × 1, not 4 × 8 × 12.
For unusual layouts — an L-shape, a keyhole, a curved corner — split the bed into rectangles or sectors on graph paper and add the volumes. The University of Maryland Extension's raised-bed guide notes that small overestimates are forgiven (you can always topdress); underestimates leave you scrambling for one last bag mid-build.
The most popular raised-bed footprint in North America — the 4 by 8 foot rectangle — comes from Mel Bartholomew's 1981 book Square Foot Gardening. Mel argued the 4-foot width is the maximum reach for an adult arm from either side, so no plant ever requires stepping into the bed. Stepping in compacts the soil and undoes the whole point of building raised in the first place.
Raised-bed soil depth by crop
Depth is where new growers most often shortchange themselves. Six inches of soil sounds generous on paper, but it limits the bed to lettuce, spinach, and shallow herbs. Most vegetables need 12 inches; deep-rooted crops want 18 to 24.
Eartheasy and the University of Maryland Extension both publish near-identical depth tables. Herbs and leafy greens are happy at 6–10 inches. Beets, carrots, zucchini, and bush beans need at least 12. Tomatoes, peppers, and full-size root vegetables — parsnips, daikon, salsify — want 15 to 18 inches at minimum, and 24 if you can afford it. Strawberries and blueberries sit at 12 to 18.
- 6 inches — lettuce, spinach, arugula, basil, parsley, chives
- 9–12 inches — leafy greens, kale, chard, most herbs
- 12–15 inches — beets, carrots, beans, peas, zucchini
- 18–24 inches — tomatoes, peppers, sunflowers, parsnips
- 18 inches — strawberries and blueberries for best yields
- 24+ inches — if the bed sits on concrete or impermeable ground
Mel's Mix and other raised-bed soil recipes
Mel's Mix is the recipe Bartholomew built Square Foot Gardening around: equal parts compost, peat moss (or coconut coir), and coarse vermiculite. The compost feeds; the peat or coir holds moisture without compacting; the vermiculite keeps the structure light and well-drained. Square Foot Gardening's own resource site insists the compost component come from at least five different sources to broaden the nutrient profile.
It is not cheap. A full 32 cubic feet of Mel's Mix for a standard 4×8 bed runs around $80–$120 depending on local prices. Two budget alternatives perform respectably: 60% topsoil with 40% compost (about $50 for the same bed), or a 50/30/20 split of topsoil, compost, and a bulky amendment like fine bark or aged mulch ($30–$40). Both compress more than Mel's Mix and need topdressing more often, but they grow vegetables fine.
Bagged "potting soil" or "potting mix" is engineered for containers — it drains too fast and holds too little water for in-ground volumes. Buy "raised bed mix" or "garden soil" labelled for in-ground use, or build your own from topsoil, compost, and amendments. The U.S. Forest Service consumer guides flag this as one of the most common new-gardener mistakes.
Raised-bed soil: bags vs bulk delivery
A 2-cubic-foot bag of garden soil retails for $3–$6 in the U.S. Filling a 4×8 bed at 12-inch depth takes about 18 bags, or roughly $60–$110 in bagged product. The same volume delivered in bulk from a landscape supplier — about one cubic yard — runs $30–$50 plus a delivery fee that typically falls between $50 and $100.
The crossover sits at around 1.5 to 2 cubic yards (roughly 40–55 cubic feet, or two standard 4×8 beds at 12 inches). Below that, bags usually win on convenience even if they cost a bit more. Above that, bulk wins on both price and the absence of 30 empty plastic bags to recycle.
- Up to 30 cu ft — bagged is fine; one bed
- 30–60 cu ft — line ball; check local bulk pricing
- Over 60 cu ft — bulk delivery is almost always cheaper
- Delivery fee — usually $50–$100, flat per trip
- Minimum bulk order — commonly 1 cu yd (27 cu ft)
- Pickup option — bring a tarp; a half cubic yard fits in most pickups
One cubic yard of garden soil weighs roughly 1,000–1,500 pounds, depending on moisture and composition. That is the upper safe limit for a half-ton pickup. If you order more, plan two trips or accept delivery; an overloaded suspension is not worth the savings.
Settling and topping up raised-bed soil
Fresh raised-bed mix compresses about 10–15% in its first month, more if it is heavy on peat or compost. The calculator above adds 10% to the geometric figure so you order enough the first time. After that, expect to topdress with one to two inches of fresh compost each spring — the same one cubic foot per 6 square feet of bed surface.
Some loss is healthy: roots consume organic matter, microbes break it down further, and the structure improves year over year. A bed that sinks four inches in the first season and then stabilises is doing exactly what a living soil should do. A bed that keeps sinking year after year is shedding organic matter faster than you are adding it; double the annual compost topdressing until it holds level.
Common raised-bed soil mistakes
The single most common bag-count error is multiplying 4 × 8 × 12 (height in inches) and ordering 384 cubic feet of soil instead of 32. Always convert depth to feet first — divide inches by 12 — before multiplying.
Topsoil sold for landscape grading is often subsoil and clay, not the dark crumbly stuff a garden needs. Ask for "screened garden topsoil" or "raised bed mix" and look at a sample first. If it smells sour or comes out in clods, walk away.
Cardboard or landscape fabric under a new raised bed kills most of the existing turf before it can grow up through your soil. Two layers of overlapping cardboard breaks down inside a year and gives roots a clear path into the native soil below.
A bed that holds 32 cubic feet on day one will accept another two to three cubic feet of fresh compost in its second spring. Budget for it. Skipping the annual topdress is the fastest way to watch yields fall season over season.