Ordering soil is an exercise in unit conversion. The bed is measured in feet and inches, the bag is labelled in cubic feet, the bulk delivery is priced in cubic yards, and the weight that matters for delivery and structures is in pounds per cubic yard. Get any one conversion wrong and you order too much soil, too little, or a load the truck cannot legally carry. This guide works through the volume formula, the three conversions, the settling allowance that experienced gardeners always add, and the bulk-versus-bags decision — and it keeps topsoil, fill dirt, and mulch in their separate lanes, because they are different materials with different jobs.

The volume formula, in feet and in yards

For a rectangular bed, volume is length × width × depth, all in the same unit. Measure in feet, with depth in feet: a bed 8 feet long, 4 feet wide, and 12 inches (1 foot) deep holds 8 × 4 × 1 = 32 cubic feet. Divide by 27 to convert to cubic yards: 32 ÷ 27 = 1.19 cubic yards. For a depth in inches, divide the inches by 12 first: 6 inches is 0.5 feet, so the same bed at 6 inches deep holds 8 × 4 × 0.5 = 16 cubic feet, or 0.59 cubic yards.

Circular beds use area × depth: area = π × radius². A circular bed 4 feet across (radius 2 feet) at 1 foot deep holds π × 4 × 1 = 12.57 cubic feet, or 0.47 cubic yards. For irregular shapes, split the area into rectangles and circles, compute each, and add — or measure the area with the square-footage method and multiply by the average depth. Depth is the measurement most often guessed: dig or probe to the actual depth at several points and average, because a bed that is 10 inches deep at the edges and 14 in the middle averages 12, not 14.

Worked set for common beds, before any settling allowance: a 4 × 4 foot bed at 6 inches needs 8 cubic feet (0.30 cubic yards); at 12 inches, 16 cubic feet (0.59). A 4 × 8 foot bed at 6 inches needs 16 cubic feet (0.59); at 12 inches, 32 cubic feet (1.19). A 3-foot-diameter container at 12 inches deep needs 7.07 cubic feet (0.26 cubic yards). These are the numbers the calculator produces; the table is here so you can sanity-check it by hand.

Bags, bulk, and the settling allowance

Bagged soil is labelled in cubic feet — commonly 1, 1.5, or 2 cubic feet — and occasionally in quarts (1 cubic foot ≈ 25.7 dry quarts). Bag count is volume needed ÷ bag volume, rounded up: 32 cubic feet ÷ 1.5 = 21.33, so 22 bags. The rounding matters: a fractional bag cannot be bought, and the last bag is usually the one that finishes the job. Bulk soil is sold by the cubic yard, usually with a minimum delivery (often 1 yard) and a delivery fee; a cubic yard is 27 cubic feet, so the 32-cubic-foot bed needs 1.19 yards of bulk soil, and you will order either 1.25 yards (if the supplier sells fractions) or 1.5 yards.

Settling takes about 10% of the volume in the first weeks as soil compacts and organic matter begins to break down — more for loose, compost-rich mixes, less for dense mineral soil. Add 10% to the computed volume before ordering: the 32-cubic-foot bed becomes 35.2 cubic feet, or 1.30 cubic yards, and the bag count at 1.5 cubic feet becomes 24 bags. Over-ordering by 10% costs little and saves a second trip; under-ordering by 10% leaves a bed visibly short. For a bed that must look finished on day one, fill to the frame and keep a bag or two in reserve for topping up after the first heavy rain.

Topsoil, fill dirt, and mulch are different materials

Topsoil is the upper layer of soil, containing organic matter and the biology that supports plant growth. It is what a garden bed is filled with. Fill dirt is subsoil — excavated material from below the topsoil layer — used for levelling, raising grades, and filling holes. It is cheaper, heavier, and largely sterile: plants grow poorly in it, because it lacks organic matter and structure. Compost is decomposed organic matter, used as an amendment mixed into topsoil, not as a standalone growing medium in large volumes, because it continues to decompose and settle. Mulch — bark, wood chips, straw — sits on top of the soil to retain moisture and suppress weeds; it is not mixed in, and it is sold and measured separately, by the cubic yard or by the bag, like soil.

Buying the wrong material is the most expensive soil mistake. Fill dirt in a vegetable bed produces stunted plants and a season of remediation; topsoil used for levelling under a shed wastes money on biology that will be buried and starved of air. If a supplier’s price seems too good for topsoil, ask whether it is screened topsoil, unscreened topsoil, or fill dirt — the three are priced differently for a reason, and the answer decides whether the bed grows.

Weight, delivery, and structural limits

Soil weight varies with moisture and composition: dry topsoil about 2,000 pounds per cubic yard, moist 2,200 to 2,400, wet up to 2,700. A 1.19-cubic-yard order therefore weighs roughly 2,400 to 3,200 pounds — more than a tonne and a half — which is why bulk soil is delivered, not collected in a car. A standard dump truck carries 10 to 14 cubic yards, but its legal payload governs: wet soil can exceed the payload before the bed is full, and the supplier, not the customer, decides how much goes on the truck.

For raised beds on decks, balconies, and roofs, check the structure against the wet weight. A bed of 2 cubic yards of wet soil weighs over 5,000 pounds before the timber, the water in the soil after rain, and the gardener standing on it. Most residential decks are not rated for that concentration of load. Ground-level beds have no such constraint, which is one more reason to keep large raised beds on the ground. If the bed must be elevated, spread the load, use lightweight growing mixes, and get a structural opinion — the cost of the opinion is trivial against the cost of a failure.

How deep should the soil be?

Depth decides what will grow. Shallow-rooted herbs and lettuce manage in 6 inches. Most vegetables need 12 inches of good soil. Root crops — carrots, parsnips — need 12 to 18 inches of loose, stone-free soil, or they fork and stunt. Shrubs and small trees need 18 to 24 inches. Lawns establish on 4 to 6 inches of topsoil over prepared subsoil. Deeper is not always better: soil below the root zone stays wet, compacts, and contributes nothing, while costing money and weight.

For an existing bed being topped up, measure what is there before ordering: scrape back the mulch, probe the depth in several places, and order only the difference plus settling. Topping up 2 inches on a 4 × 8 foot bed needs 5.33 cubic feet (0.20 cubic yards) — a few bags, not a bulk delivery. The calculator handles top-ups the same way as new beds: enter the existing depth and the target depth, and it computes the difference.

Frequently asked questions

How much soil do I need for a 4 × 8 raised bed?

At 12 inches deep: 32 cubic feet, or 1.19 cubic yards — 22 bags of 1.5 cubic feet. Add about 10% for settling (1.30 cubic yards, or 24 bags). At 6 inches deep, halve those figures: 16 cubic feet, 0.59 cubic yards, 11 bags.

How many bags of soil make a cubic yard?

It depends on the bag size: 27 bags of 1 cubic foot, 18 bags of 1.5 cubic feet, or 13.5 bags (so 14) of 2 cubic feet. Bag counts always round up, and adding 10% for settling raises the counts to 30, 20, and 15 respectively.

What is the difference between topsoil and fill dirt?

Topsoil is the organic-rich upper layer that supports plant growth. Fill dirt is subsoil used for levelling; it is cheaper and largely sterile, and plants grow poorly in it. Use topsoil for beds and fill dirt for raising grades and filling holes.

Should I buy soil in bulk or in bags?

Bags for under about 1 to 2 cubic yards, bulk above it. Bulk is much cheaper per cubic foot but carries a delivery fee and needs a dumping spot. Below the break-even, bags are cheaper all-in and far easier to handle.

How deep should soil be in a raised bed?

6 inches for shallow herbs and lettuce, 12 inches for most vegetables, 12 to 18 inches for root crops, 18 to 24 inches for shrubs. Deeper than the root zone wastes money and adds weight without benefit.

Does wet soil change the volume I need?

No — water changes the weight, not the volume, by much. But wet soil is far heavier (up to 2,700 lb per cubic yard against about 2,000 dry), which matters for delivery payloads and for structures under raised beds. Order by volume; check loads by wet weight.