Soil Calculator – How Much Soil Do You Actually Need?

The picture: your bed in cross-section

A side view of the fill. The brown layer is your soil at the depth you chose — change the depth above and watch the bed fill up or empty out. Underneath, the coverage table every gardener ends up needing.

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How it works in three steps

How the numbers are worked out

Going deeper

Four beds, four shopping lists

Every figure below was run through the calculator above, starting from its default bed — 4 ft by 8 ft, filled 6 inches deep with screened topsoil — and changing one thing at a time. The volume never depends on the material: the same bed always needs the same cubic yards. What the material changes is the weight, and by enough to decide whether a load is safe to carry.

The default row is worth unpacking because it contains the whole method. The bed is 32 square feet (4 × 8), the depth is 6 inches or 0.5 feet, so the volume is 32 × 0.5 = 16 cubic feet, and 16 ÷ 27 = 0.59 cubic yards. The weight range comes from the page’s material table: screened topsoil weighs 2,000–2,700 lb per cubic yard, and the calculator multiplies the volume by that range rather than pretending one exact number exists — giving the displayed 1,185–1,600 lb for this bed. Bag counts are the volume divided by the bag size, always rounded up, because bags are sold whole: 16 ÷ 1.5 = 10.67, so 11 bags of the 1.5 cu ft size, and likewise 22 bags of 0.75 cu ft, 16 of 1 cu ft, and 8 of 2 cu ft.

Doubling the depth doubles everything, exactly. At 12 inches the same bed needs 1.19 cubic yards (32.0 cubic feet), weighs 2,370–3,200 lb in topsoil, and takes 22 bags of the 1.5 cu ft size — every figure twice the 6-inch row, as it must be, since volume is area times depth and depth doubled. That linearity is useful for planning: a bed filled twice as deep costs twice the material and twice the weight, and a bed twice the area does the same. It also explains why depth is the input to get right before ordering: an error of two inches on a 6-inch fill is a third more or less material, while an error in length or width of a few inches barely moves the total.

The material rows are where the shopping decision actually lives. The same 0.59 cubic yards weighs 1,185–1,600 lb as topsoil, 593–948 lb as compost, and only 237–474 lb as wood mulch — the mulch load is roughly a fifth of the topsoil load for the identical volume. That is why the material choice matters far more for transport than for volume: a cubic yard of topsoil can quietly exceed a small pickup’s payload, while the same volume of mulch would not trouble it, and the page’s own guidance is blunt about the awkward middle — a full yard fits a small pickup’s bed easily but can exceed the payload on the door sticker. If a load is close to the vehicle’s limit, the page’s advice is to ask the yard for a scale ticket instead of trusting any chart, including this one, because moisture alone can push a yard of topsoil from near 2,000 lb when dry past 3,000 lb after a rainy week in an open stockpile. Moisture moves the weight, not the volume: the bed still needs 0.59 cubic yards whether the pile is dry or soaked.

The settling row answers the question every new raised bed asks a month later: why has the surface dropped? Loose soil is part air, and watering, rain and the soil’s own weight collapse those pockets while organic matter slowly breaks down, so a new bed can drop by a fifth or more in its first season. The calculator’s settling checkbox adds 10% before bags and weight are worked out: the default bed becomes 0.65 cubic yards (17.6 cubic feet) weighing 1,304–1,760 lb. Ten percent is a planning pad, not a guarantee against a fifth of settling — topping up later with compost is normal, and a bed that was filled exactly to the brim on day one should be expected to need it.

Reading the coverage table backwards — and buying in bulk or bags

The calculator has two directions, and confusing them is the most common ordering mistake. The bed direction answers “how much do I need for this bed?” — area times depth, converted to cubic yards, bags and weight. The coverage direction answers the opposite question: “how deep will this amount go over this area?” Its default is 1 cubic yard spread over 108 square feet, which sits 3.0 inches deep. Those two answers are consistent, not contradictory: 1 cubic yard is 27 cubic feet, and 27 ÷ 108 = 0.25 feet, which is 3 inches. The page’s own coverage table generalises the same division: 1 cubic yard covers 324 sq ft at 1 inch deep, 162 at 2 inches, 108 at 3 inches, 81 at 4 inches, 54 at 6 inches, and 27 at 12 inches — each row is 27 cubic feet divided by the depth in feet. The metric table works the same way: 1 cubic metre covers 40 m² at 2.5 cm deep, 20 m² at 5 cm, 13.3 m² at 7.5 cm, 10 m² at 10 cm, 6.7 m² at 15 cm, and 3.3 m² at 30 cm.

Use the coverage direction when you already have material — a delivered yard, a pile left from another job — and want to know how far it goes. Use the bed direction when you have a bed and want to know what to order. And use the weight table when you have a vehicle: the page’s material ranges are topsoil (screened) 2,000–2,700 lb per cubic yard, garden soil or raised-bed blend 2,200–2,700, compost 1,000–1,600, wood mulch 400–800, and dry sand 2,600–3,000, with per-cubic-foot equivalents of 74–100, 81–100, 37–59, 15–30 and 96–111 lb respectively. Sand is the heaviest load on the page, heavier than topsoil at the top of its range, which surprises people who think of sand as light because a single handful is. A cubic yard of it is not a handful.

The bulk-or-bags decision, from the page’s own guidance, is decided by volume. Below about half a cubic yard, bags win on convenience: no delivery fee, no pile on the driveway, and you can carry them to exactly where they are needed. Above a cubic yard the balance tips hard — that is 27 to 36 bags to lift, load and open, while a bulk tip lands it in one go. The default bed at 0.59 cubic yards sits in the awkward middle, where either choice is defensible: 11 bags of the 1.5 cu ft size is a manageable afternoon, while a bulk delivery of just over half a yard may carry a minimum-load fee. At 12 inches deep the same bed needs 1.19 cubic yards and 22 bags of the same size, and bulk starts to look like the obvious answer — unless the weight, 2,370–3,200 lb of topsoil, is more than the vehicle or the driveway can take, in which case bags, carried in trips, are the safer way to move the same material.

Depth, shape, and the numbers that do not change

Three things about this calculator never change, whatever bed you enter, and they are worth knowing because they stop three common errors. First, the material never changes the volume. Compost, mulch, topsoil and sand all fill the same 0.59 cubic yards in the default bed; only the weight and the bag count’s underlying density differ, and the bag count does not differ at all, since bags are sold by volume. Anyone who orders “less compost because it is lighter” is ordering the wrong volume for the bed. Second, the conversion constants never change: 27 cubic feet to a cubic yard, 0.45 cubic metres and 453 litres for the default bed’s 16 cubic feet. Third, rounding always goes up for bags and never down for volume: 10.67 bags becomes 11, never 10, because a bed filled to 10 bags’ worth is a bed filled short, and the shortfall is discovered at the far end of the bed, where it is hardest to fix.

Shape matters only through area. A rectangle is length times width; a circle is π times half the diameter squared. The calculator handles both, and the rest of the arithmetic — area times depth, divided by 27, multiplied by the material’s density range — is identical. What shape does change is waste: a round bed cut from rectangular boards, or a bed fitted around existing plants, will use material less tidily than the arithmetic suggests, which is another reason the 10% settling pad is worth ticking even when the bed is not new. The pad costs little — on the default bed it adds 1.6 cubic feet and roughly 120–160 lb — and it covers both settling and the small shortfalls of real beds, real boards and real pouring.

Finally, a note on what “estimated” means on this page. The volume is exact arithmetic from your measurements: if the bed is truly 4 by 8 feet at 6 inches, it truly needs 16 cubic feet. The weight is an estimate, honestly ranged, because soil is not a manufactured product and its density changes with composition and moisture. Treat the two halves of the answer differently: order the volume with confidence, and treat the weight as a range to check against the vehicle’s payload, the trailer’s rating, and the yard’s scale ticket. A shopping list that is exact where it can be exact and ranged where it must be ranged is more useful than one that pretends to know what it cannot.

Ordering for more than one bed is where the arithmetic pays off twice. Two default beds — each 4 by 8 feet at 6 inches — need exactly the same volume as one bed at 12 inches: 32 cubic feet, or 1.19 cubic yards, weighing 2,370–3,200 lb in topsoil and taking 22 bags of the 1.5 cu ft size. That equivalence is useful because it lets you order once for several beds instead of once per bed, and a single bulk delivery for the combined volume is usually cheaper and simpler than separate bag runs — provided the combined weight is within what the vehicle and the driveway can take. If it is not, the same arithmetic tells you how to split the order: one bed at a time, 0.59 cubic yards and 11 bags each, carried in trips that stay inside the payload. The calculator answers per bed; you do the multiplying, and the multiplying is exact because volume scales linearly with area and depth.

One more practical point: measure twice, in the units the bed was actually built in. A bed built from 8-foot boards is 8 feet long, but a bed built from metric timber or against a wall that is not quite square may be a few inches different in each direction, and those inches compound across the area. The calculator accepts metric inputs alongside imperial ones for exactly this reason — length and width in metres, depth in centimetres — and converts them before the volume is worked out, so a bed measured in centimetres does not need hand conversion first. Whatever units you measure in, measure the inside of the bed, not the outside: the boards themselves do not need filling, and on a small bed the thickness of the timber is a noticeable fraction of the area. A bed that is 4 by 8 feet outside with 2-inch boards is closer to 3 feet 8 inches by 7 feet 8 inches inside, and that difference, across the whole bed, is material you would otherwise order and never use.

Soil types and weight: why the same volume weighs differently

A cubic yard of soil does not have one weight. Dry topsoil weighs roughly 2,000 pounds per cubic yard; moist topsoil, 2,200 to 2,400; wet, compacted soil can reach 2,700 or more. Fill dirt is usually lighter and less consistent, garden soil with compost lighter still, and sand heavier — dry sand runs about 2,600 pounds per cubic yard. The calculator on this page shows a weight range rather than a single number for exactly this reason: the moisture and composition of the delivered material move the weight by hundreds of pounds per yard, and a single figure would be false precision.

Weight matters for two decisions: delivery and structure. A bulk delivery is priced by the yard but carried by the truck — a standard dump truck carries about 10 to 14 cubic yards, and its legal payload may be the binding constraint on wet soil. A raised bed on a deck or roof must be checked against the structure’s load capacity with the wet weight, not the dry: a bed holding 2 cubic yards of wet soil carries over 5,000 pounds, concentrated on a small footprint. When in doubt, use the top of the weight range for structural checks and the middle for delivery planning.

Bulk or bags: the break-even, honestly calculated

Bags win for small jobs and lose fast as volume grows. A 1.5-cubic-foot bag covers a 4 × 8 foot bed to 6 inches deep only after about 11 bags (16 cubic feet ÷ 1.5), and a 2-cubic-foot bag after 8. Bulk soil, delivered by the cubic yard, is typically several times cheaper per cubic foot than bagged soil, but it carries a delivery fee that dominates small orders and requires somewhere to dump a pile. The break-even usually sits around 1 to 2 cubic yards: below it, bags are cheaper all-in and far easier to handle; above it, bulk wins and the saving grows with every additional yard.

Bag counts always round up — you cannot buy part of a bag, and settling takes 10% or so of the volume in the first weeks, which is why the calculator offers a settling allowance. Buy the extra bag rather than the exact count: returning for one more bag costs more time than the bag costs money, and a bed that settles below its frame looks unfinished and grows poorly at the edges.

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