Topsoil vs Compost vs Garden Soil: Which to Buy (and How Much)

The short answer

Buy by project, not by bag: raised beds want a 50–60% topsoil / 40–50% compost mix; new in-ground beds want compost tilled into the native soil; lawn topdressing wants screened compost in thin layers; leveling and filling want topsoil (with fill dirt underneath for real depth — see our topsoil vs fill dirt guide). The flagship math: a 4×8×1 ft raised bed is 32 cu ft / 27 = 1.19 cubic yards — about 22 bags of 1.5 cu ft soil, or 16 of the 2 cu ft bags.

The 60-second material decoder

Topsoil is screened native soil — the mineral base with sand, silt, clay, and some organic matter. It's what lawns and beds actually grow in, and it's the cheapest option in bulk. Compost is decomposed organic matter — dark, rich, and full of nutrients. It's an amendment, not a soil: mixed into topsoil it improves structure and feeds plants; used alone it holds too much water, settles 20–30% as it keeps breaking down, and can burn young roots. Garden soil is a bagged blend of the two plus extras (peat, bark fines, sometimes starter fertilizer) — formulated so you can plant straight into the bag's contents. Convenient, but you pay for the convenience per yard (see the cost table below).

Project → material → quantity

Raised beds: the worked example

The classic 4 × 8 × 1 ft raised bed: 4 × 8 × 1 = 32 cu ft / 27 = 1.19 cubic yards. A 50/50 mix means ~0.6 yards of topsoil + ~0.6 yards of compost (a 60/40 split is 0.71 + 0.47 — either works; measure by bucketfuls, not precision). Bagged route: 32 cu ft / 1.5 = 21.3 → 22 bags of 1.5 cu ft soil, or 32 / 2 = 16 bags of 2 cu ft. Deeper beds scale linearly — a 4 × 8 × 2 ft bed needs 2.37 yards (see the table).

Money tip for deep beds: the bottom half of a 2-foot bed can be cheap filler — logs, branches, and leaves (hugelkultur style) or plain fill material — with your good mix in the top 10–12 inches where roots actually live. A 4 × 8 × 2 ft bed needs only ~1.2 yards of mix this way instead of 2.37.

New in-ground beds: amend, don't replace

For a bed dug into native soil, you don't need a bed-full of bagged product — you need 2–3 inches of compost tilled 6–8 inches deep into what's there. Worked example: a 10 × 6 ft bed = 60 sq ft; a 2-inch compost layer is 60 × (2/12) = 10 cu ft / 27 = 0.37 cubic yards — about 7 bags of 1.5 cu ft compost (10 / 1.5 = 6.67). Only if your native soil is truly hopeless (construction rubble, heavy clay pan) does full replacement with topsoil make sense — and then it's bulk topsoil by the yard, not bags.

Topdressing and leveling: compost vs topsoil

Topdressing a lawn (feeding it, not leveling) wants screened compost at ¼–½ inch: 1,000 sq ft at ½ inch = 1,000 × (0.5/12) / 27 = 1.54 cubic yards of compost. Spread it thin, water it in, let the grass grow through. Leveling wants topsoil — compost keeps decomposing and the low spot comes back. 2,500 sq ft at 1 inch of topsoil: 2,500 × (1/12) / 27 = 7.72 cubic yards, about 9.26 tons at topsoil's 1.2 tons per yard — a dump-truck delivery, and a reminder that “just an inch” is heavy on a big lawn.

Cost per yard: bulk vs bagged

The bagged-equivalent math: a 1.5 cu ft bag is 1.5 / 27 = 0.0556 yards, so 18 bags make a yard. At $5 a bag that's $90 per yard — roughly double bulk. The crossover is around 1–1.5 yards: below that, bags win on convenience (no delivery minimum, no shoveling a pile); above it, bulk delivery is decisively cheaper. That 4×8 raised bed at 1.19 yards sits right on the line — bulk if you have other projects, bags if this is the only one.

The “bagged lies” label math

Bag labels state volume at fill, and bags are filled fluffy — the contents settle. Three things to check: 1. The actual cubic feet, printed small on the bag (a “1.5 cu ft” bag is the standard unit of garden math — 18 per yard, 12 per half yard). 2. Expanded vs. compressed volume — peat-heavy mixes list compressed volume; fluffed, they're bigger, but you pay for compressed. 3. Moisture weight — a rain-soaked bag can weigh half again as much without containing more soil. When comparing bag prices, divide price by the stated cubic feet: the $4 bag that holds 1 cu ft ($4/cu ft) loses to the $5.50 bag that holds 2 cu ft ($2.75/cu ft) every time.

Common mistakes

Pure compost beds. Settles, holds water, burns seedlings — always blend with topsoil. Topsoil under concrete or pavers where fill dirt belongs — its organic content settles. Buying garden soil in bulk quantities bagged. Past ~1.5 yards the markup is painful. Leveling with compost. The low spot returns as the compost decomposes. Skipping the math on bags. Count bags from the yard figure (yards × 18 for 1.5 cu ft bags), not from vibes — the 4×8 bed is 22 bags, not “a trunkful.”