Topsoil vs Compost vs Garden Soil: How Much of Each
Three bags sit shoulder to shoulder on the garden center rack: topsoil, compost, garden soil. The photo on the front of each is close to interchangeable — dark, crumbly earth, something green growing out of it. The prices are not. At my local big-box store this spring, topsoil ran $2.48 for a 1-cubic-foot bag, compost $4.98 for 1.5 cubic feet, and garden soil $7.48 for 1.5 cubic feet. Per cubic foot that works out to $2.48, $3.32, and $4.99 — the same-looking dirt at one, one-and-a-third, and two times the price.
They are three different products for three different jobs, and grabbing the wrong one either wastes money or gives you a result that sinks, slumps, or settles. The good news is that sorting them out takes about two minutes, and the volume math is identical for all three.
What is actually in each bag
Topsoil is screened mineral soil: sand, silt, and clay with the rocks and roots sifted out, sometimes with a little organic matter mixed in. It is the cheapest of the three because it is, bluntly, dirt that has been run through a screen. That is not an insult — when the job calls for volume and stability, plain dirt is exactly what you want.
Compost is fully decomposed organic matter: leaves, yard waste, manure, food scraps, broken down until it is dark and uniform. It is an amendment, meant to be blended into soil, and it keeps shrinking as it continues to break down. Used alone as fill, a pile of it will lose a noticeable fraction of its height within a season.
Garden soil is a pre-blended mix — typically topsoil plus compost plus bark fines or peat — designed to go straight into a planting bed. You are paying someone else to do the mixing, which is why it costs roughly twice what topsoil does per cubic foot. (You will also see “raised bed soil” and “potting mix” on the same shelf. Potting mix is the odd one out: it usually contains no actual soil and is built for containers.)
Match the bag to the job
Topsoil is the fill product: leveling low spots in a lawn, filling holes, building up an area, or providing the bulk bottom layer in a large raised bed. Compost is the mixing product: blended into beds or spread an inch or two thick over the surface. Garden soil is the convenience product: new planting beds and big planters when you do not want to buy two ingredients and blend them yourself.
The difference between those bags cost me real money before it taught me anything. Last spring I set out to level the dips in my back lawn — about 120 square feet of low spots averaging 2 inches deep, which is 120 × 2/12 = 20 cubic feet of fill. I grabbed ten bags of garden soil at $7.48 each because the bag said “soil” and the picture looked right: $74.80 for 15 cubic feet. Halfway through spreading it I finally read the back label — compost, bark fines, peat. A planting mix. Fluffy, rich, and destined to decompose and settle, which is the opposite of what you want under grass you are trying to level. Plain topsoil, at $2.48 per cubic foot versus the garden soil’s $4.99, was the right product the whole time. I finished the job with five 1-cubic-foot topsoil bags for $12.40, bringing my total to $87.20. Twenty bags of topsoil from the start would have cost $49.60. That $37.60 gap bought me nothing except a lesson about reading the back of the bag instead of the front.
The ratios people actually use
When you blend your own bed mix instead of buying garden soil, two recipes cover most situations. Landscape suppliers’ default for a new in-ground bed is around 70/30 — seventy percent topsoil, thirty percent compost by volume. For raised vegetable beds, people commonly run richer, anywhere from 60/40 down to 50/50. If you are filling a standard raised bed, the bag-by-bag breakdown is in our guide to soil for a 4×8 raised bed.
For amending an existing bed, the usual move is a 1- to 2-inch layer of compost spread over the surface and worked in. Two inches across a 4×12 bed is 48 × 2/12 = 8 cubic feet — six 1.5-cubic-foot bags, with a cubic foot to spare.
One honest limit: I can tell you exactly how many bags a 2-inch layer takes. Whether your particular soil wants 2 inches of compost or none at all is a soil-test question, and no shopping calculator should pretend to answer it.
The volume math for a mixed order
Every one of these products obeys the same arithmetic: area in square feet × depth in feet = cubic feet, and 27 cubic feet make a cubic yard. Our mulch and soil calculator runs that conversion and the bag counts for any depth — the math does not care whether the bag says mulch, soil, or compost.
Worked example: a new 10×20-foot planting bed, built up 6 inches. That is 200 × 0.5 = 100 cubic feet, or 100 ÷ 27 = 3.7 cubic yards. At a 70/30 blend, you need 2.6 yards of topsoil and 1.1 yards of compost. Bulk yards usually sell in half-yard increments, so the realistic order is 2.5 and 1.5, or 3 and 1 if your soil is already decent.
Buying that volume in bags is brutal: 100 cubic feet is 67 bags of garden soil at 1.5 cubic feet each, which at $7.48 a bag comes to $501.16. The same volume delivered in bulk — at my local yard’s prices of $38 a yard for topsoil and $48 for compost — runs $98.80 plus $52.80 plus a $65 delivery fee, or $216.60. Less than half, and nobody has to wrestle 67 bags.
Weight is the other reason bulk gets delivered. Typical bulk densities run about 1.0–1.1 tons per cubic yard for topsoil and 0.9 for compost (shredded mulch, for comparison, is a featherweight 0.4). That 3.7-yard order weighs somewhere north of 3.5 tons. Plan on a dump truck, not a weekend of trunk loads.
Bag-to-yard cheat sheet
A cubic yard is 27 cubic feet, so the conversion depends entirely on bag size — and soil bags are smaller than mulch bags. At 1 cubic foot per bag, a yard is 27 bags. At 1.5 cubic feet, 18 bags. Mulch’s standard 2-cubic-foot bag makes a yard 13.5 bags. Brands vary, so check the printed volume every time; two bags sitting side by side at the same price can differ by half a cubic foot.
The short version
Cheap dirt for volume, expensive dirt for planting, compost for blending in. Topsoil fills and levels, compost amends, garden soil saves you the mixing at double the per-foot price. The photograph on the front of the bag tells you nothing — the volume and ingredient list on the back, divided by the price, tells you everything.