CalculatorMulchTopsoil vs Garden Soil vs Compost vs Potting Mix
What actually separates these five products?
The proportion of mineral soil to organic matter, and how coarse the whole thing is. Everything else — the name on the bag, the price, the weight — follows from that.
| Product | What it is | Typical lb per cubic yard | Belongs in | Max share of a growing mix |
|---|---|---|---|---|
| Fill dirt | Unscreened subsoil, little organic matter | 2,200–2,900 | Grade changes, backfill below root zone | 0% |
| Screened topsoil | Native upper-layer soil, sieved | 2,000–2,700 | Lawn base, bulk of a bed, levelling | 50–60% |
| Garden soil | Topsoil pre-blended with compost | ~2,035 (calculated) | In-ground beds and borders | 100% as sold |
| Compost | Decomposed organic matter | 1,000–1,600 | Amendment, topdressing, mix component | 25–30% |
| Potting mix | Peat or coir, bark, perlite — no soil | Light; sold in dry quarts | Containers and seed starting only | Containers only |
The garden soil figure is worth explaining, because nobody publishes it. It is a blend, so its weight is a blend too: 70 percent topsoil at a midpoint 2,350 lb per yard plus 30 percent compost at a midpoint 1,300 gives 2,035 lb per cubic yard. That is meaningfully lighter than straight topsoil and it is why a yard of garden soil is a slightly less alarming thing to put in a pickup. All the underlying ranges are in the density and truck-load guide.
Which one does my job need?
Match the product to the depth and the purpose, then calculate the volume once.
| Job | Product | Depth | Volume for 500 sq ft |
|---|---|---|---|
| New lawn from seed or sod | Screened topsoil | 4–6 in | 6.17–9.26 cu yd |
| Levelling and lawn repair | Screened topsoil | 0.5–1 in | 0.77–1.54 cu yd |
| Topdressing an established lawn | Compost | 0.25–0.5 in | 0.39–0.77 cu yd |
| New in-ground planting bed | Garden soil, or topsoil + compost | 6–12 in | 9.26–18.52 cu yd |
| Raised bed | Blend (see below) | 6–24 in | by bed, not by area |
| Raising a grade or filling a hole | Fill dirt | as required | by depth |
| Pots, troughs, baskets | Potting mix | full container | by container volume |
Two rows deserve a second look. Topdressing at a quarter to half an inch sounds trivial and is not — over 500 sq ft it is still up to three-quarters of a cubic yard of compost, roughly 1,000 lb to move by barrow. And the new lawn row is where budgets die: 500 sq ft at 6 inches is 9.26 cubic yards weighing about 11 tons, which is a delivered load and not a weekend of pickup trips. That arithmetic is in mulch weight and truck loads, and the volumes come out of the topsoil calculator.
Why does compost have a ceiling?
Because it keeps disappearing. Compost is organic matter partway through decomposition, and the process does not stop when you put it in a bed — the material continues breaking down, losing volume and releasing nutrients for a year or more. A bed that is half compost drops noticeably every season and needs topping up indefinitely.
Three other things go wrong above roughly 30 percent:
- Water. Compost holds a great deal of it. In a wet spring, a compost-heavy bed stays saturated and roots suffocate.
- Salts and nutrients. Finished compost carries soluble salts. In the open ground rain leaches them; in a confined raised bed at high concentration they can scorch seedlings.
- Structure. Mineral particles are what hold a bed open at a stable volume. Take them out and the bed behaves like a compost heap, because that is what it is.
The exception is a deliberately soil-less mix such as Mel’s Mix, where a third compost is balanced by a third aeration material — a different system with different rules, covered in how to fill a raised bed.
What does the bag versus bulk premium look like?
This is where the money actually is, and it dwarfs the difference between the products themselves.
| Product | Bag | Bags per cubic yard | Bag price | Cost per cubic yard | Bulk per yard |
|---|---|---|---|---|---|
| Screened topsoil | 40 lb (0.5 cu ft) | 54 | $2.99 | $161.46 | ~$30 |
| Compost | 1 cu ft | 27 | $5.49 | $148.23 | ~$45 |
| Garden soil | 1 cu ft | 27 | $5.99 | $161.73 | ~$50 |
| Raised bed mix | 1.5 cu ft | 18 | $9.98 | $179.64 | ~$55 |
Topsoil is the extreme case: a factor of more than five, for material that came out of the same screening plant. The 40 lb bag is the culprit — it looks like a lot because it is heavy, but at roughly half a cubic foot it takes 54 of them to make a cubic yard. Anyone filling a bed from 40 lb bags is paying a premium of well over a hundred dollars per yard for the privilege of carrying it in small pieces.
Bags are still right for small quantities, awkward access and anything you might return. Past a cubic yard, the delivery fee is the cheapest line on the invoice.
What if you are not sure which to buy?
Work from depth. Anything you are placing more than 4 inches deep should be mostly mineral — topsoil, garden soil or fill dirt depending on whether plants are going in it. Anything you are placing less than 2 inches deep is a surface treatment, and that is compost’s job. Containers are always potting mix, without exception.
If a supplier offers a single unnamed “soil” by the yard, ask two questions: is it screened, and does it contain compost. Those two answers place it in the table above, whatever it is called on the sign. Then size the order with the cubic yards calculator before you agree to anything.
Frequently asked questions
Can I use garden soil in pots and containers?
It is a poor idea. Garden soil contains mineral particles that pack tight in the confined volume of a pot, so drainage collapses and roots sit wet; it also weighs several times what a potting mix does, which matters on a balcony or a hanging basket. Potting mix is engineered around porosity, which is why it is soil-less. The reverse substitution is just as bad — potting mix in an in-ground bed decomposes and vanishes within a season or two.
Is topsoil the same as fill dirt?
No, and the price difference tells you so. Topsoil is the biologically active upper layer, screened to remove stones and roots, and it will grow things. Fill dirt is subsoil taken from below that layer, usually unscreened, with little organic matter and often high clay content. Fill dirt is the right and cheap answer for raising a grade, backfilling a hole or building up under a patio, and the wrong answer everywhere a plant is going.
How much compost can I add to a bed?
Around 25 to 30 percent of the finished volume, which for a bed dug to 8 inches means an inch to two inches worked in. More than that and you get a mix that holds too much water, keeps shrinking as the organic matter breaks down, and can push nutrient and salt levels higher than seedlings tolerate. Compost is an amendment; the mineral fraction is what gives a bed its structure.
Why is bagged soil so much more expensive than bulk?
Because the bag, the filling line, the pallet and the retail shelf all cost more than the material inside them. A cubic yard of screened topsoil is around $30 delivered in bulk, and the same cubic yard assembled from 54 bags of 0.5 cu ft costs roughly $161. Bags earn that premium on small jobs, where they save a delivery fee and let you carry material through a house or a gate. On anything above a yard they stop making sense.
How we calculate this
Sources: