COMPARISON
Compost vs topsoil
Topsoil is the growing medium; compost is an amendment you mix into it. Topsoil is mineral soil — the stuff roots actually grow in — while compost is decomposed organic matter that improves the structure, drainage and nutrient holding of soil that is already there. Buy one to create or raise a growing layer; buy the other to improve one.
The practical difference when you are ordering
| Topsoil | Compost | |
|---|---|---|
| What it is | Mineral soil, sometimes blended with organic matter | Decomposed organic matter |
| Main use | Filling, raising, or building a growing layer | Improving soil that is already there |
| Typical depth | Set by how deep the roots need to go | Set by how much you are incorporating or dressing |
| Can you plant straight into it | Yes | Usually not on its own — it is a mix-in, and it shrinks as it finishes decomposing |
| Bag size you will meet | Often 0.75 cubic feet | Commonly 1, 1.5 or 2 cubic feet |
That last row matters more than it looks. Compost continues to decompose after you spread it, so a layer of pure compost settles and shrinks; topsoil does not lose volume the same way. If you are filling a bed to a finished grade, that difference is the whole reason the two products are not interchangeable.
Sizing an order of either one
The arithmetic is identical, which is why people assume the products are too:
cubic feet = square feet × depth in inches ÷ 12cubic yards = square feet × depth in inches ÷ 324
What differs is the depth you put in. A bed being filled with topsoil might be measured in many inches; the same bed being amended with compost might be a dressing measured in a couple. Run each through the topsoil calculator or the compost calculator with its own depth rather than assuming one order covers both jobs.
Worked comparison, same bed, same day
A 200 sq ft bed:
- Raising the grade with topsoil at 6 inches: 200 × 6 ÷ 12 = 100 cubic feet = 3.70 cubic yards
- Amending the same bed with compost at 2 inches: 200 × 2 ÷ 12 = 33.33 cubic feet = 1.23 cubic yards
Three times the volume for the topsoil, because the job is three times the depth. Neither number is wrong; they answer different questions.
Where a blend makes sense
Many suppliers sell a raised-bed or garden mix that is topsoil and compost blended. That is a reasonable product for filling a new bed, and the volume calculation is the same as for topsoil — you are filling a box. What you cannot do is assume a bag labelled "garden soil" is either pure topsoil or pure compost; read what the label says it contains, because that determines whether it is a fill product or an amendment.
How to decide in one question
Ask what is missing. If there is no soil there — a new bed, a raised box, a hollow where a stump came out — you need topsoil. If there is soil there and it drains badly, crusts, or feeds plants poorly, you need compost. If the answer is both, you are buying both, and each gets its own depth and its own calculation.
Related calculators
Compost calculator · Topsoil calculator · Mulch coverage calculator for the same area-and-depth arithmetic on a surface cover.
Note: bag sizes quoted are those seen on published product listings; depth is left to your planting plan rather than quoted from a source that would not fit your site.