Compost is the cheapest, most effective way to improve almost any soil, and how much you need comes down to the area you are amending and how deep you work it in. This calculator turns those two numbers into a volume, then into bags and a bulk figure, so you can choose the cheaper route for the size of your garden.
How compost volume is calculated
Cubic yards = cubic feet ÷ 27
Bags = cubic feet ÷ bag size, rounded up
As with all soil amendments, depth is in inches while area is in feet, so it is converted before multiplying. The depth you choose depends entirely on the job — a thin topdressing or a deep bed amendment.
How deep to apply compost
- Topdressing a lawn: a thin quarter to half inch, raked in so it does not smother the grass.
- Amending a vegetable or flower bed: 2 to 3 inches, dug into the top several inches of soil.
- Building a new bed: a deeper layer, often blended with topsoil to a full planting depth.
Application depth guide
| Use | Depth |
|---|---|
| Lawn topdressing | ¼–½ in |
| Established bed refresh | 1–2 in |
| New bed amendment | 2–4 in worked in |
| Raised bed fill | blend to full depth |
A worked example
A 20×10 ft vegetable bed amended at 2 inches, with 1.5 cu ft bags:
- Volume = 20 × 10 × 0.167 = 33 cu ft
- Cubic yards = 33 ÷ 27 = 1.2 yd³
- Bags = 33 ÷ 1.5 = 22 bags — or a bulk yard, which would be cheaper
Bagged versus bulk compost
Bagged compost is screened, weed-free and convenient for raised beds and small gardens, but expensive by volume. Bulk compost, delivered by the yard, is far cheaper and ideal for larger beds and lawns, though quality varies more — ask whether it is fully matured, since unfinished compost can rob nitrogen from the soil as it breaks down. The crossover is around one to one-and-a-half cubic yards.
Compost as part of soil building
Compost works alongside topsoil and mulch in a healthy garden: topsoil provides the body, compost the fertility and structure, and mulch on top conserves moisture and suppresses weeds. The related topsoil and mulch calculators size those layers. Amend with compost before planting or laying sod — the related sod calculator covers the lawn — and the whole soil profile is ordered together.
Quality and maturity matter
Not all compost is equal, and immature compost can harm rather than help. Finished, mature compost is dark, crumbly and smells earthy, with no recognisable food scraps or ammonia odour. Unfinished compost still breaking down robs nitrogen from the soil as microbes continue decomposing it, temporarily starving plants — and it can carry weed seeds if it never reached hot composting temperatures. When buying bulk, ask whether it is fully matured and screened; when making your own, let it finish completely before spreading on beds. The volume maths is the same regardless, but the quality determines whether the compost feeds your plants or competes with them.
Mixing in versus topdressing
How you apply compost depends on the planting. For garden beds being prepared or amended, work the compost into the top several inches of soil so roots find it throughout the root zone, rather than layering it on top where only the surface benefits. For established beds, perennials and around shrubs, a thin surface layer as mulch works well — worms, rain and time carry it down without disturbing roots. On lawns, a thin topdressing raked in feeds the grass and improves the soil without smothering it. Match the method to whether you can disturb the soil or must work around living plants.
Building raised beds and new gardens
For raised beds and brand-new gardens, compost is rarely used neat — it is blended with topsoil to create a balanced growing medium. A common mix is roughly a third compost to two-thirds topsoil, giving fertility and structure without the medium being so rich it holds excess water or burns seedlings. The related topsoil calculator sizes the bulk of the fill, and this calculator sizes the compost share. Mix the two thoroughly rather than layering, fill the bed allowing for some settling, and top up after the first watering settles the blend.
Compost in a healthy soil system
Compost is one part of building living soil alongside topsoil and mulch. Topsoil provides the body and mineral content, compost adds organic matter, fertility and the microbial life that makes nutrients available, and mulch on top conserves moisture and suppresses weeds while slowly breaking down to feed the soil further. The related topsoil and mulch calculators size those layers. Amend with compost before planting or laying sod — the related sod calculator covers the lawn — and the whole soil profile is planned and ordered together, giving plants the best possible start and improving the ground year on year.
Estimating cost and making your own
Compost is cheap by the bag and cheaper still in bulk, and the calculator's volume figure lets you price both routes and find the crossover, usually around a cubic yard. But the lowest-cost compost is the kind you make yourself from kitchen and garden waste, which over time supplies a steady stream of free soil amendment and diverts waste from the bin — a compost bin or pile is a modest setup that pays back season after season. When buying, prioritise mature, screened compost over cheap, unfinished material that can rob nitrogen and carry weeds. Whether bought or homemade, compost works alongside topsoil and mulch in a healthy soil system, and the related calculators size those layers. Amend beds before planting, blend compost into raised-bed fill, topdress lawns thinly, and the soil improves year on year — few garden investments return as much for as little as building organic matter into the ground with compost.
Frequently asked questions
How much compost do I need?
For amending soil, work in a 1 to 3 inch layer. Measure area × depth and divide by 27 for cubic yards. A 20×10 ft bed at 2 inches needs about 1.2 cubic yards.
How deep should I apply compost?
Topdressing an established bed or lawn needs only ¼ to 1 inch; building or heavily amending a bed uses 2 to 4 inches worked into the soil. New raised beds may use a compost-soil blend at full depth.
How many bags of compost in a cubic yard?
At a 1.5 cu ft bag size, a cubic yard (27 cu ft) is 18 bags. Bulk delivery is more economical once you need a yard or more.
Should compost be mixed in or left on top?
For garden beds, work compost into the top several inches of soil so roots reach it. As a mulch or topdressing on lawns and around established plants, a thin layer on the surface works fine and breaks down in.