Compost Calculator
Compost is decomposed organic matter you mix into soil to feed it — not a top layer like mulch. Work out how many cubic yards or bags you need below.
Sets a typical depth — tweak anything below.
For a 20 ft × 10 ft area at 2 in deep, order about 1.36 cubic yards (0.69 US tons) of compost.
Estimates only. Densities vary by moisture, compaction and supplier — confirm quantities before ordering.
What is compost?
Compost is organic matter — leaves, bark, food waste, manure — broken down by bacteria until it's dark, crumbly and stable. It isn't soil and it isn't mulch: it's a soil amendment. It holds water, feeds soil life, and loosens up clay, but it keeps decomposing after you spread it, which is why it shrinks and needs topping up.
What is compost used for?
Mix it into beds before planting, blend it with topsoil to fill a raised bed, or spread a thin layer over a lawn as topdressing. The rule of thumb is that compost goes in the soil, while mulch goes on top of it.
How much does compost weigh?
A cubic yard of compost weighs about 0.51 US tons — roughly 600 kg/m³, or about 37 lb/ft³. Moisture and compaction shift this, so the calculator applies this density and lets you change it in the extra options if your supplier quotes a different figure.
Coverage by depth
| Depth | Coverage per cubic yard | Bags per 100 sq ft (1.5 cu ft each) |
|---|---|---|
| 1 inch | 324 sq ft | 5.6 bags |
| 2 inches | 162 sq ft | 11.1 bags |
| 3 inches | 108 sq ft | 16.7 bags |
| 4 inches | 81 sq ft | 22.2 bags |
A cubic yard is 27 cubic feet, so it holds 18.0 bags. 1–2 inches dug into a bed is typical; a quarter inch is all a lawn topdressing needs.
Common mistakes to avoid
- Guessing the depth. Too shallow and the material shifts and shows the ground beneath. Most surface layers want 2–4 inches; load-bearing bases are built up thicker.
- Forgetting to convert inches to feet. Depth is quoted in inches but volume math is in feet — 4 inches is 0.33 ft, not 4. The calculator handles this, but hand estimates often don't.
- Skipping the waste allowance. Ground is never perfectly flat and material settles once it's placed. Order 5–10% extra so a small shortfall doesn't trigger a second delivery.
- Using the wrong density. Each material weighs differently, so tonnage can be off by 20%+ if you use a generic figure. Pick the right material and adjust the density if your supplier differs.
- Buying bags for a big job. Bagged material costs far more per cubic yard than bulk. Past roughly one cubic yard, a bulk delivery is almost always cheaper.
Frequently asked questions
How much compost do I need?+
For amending an existing bed, spread 1–2 inches over the surface and dig it into the top 6–8 inches — that's about 0.3 to 0.6 cubic yards per 100 square feet. For filling a raised bed, compost is usually 30–40% of the total volume, not all of it. For topdressing a lawn, a quarter inch is plenty. Enter your area and depth above for the exact figure.
What's the difference between compost and mulch?+
Compost goes in the soil; mulch goes on top of it. Compost is fully broken down and feeds the soil, so you mix it in. Mulch is coarse wood or bark that sits on the surface to block weeds and hold moisture, and it's meant to break down slowly. Use compost as a surface layer and it'll grow weeds beautifully — it's basically a seedbed. Use mulch dug into soil and it steals nitrogen from your plants as it rots.
Can you plant directly in pure compost?+
You can, but don't. Pure compost is too rich for most plants, holds too much water, and shrinks dramatically as it finishes breaking down — fill a raised bed with it and the bed will be noticeably lower by next season. Blend it with topsoil, roughly 30–40% compost to 60–70% topsoil, and you get the nutrition without the collapse.
How much does a yard of compost weigh?+
Around 1,000 lb a cubic yard — about half a US ton, and roughly double what a yard of mulch weighs but well under half a yard of gravel. Moisture swings it a lot: compost is sold damp and a wet yard can push well past 1,400 lb. A half-ton pickup can take about a yard of compost, which is more than it can take of gravel.
How deep should compost be on a lawn?+
A quarter inch, half an inch at the absolute most, raked in so the grass blades still show through. This is the one people get wrong — bury a lawn under 2 inches of compost and you smother it. Topdressing is meant to be a dusting you repeat each year, not a layer.