COMPOST
How much compost do I need?
Cubic feet of compost = square feet × depth in inches ÷ 12. Measure the area you are spreading over, decide the depth in inches, and that is the volume. Bags are the same number divided by the bag volume, rounded up.
Worked example
A 300 sq ft bed at 2 inches: 300 × 2 ÷ 12 = 50 cubic feet. In cubic yards that is 50 ÷ 27 = 1.85 cubic yards.
In bags, divide and round up:
| Bag volume | Exact bags | Bags to buy |
|---|---|---|
| 1 cu ft | 50.0 | 50 |
| 1.5 cu ft | 33.3 | 34 |
| 2 cu ft | 25.0 | 25 |
One bed, two dozen bags or four dozen, decided entirely by the package. Check the cubic feet on the bag before you decide the price is good.
Depth is the input that changes the answer most
Going from 1 inch to 3 inches triples the order for the same ground. That is why the depth has to come from what you are doing rather than from a default: a light dressing over a lawn, compost worked into a vegetable bed, and compost used to fill a raised box are three different depths and three very different volumes. Work out the depth you actually want, then multiply.
How much does a cubic yard of compost cover?
One cubic yard is 27 cubic feet, so it covers 324 ÷ depth in inches square feet:
- 1 inch: 324 sq ft
- 2 inches: 162 sq ft
- 3 inches: 108 sq ft
- 6 inches: 54 sq ft
Those follow from the same formula and are worth keeping, because bulk compost is quoted in cubic yards and this is the fastest way to sanity-check a quote.
Compost is sold by volume — but it shrinks
Unlike topsoil, compost keeps decomposing after it is spread. A layer of pure compost settles noticeably in the first season, so the depth you order is not always the depth you still have next year. If you are filling to a finished grade, plan for that settlement; if you are working compost into existing soil, it does not matter, because the volume is being mixed rather than stacked.
Ordering checklist
- Net area, with paths, patio and unplanted strips taken off.
- Depth in inches, chosen for the job.
- Bag volume read off the package.
- Whether it is mature and screened. Ask the supplier; immature compost can heat and damage roots.
- Bulk or bagged. Past about a cubic yard, bulk is usually the simpler order, and the bag-to-yard conversion shows how to compare them.
Related calculators
Compost calculator · Compost vs topsoil · Topsoil calculator
The arithmetic is from the definitions 1 cubic yard = 27 cubic feet and 1 foot = 12 inches. Depth is deliberately left to your own plan rather than quoted from a source that would not fit your site.