COMPOST
Compost calculator
Cubic yards of compost = square feet × depth in inches ÷ 324, and cubic feet = square feet × depth in inches ÷ 12. Enter your area and the depth you intend to work in, and the answer is the volume a supplier will quote in.
Worked example
A 500 sq ft vegetable plot with 2 inches of compost worked into the top layer: 500 × 2 ÷ 12 = 83.33 cubic feet, and 500 × 2 ÷ 324 = 1.54 cubic yards.
Bagged compost is sold whole, so divide by the bag volume and round up. A 1 cubic foot bag gives 84 bags; a 1.5 cubic foot bag gives 56; a 2 cubic foot bag gives 42. Read the bag before ordering — the same plot is 42 bags or 84 depending on nothing but the package.
Raised beds: the depth is the whole story
A raised bed is the case where compost volume gets large, because you are filling a box rather than dressing a surface — so a compost calculator for raised beds is answering a fill question, not a dressing question. Take a 4 ft × 8 ft bed, which is 32 sq ft:
- At a 1-inch dressing: 32 × 1 ÷ 12 = 2.67 cubic feet
- At 2 inches: 5.33 cubic feet
- At 6 inches — a partial fill: 16 cubic feet
- At 12 inches — filling the box: 32 cubic feet, which is 1.19 cubic yards
The arithmetic does not change; only the depth does. What that depth should be is a growing decision, not a calculator one: a dressing, an incorporation layer and a full fill are three different jobs, and the right figure comes from what you are planting and from your soil test. Measure the depth you actually need, then use the formula.
Compost is not topsoil, and the answer is usually not the same
They are sold in the same aisle and both come in bags and by the yard, but they do different jobs and are not interchangeable by volume. If your question is really about filling a bed to a finished grade rather than feeding the soil, the topsoil calculator covers that case, and the difference is set out on compost vs topsoil.
Bag or bulk
Compost is lighter than topsoil per cubic foot and usually lighter than mulch, so the delivery question is less about weight and more about how many bags you want to carry. Roughly: a cubic yard is 27 cubic feet, which is 27 bags of 1 cubic foot, 18 of 1.5, or 14 of 2. Past about a cubic yard, bulk is usually the simpler order — the bag-to-yard conversion shows the same arithmetic used for mulch.
Ordering checklist
- Area in square feet, with paths and unplanted strips subtracted.
- Depth in inches, and whether that is a surface dressing or something you are digging in.
- Bag volume off the package, not assumed.
- Whether the compost is finished. Unfinished compost continues to heat and can harm roots; buy from a supplier who states it is mature and screened.
- Salt and herbicide residue, if the compost is from manure or green waste — ask for the supplier's test data rather than assuming.
Related calculators
The mulch calculator runs the same volume engine for a surface cover, the coverage calculator subtracts a bare circle from a rectangular bed, and the data downloads page publishes the underlying coverage tables as CSV.
The arithmetic is from the definitions 1 cubic yard = 27 cubic feet and 1 foot = 12 inches. Depth guidance is deliberately left to your planting plan or soil test rather than quoted from a source that would not fit your site.