Compost Calculator

Measure the beds, pick the rate for what you are actually doing, and get compost in cubic yards and bags. Every rate here comes with the depth its source says to work it in — spreading is only half the instruction.

How this is calculated

Areas are summed and multiplied by the layer depth: volume = total area × depth (at depth d inches, cubic yards = square feet × d ÷ 324).
Bags = volume ÷ your bag size, rounded up to whole bags.
Weight = volume × 1,000–1,500 lb per cubic yard, shown as a range because moisture dominates it.
No waste or settling factor is applied. Compost does break down and lose volume, but no source quantifies it for a surface application, so nothing is invented.

Data used, with sources

Value	Used as	Source
3–4 in — new vegetable or flower bed, first application	spread on the surface, then incorporate into the top 8–12 in	Oregon State University Extension Service ; Colorado State University Extension
0.25–1 in — established vegetable or flower bed	annual topdressing (¼ in is the routine rate; 1 in a year is the documented ceiling)	Oregon State University Extension Service ; Colorado State University Extension
1–2 in — lawn, one-time application	incorporate into the top 6 in	Colorado State University Extension
3–4 in — before installing landscape plants	spread, then mix into the top 6–8 in	Oregon State University Extension Service
Bulk density 1,000–1,500 lb/cu yd	weight shown as a range — Oregon State’s rule of thumb: screened compost at 50 % moisture runs about 1,000 lb per cubic yard, and very wet compost can exceed 1,500. The published band is therefore typical-to-very-wet; no source gives a floor, and dry compost runs lighter. Moisture is what moves this number, which is why compost is sold and applied by volume — treat any weight as a delivery-planning figure, not an order quantity.	Oregon State University Extension Service

Tons here means US short tons (2,000 lb).

The rate depends on what you are doing

Two extension services publish the same numbers independently, which is unusual enough to be worth trusting. Oregon State and Colorado State both put a first application for a new vegetable or flower bed at 3 to 4 inches, incorporated into the top 8 to 12 inches of soil. Both put the annual rate for an established bed an order of magnitude lower — Colorado State at a quarter-inch, Oregon State at a quarter-inch to 1 inch, with 1 inch named as the most to add in a year. Colorado State adds the case the others skip: a one-time lawn application at 1 to 2 inches, worked into the top 6. That is a different job from topdressing an established lawn, which the topsoil calculator carries at the much thinner cited depth.

The gap between 3 inches and a quarter-inch is the single biggest error available here, and it runs in the expensive direction. If you build a bed at the new-bed rate and then repeat it every spring, you are applying twelve times the maintenance rate indefinitely.

Spreading is half the instruction

Every cited rate on this page is published as a pair: a layer to spread, and a depth to work it into. Colorado State is explicit about why the second half matters — cultivate to the indicated depth, because "on compacted/clayey soils, anything less may result in a shallow rooting depth predisposing plant to reduced growth, low vigor, and low stress tolerance". Compost left sitting on top of a compacted soil is a mulch, not an amendment.

Colorado State also notes the corollary that makes the arithmetic honest: if your actual incorporation depth differs from the one the rate assumes, the rate has to change with it. A 3-inch layer mixed into 12 inches of soil is a very different amendment from the same layer mixed into 4.

Common mistakes

Filling a raised bed with compost alone. Oregon State says not to: compost lacks the mineral fraction soil needs, and the bed slumps as it decomposes. Use a mix — the raised bed soil calculator sizes each ingredient of a cited one.
Applying the new-bed rate every year. The establishment rate is 3–4 inches once; the maintenance rate is a quarter-inch a year.
Spreading without incorporating. The cited rates assume the compost is cultivated in to a stated depth.
Ignoring what the compost is made of. The published rates assume plant-derived compost; manure- and biosolids-based products, and anything high in salts, need substantially reduced rates.
Ordering by weight. Compost is sold by volume because its weight is mostly a moisture measurement.

Frequently asked

How much compost do I need for 1,000 square feet?

About 3.09 cubic yards per inch of depth. Colorado State Extension rounds that to "three cubic yards (=81 cubic feet) covers 1,000 square feet approximately 1 inch deep" — the exact arithmetic is 3.09, and we show the exact figure. Multiply by your layer depth: a new bed at 3 inches needs about 9.26 cubic yards over that area. The same check works on Oregon State's example — they say 7.5 cubic yards covers 800 square feet about 3 inches deep, and the exact figure is 7.41.

How deep should I spread compost?

It depends on whether the ground is new to you or already in production, and two extension services agree on the numbers. For a first application when installing a vegetable or flower garden, spread 3 to 4 inches and work it into the top 8 to 12 inches of soil. For a bed already established, a quarter-inch a year is the routine rate, and Oregon State puts 1 inch as the most to add annually. For a one-time lawn application, Colorado State gives 1 to 2 inches worked into the top 6 inches.

How many bags of compost is a cubic yard?

Pure arithmetic by bag size: a cubic yard is 27 cubic feet, so 27 one-cubic-foot bags, or about 14 two-cubic-foot ones. Oregon State notes bags "often contain 1-2 cubic feet", and gives a useful yardstick — a single 1-cubic-foot bag covers 12 square feet an inch deep. Covering 100 square feet 3 inches deep takes 25 one-cubic-foot bags, which is the point most people switch to bulk.

How much does a cubic yard of compost weigh?

Around 1,000 lb as a rule of thumb — Oregon State's compost-analysis guide puts screened compost at 50 % moisture near that, with very wet compost passing 1,500 lb per cubic yard. Moisture, not the material, is what moves the number, so treat any weight as a trailer-and-axle planning figure. Compost is bought and applied by volume for exactly this reason.

Can I use compost on its own as soil?

No, and this is the most expensive mistake on this page. Oregon State is unambiguous: "Don't fill raised beds with compost alone. Fill beds with a mix of native soil, compost or a soil mix. Compost lacks the mineral component of earthen soil and planting mixes." Compost is an amendment. A bed filled with pure compost slumps as it decomposes, dries out fast, and holds nutrients in a way that does not resemble soil. If you are filling a bed, use the raised bed soil calculator, which sizes each ingredient of a cited mix.

Can you add too much compost?

Yes. Colorado State scales its rates down over repeated years and warns that the published rates assume plant-derived compost: "For compost made with manure or biosolids and compost known, by soil test, to be high in salts, application rates will need to be reduced substantially." Excess compost drives up soil phosphorus and salts, and the surplus nutrients leave in runoff. If you are applying every year, the routine quarter-inch — not the 3 inches you used to build the bed — is the rate that matches the guidance.

Sources

Interpreting Compost Analyses (EM 9217) — Oregon State University Extension Service, accessed Aug 11, 2026.
How to use compost in gardens and landscapes (EM 9308) — Oregon State University Extension Service, accessed Aug 11, 2026.
CMG GardenNotes #241: Soil Amendments — Colorado State University Extension, accessed Aug 11, 2026.

Quantities on this page are planning estimates, not engineering advice or a purchase guarantee — product sizes vary by producer, so confirm coverage with your supplier before ordering. Application rates assume plant-derived compost and a soil test you have actually read. Nutrient and salt loading are governed by that test, not by this estimate.