Compost Calculator
How much compost you need depends on the job: a quarter inch over a lawn, an inch or two for a new lawn, three or four inches worked into a new bed. Pick the job (or enter your own depth) and the area for the cubic yards to order in bulk and the number of 1 or 1.5 cu ft bags.
What you'll need
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Worked example: 20 × 10 ft new bed, 3 in of compost
| Step | Working | Result |
|---|---|---|
| Volume | 20 × 10 × 3 ÷ 12 | 50 cu ft |
| Cubic yards | 50 ÷ 27 | 1.85 yd³ |
| 1 cu ft bags | 50 ÷ 1 | 50 bags |
| 1.5 cu ft bags | 50 ÷ 1.5 | 34 bags |
How deep to spread compost
| Job | Depth | Per 1,000 sq ft |
|---|---|---|
| Lawn topdressing | 1/4–1/2 in | 0.77–1.54 yd³ |
| New lawn, before seed or sod | 1–2 in | 3.09–6.17 yd³ |
| Existing bed, yearly | 1/4–1 in | 0.77–3.09 yd³ |
| New bed, worked into 8–12 in | 3–4 in | 9.26–12.35 yd³ |
Depths are from Oregon State University Extension's How to Use Compost in Gardens and Landscapes (EM 9308), which also gives its own check: 7.5 cubic yards covers 800 sq ft about 3 in deep.
Building the bed from scratch? Size the soil with the raised bed soil calculator, and laying a new lawn after the compost, the sod calculator counts the rolls and pallets.
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Frequently asked questions
- How much compost do I need?
- Area × depth in feet ÷ 27 gives cubic yards. A 20 × 10 ft new bed with 3 in of compost needs 1.85 yd³ (50 cu ft, or 50 one-cubic-foot bags). Topdressing 1,000 sq ft of lawn 1/4 in deep takes 0.77 yd³.
- How much compost should I put on my lawn?
- Oregon State University Extension recommends topdressing 1/4 to 1/2 in deep — about 0.77 to 1.54 cubic yards per 1,000 sq ft — ideally after core aeration so it reaches the root zone. For a new lawn, work 1 to 2 in into the soil before seeding or laying sod.
- How much compost do I add to a garden bed?
- For a new bed, spread 3 to 4 in and work it into the top 8 to 12 in of soil (OSU Extension). For an existing bed, add 1/4 to 1 in a year on the surface. Oregon State's own check: 7.5 cubic yards covers 800 sq ft about 3 in deep.
- How many bags of compost make a yard?
- A cubic yard is 27 cubic feet, so 27 one-cubic-foot bags or 18 bags of 1.5 cu ft. Beyond about 2 yards, bulk delivery is usually cheaper than bags.
- Is compost the same as topsoil?
- No. Compost is decomposed organic matter used to improve soil; topsoil is mineral soil that holds plants up. Beds and lawns are usually built with topsoil and improved with a few inches of compost — the topsoil calculator sizes the soil.
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