How to use the compost calculator
- Enter the area as a rectangle, circle, triangle or known total.
- Enter the depth of compost you will spread before mixing it in. Fractions work: 0.25 for a quarter inch.
- Pick a moisture level if you want the weight. Compost volume does not change with moisture, but its weight does a lot.
- Choose a bag size to compare bagged compost with bulk delivery.
The compost formula
Cubic yards = area (sq ft) × depth (in) ÷ 12 ÷ 27
Weight (lb) = cubic feet × bulk density (lb/ft³)
Colorado State University Extension puts it simply: 3 cubic yards (81 cubic feet) covers 1,000 sq ft about 1 inch deep. For weight, Oregon State University Extension gives a rule of thumb of about 1,000 lb per cubic yard for screened compost at 50% moisture, and a range from 800 to more than 1,600 lb per cubic yard depending on moisture and particle size. The default here is 37 lb/ft³ (1,000 lb/yd³).
How deep to spread compost
Recommendations from Oregon State, Colorado State and Washington State University Extension:
- New vegetable or flower beds: 3–4 inches, worked into the top 8–12 inches of soil.
- Existing garden beds, yearly: 1/4 to 1 inch.
- New landscape plantings: 3–4 inches mixed into the top 6–8 inches.
- Lawn before seeding or sod: 1–2 inches mixed into the top 6 inches (one-time).
- Topdressing an existing lawn: 1/4 to 1/2 inch of fine, screened compost, ideally after core aeration.
- Trees and shrubs: 1–3 inches of coarse compost, kept 3–6 inches away from trunks.
Worked example
A new 20 × 10 ft vegetable bed with 3 inches of compost, plus 5%:
- Area: 200 sq ft
- Volume: 200 × 3 ÷ 12 = 50 cu ft; with 5% = 52.5 cu ft
- Cubic yards: 52.5 ÷ 27 = 1.94 yd³
- Weight at 37 lb/ft³: about 1,940 lb, just under a ton
- Bagged: 53 bags of 1 cu ft
Lawn topdressing: 1/4 inch over a 5,000 sq ft lawn is 5,000 × 0.25 ÷ 12 = 104 cu ft, or 3.9 cubic yards.
Coverage of one cubic yard
- 1/4 inch: 1,296 sq ft
- 1/2 inch: 648 sq ft
- 1 inch: 324 sq ft
- 2 inches: 162 sq ft
- 3 inches: 108 sq ft
Tips for buying compost
- Bulk is cheaper above about a yard. One cubic yard equals 27 bags of 1 cu ft.
- Ask for a test sheet. Composts made with manure or biosolids can be high in salts; Colorado State recommends lower rates for those.
- Check moisture yourself: weigh 5 gallons of packed compost and multiply by 40 to get pounds per cubic yard, a field method from Washington State University.
- Mixing compost into new soil? The topsoil calculator covers the soil itself.
Frequently asked questions
How much compost do I need for my garden?
Multiply the bed area by the depth in inches and divide by 324 to get cubic yards. A 200 sq ft bed at 3 inches needs about 1.85 cubic yards, or 1.94 with 5% extra.
How much does a cubic yard of compost weigh?
About 1,000 lb for screened compost at 50% moisture, according to Oregon State University Extension, and anywhere from 800 to more than 1,600 lb depending on moisture.
How much compost do I need to topdress a lawn?
At 1/4 inch, one cubic yard covers about 1,300 sq ft, so a 5,000 sq ft lawn needs roughly 3.9 cubic yards.
How many bags of compost are in a cubic yard?
There are 27 bags of 1 cu ft, 36 bags of 0.75 cu ft or 18 bags of 1.5 cu ft in one cubic yard.
How deep should compost be in a raised bed?
For a new bed, mix 3–4 inches of compost into the soil; each year after, add 1/4 to 1 inch. Do not fill raised beds with compost alone; use a soil mix.