How it works
compost volume = area (sq ft) × depth (ft)
The calculator multiplies length by width to get the area, converts your chosen depth from inches to feet by dividing by 12, and multiplies the two to get cubic feet. Cubic yards are that figure divided by 27, and litres use the exact 28.3169 litres per cubic foot. The maths lines up with the extension rule of thumb that one cubic yard of compost covers about 324 square feet one inch deep, 162 square feet two inches deep, or 108 square feet three inches deep. For depths, extension services suggest roughly a quarter-inch to top-dress an established lawn, a one- to two-inch layer tilled into a bed to amend it, and up to three inches when establishing a new bed. Bags are counted by rounding the cubic-foot total up to whole 1-cubic-foot bags; once you pass about a cubic yard, bulk compost is usually cheaper.
Sources
- Penn State Extension — Using Composts to Improve Turf Top-dress established lawns with about a ¼-inch layer of compost; amend soil with a 1- to 2-inch layer incorporated 4 to 6 inches deep.
- NC State Extension — Organic Lawn Care One cubic yard of compost covers about 324 ft² at 1 inch deep and about 108 ft² at 3 inches deep.
FAQ
How much compost do I need?
It depends on the area and how deep you spread it. A quarter-inch top-dressing over 1,000 square feet is about 21 cubic feet (0.77 cubic yard); a two-inch amendment over the same area is roughly 167 cubic feet (6.2 cubic yards). Enter your area and pick a depth above for an exact number.
How thick should I spread compost on a lawn?
Extension guidance is a thin layer, about a quarter to a half inch per application, so the grass blades still show through. Thicker layers on established turf can smother the grass. Repeat a light top-dressing each season rather than burying the lawn in one go.
How much compost to amend a garden bed?
To improve an existing bed, spread a one- to two-inch layer of compost and work it into the top 4 to 6 inches of soil. For a brand-new bed in poor ground, up to a three-inch layer is reasonable. The depth selector above matches these rates.
How many bags of compost are in a cubic yard?
A cubic yard is 27 cubic feet, so it equals about 27 standard 1-cubic-foot bags. Once your job needs a cubic yard or more, ordering bulk compost by the yard is usually far cheaper than buying dozens of bags.
How much area does a yard of compost cover?
A cubic yard is 27 cubic feet, so it covers about 324 square feet at one inch deep, 162 square feet at two inches, or 108 square feet at three inches (324 divided by the depth in inches). Halving the depth doubles the area a yard covers, which is why thin top-dressings stretch so much further than deep amendments.
Can you use too much compost?
Yes. Piling on more compost than the soil needs can build up excess nutrients such as phosphorus and, over years, affect water quality. Match the depth to the job — a thin top-dressing for lawns, a modest layer to amend beds — rather than adding as much as possible.
Volumes are estimates; compost settles and finished compost varies in density. Application rates are general extension guidance — match them to your soil test and plants. General gardening guidance, not professional advice.