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hortyard

Compost calculator

Enter the area and how thick a layer you want to spread. You get cubic yards and bags.

You need 0.25 cubic yards of compost (6.7 cubic feet), or 4 bags of 2 cu ft, or 3 bags of 3 cu ft. Example: one bed 20 by 4 ft, 1 in deep.

Estimates. Bag sizes, material density and settling vary; confirm quantities with your supplier.

Good to know

For a new garden bed, guides suggest spreading 1 to 4 inches of compost and mixing it into the top 6 to 8 inches of soil.

On beds that already have good soil, 0.25 to 2 inches a year keeps organic matter up.

On a lawn, a top-dressing is thin: 0.25 to 0.5 inches, raked into the grass.

Penn State: mix 1 to 2 inches of compost into the top 6 to 8 inches of soil for gardens or areas to be planted with turf.

Oregon State: new landscape beds 3 to 4 inches mixed 6 to 8 inches deep; new vegetable beds 3 to 4 inches mixed into the top 8 to 12 inches; existing vegetable beds 1/4 to 1 inch per year.

The arithmetic

Volume is area times depth. With the area in square feet and the depth in inches, multiply them and divide by 12 for cubic feet, then by 27 for cubic yards. A bag count is the cubic feet divided by the bag size, rounded up.

Questions

How much compost for a new 4 by 8 foot bed?

The bed is 32 square feet. Two inches of compost is 32 times 2 divided by 12, or about 5.3 cubic feet: three bags of 2 cubic feet, mixed into the top of the soil.

How much compost to top-dress a lawn?

Lawn top-dressing is thin. At a quarter inch, 1,000 square feet takes 1,000 times 0.25 divided by 324, about 0.77 cubic yards, raked in so the grass blades still show.

Can I use my own compost?

Yes. Finished compost is dark, crumbly and smells of soil, and the original scraps are no longer recognizable. Screen out large pieces before spreading it on a lawn.

Typical values from these university extension guides: