Compost Calculator – How Much Compost Do I Need?

How to use the compost calculator

Calculate compost

Compost weighs about 0.55 tons (1,100 lb) per cubic yard and is usually sold in 1 cu ft bags. A quarter-inch layer top-dresses a lawn; 1 to 2 inches worked in feeds a garden bed. Estimates only - confirm quantities with your supplier.

Choose the shape of your area: rectangle, circle or triangle.
Enter the length and width (or diameter) in feet.
Enter the depth in inches. A quarter inch for a lawn top dressing, 1 to 2 inches worked into garden beds, 2 to 3 inches for new beds.
Read off the cubic yards, cubic feet and bags. For lawns, measure the whole lawn and use 0.25 inches, the usual top-dressing depth.

How deep should compost be?

Use	Depth	Notes
Lawn top dressing	1/4 in	Rake in so grass blades show through
Annual top-up for garden beds	1 in	Lay on top, worms do the mixing
Working into existing beds	1–2 in	Fork into the top 6 in of soil
New garden beds	2–3 in	Mix into the top 8 in
Around trees and shrubs	2 in	Keep it off the trunk, like mulch

Coverage guide: one cubic yard of compost covers 324 sq ft at 1 inch deep, 162 sq ft at 2 inches, or 1,296 sq ft at the quarter-inch depth used for lawns.

Compost or topsoil: what is the difference?

Topsoil is mostly mineral soil: it gives you depth and structure but little food. Compost is decayed organic matter: it feeds soil life, improves drainage in clay and water holding in sand, and slowly releases nutrients. Beds usually want both, topsoil for bulk and a layer of compost worked in or spread on top. If you are filling a raised bed, our Raised Bed Soil Calculator breaks the blend down for you.

Tips for buying and spreading compost

Finished compost should smell earthy and look dark and crumbly. If it smells sour or you can still recognise the ingredients, it is not finished.
Weed-free matters: hot-composted bagged or certified bulk compost saves you a season of weeding. Cheap compost full of weed seeds is no bargain.
Spread compost in spring or autumn, when soil life is active and beds are being prepared or put to rest.
More is not always better. Beyond about a quarter of the soil volume, extra compost can overload the soil with nutrients, especially phosphorus.

Frequently Asked Questions

How much compost do I need for a lawn top dressing?

A quarter inch deep. That works out to about 0.77 cubic yards (21 cubic feet) per 1,000 sq ft of lawn. Rake it in so the grass shows through, and water it down.

How many bags of compost are in a cubic yard?

A cubic yard is 27 cubic feet, so 27 bags of 1 cu ft, or 18 bags of 1.5 cu ft. Bag sizes vary, so check the label: this calculator counts 1 cu ft bags.

How much does a cubic yard of compost weigh?

About half a ton: roughly 1,100 to 1,200 lb (0.55 tons) for screened, reasonably dry compost. Wet compost can weigh noticeably more.

Can you use too much compost?

Yes. A layer worked in at 1 to 3 inches is plenty. Very thick applications can push soil phosphorus too high and, if the compost is unfinished, the smell and flies will tell you about it first.

Compost vs topsoil: which do I need?

Topsoil to build depth and level, compost to feed and improve soil. Most gardens need a base of soil with compost on or in the top few inches, not a bed of pure compost.

How much compost for a raised bed?

For a top-up, 1 inch over the surface. For a new fill, make compost about 40 percent of the mix: an 8 × 4 bed at 10 inches deep needs about 0.43 cubic yards of compost in a 60/40 topsoil-compost blend.

When is the best time to add compost?

Spring before planting, and autumn after clearing beds. Either works; the soil biology does the mixing over the following weeks.

Does compost shrink as it settles?

Freshly spread compost settles and keeps decomposing, so the layer thins over the season. That is normal, and it is why the calculator adds an extra allowance on top of the neat volume.