Compost Calculator

1. Enter the length and width of your bed or lawn in feet.
2. Set the application depth in inches. Use a thin layer for lawns and a deeper layer for beds.
3. Enter your bag size in cubic feet, and optionally the price per bag and bulk price per cubic yard.
4. Read the compost needed in cubic feet and cubic yards, the bag count, and the cost each way.

Compost is decomposed organic matter that feeds soil life, holds moisture and improves structure. A compost calculator tells you how much to buy so you do not guess at the store or over-order a bulk load.

Gardeners use compost two main ways. They top-dress lawns with a thin layer to improve the soil without smothering grass, and they amend garden beds with a deeper layer that gets mixed into the root zone. Each use calls for a different depth, which changes the volume you need.

Because compost is sold by the bag or by the cubic yard, the key step is turning your area and depth into a volume. For related jobs see the mulch calculator and the topsoil calculator, or browse the construction calculators hub.

The calculator converts your area and depth into a volume, then divides by bag size and multiplies by price.

1. Multiply length by width to get the area in square feet.
2. Divide the depth by 12 to turn inches into feet, then multiply by the area for cubic feet.
3. Divide cubic feet by 27 for cubic yards, and divide by bag size (rounding up) for bags.

Why Divide by 12 and 27

Depth is measured in inches but area is in feet, so dividing the depth by 12 keeps the units consistent. There are 27 cubic feet in a cubic yard, so dividing by 27 converts the volume to the unit bulk suppliers use.

Say you are amending a garden bed that is 20 feet by 10 feet to a depth of 2 inches, using 1.5 cubic foot bags at $5 each, with bulk at $40 per cubic yard.

Area: 20 x 10 = 200 square feet.

Cubic feet: 200 x (2 / 12) = 200 x 0.1667 = 33.33 cubic feet.

Cubic yards: 33.33 / 27 = 1.23 cubic yards.

Bags: 33.33 / 1.5 = 22.2, rounded up to 23 bags.

Cost: bagged is 23 x $5 = $115, while bulk is 1.23 x $40 = about $49. For this size the bulk load is far cheaper, which is typical once you pass about a cubic yard.

The right depth depends on the job. Too little does not help; too much can smother plants or waste money.

When in doubt, start on the lighter side for lawns and the heavier side for brand-new beds. You can always top up next season.

Compost comes bagged from garden centers or in bulk by the cubic yard from landscape suppliers. The calculator prices both so you can compare.

When Bags Make Sense

Bags are easy to carry, store and spread for small jobs. For a few cubic feet, or a spot you can only reach by hand, bags win on convenience even though the per-yard price is higher.

When Bulk Wins

Once you need more than about a cubic yard, bulk compost is usually much cheaper per unit, as the example above shows. You do need somewhere to dump it and a way to move it, such as a wheelbarrow.

Watch Delivery Fees

Bulk prices often exclude delivery. Add any delivery fee to the bulk cost before deciding, especially for a single yard where the fee can rival the compost.

1. Mixing Up Inches and Feet

Depth is in inches but area is in feet. Forgetting to divide the depth by 12 overstates the volume by twelve times.

2. Forgetting the 27

A cubic yard is 27 cubic feet, not 3. Dividing by 3 instead of 27 badly undercounts bulk orders.

3. Not Rounding Bags Up

You cannot buy a fraction of a bag. Always round the bag count up so you finish the job.

4. Ignoring Settling

Compost settles and some is lost to uneven ground. Buy about 5 to 10 percent extra.

5. Over-applying on Lawns

A thick layer can smother grass. Keep lawn top-dressing to about half an inch or less.

Planning a Garden Bed

Before buying, size the exact volume so you make one trip and avoid leftover bags.

Top-dressing a Lawn

Thin layers over a large area add up fast. The calculator shows whether bags or a bulk load is smarter.

Budgeting a Project

Comparing bagged and bulk cost up front helps you set a realistic budget and spot when delivery pays off.

Filling Raised Beds

For deep raised beds, pair this with the raised bed soil calculator to blend compost with soil.

The volume math is exact, but a few real-world factors shift the amount you should buy.

Settling and Compaction

Fresh compost settles over weeks. A layer spread a touch deep ends up thinner once it beds in.

Uneven Ground

Low spots and slopes soak up more than a flat surface, so rough ground needs a bit extra.

Bag Fill Variation

Bags are sold by volume but can pack down or dry out, so actual coverage per bag varies slightly.

Irregular Shapes

This tool assumes a rectangle. For odd shapes, split the area into rectangles and add the results.

The calculator is exact for a rectangular area at a uniform depth. It is a planning estimate, not a guarantee of exact coverage.

Buy about 5 to 10 percent more than the calculated figure to cover these factors.

Multiply the bed length by width for the area, multiply by the depth in feet, and that is the cubic feet. Enter your numbers above for the exact volume, bags and cost.

For lawn top-dressing, spread a thin layer of about 0.25 to 0.5 inch and rake it in so the grass blades still show. More than half an inch can smother the grass.

For established beds use about 1 to 2 inches a year. When amending a new bed or poor soil, 2 to 4 inches worked into the top few inches of soil is typical.

A cubic yard is 27 cubic feet. That is 18 bags at 1.5 cubic feet each, or 27 bags at 1 cubic foot each. The calculator rounds up to whole bags.

Bulk is usually much cheaper per unit once you need more than about a cubic yard, but you pay for delivery and handling. Bags win on convenience for small jobs.

Divide the cubic feet by 27, since there are 27 cubic feet in one cubic yard. For example, 54 cubic feet is 2 cubic yards.

Compost can be applied in spring before planting, in fall after harvest, or as a top-dressing during the growing season. Any time the ground is workable is fine.

Buy about 5 to 10 percent more than the calculated amount to cover settling, uneven ground and bag-fill variation.

The same volume math applies to any material sold by depth. For those jobs try the dedicated mulch and topsoil calculators linked below.

Split an irregular area into rectangles, calculate each one, and add the results. For circles, estimate the area first and use the depth as normal.

• Cornell Waste Management Institute - Compost (Cornell University; composting science and use of finished compost).
• University of Minnesota Extension - Using compost in gardens (University of Minnesota Extension; application rates and depths for gardens and lawns).
• Oregon State University Extension - Compost application (Oregon State University Extension; soil amendment and compost guidance).