Compost Calculator

This compost calculator works out how much compost you need for a new bed, an existing garden or a lawn. Choose what you are doing and enter the size of the area. The calculator applies a sensible depth for that job, or one you set yourself, and shows the volume in cubic yards and cubic feet, the amount per 100 square feet, and the number of bags in common sizes. It saves you from the two classic compost mistakes: spreading so little that it makes no difference, and buying so much that it sits in a pile for a year.

How to use the compost calculator

1. Choose the job. Pick a new bed, a bed on poor or sandy soil, an existing bed that needs a top-dressing, or a lawn. Each choice applies a typical depth. Choose custom depth to enter your own.
2. Pick the shape and enter the size in feet. For round beds, enter the diameter. For odd shapes, split the area into rectangles and add the results.
3. Set the number of identical areas. If you have four matching raised beds, enter one bed and set this to 4.
4. Read the result. Order bulk compost by the cubic yard, or use the bag counts if you are buying bags.

How much compost to add

Compost is a soil amendment, not a soil replacement. A modest amount mixed into the top few inches improves structure, water holding and soil life. Very large amounts can push nutrient and salt levels higher than plants need, especially with manure-based composts. These rates reflect common US extension guidance for home gardens.

One inch of compost sounds thin, but it adds up quickly. Over 100 square feet it is about 8.3 cubic feet, roughly a third of a cubic yard. A two-inch application for a new bed of the same size is 16.7 cubic feet, about 0.62 cubic yards.

The compost formula

Cubic feet = area (sq ft) × depth (in) ÷ 12
Cubic yards = cubic feet ÷ 27

Worked example: a new 10 by 4 bed

A new bed measures 10 feet by 4 feet, an area of 40 square feet. At 2 inches, it needs 40 × 2 ÷ 12 ≈ 6.7 cubic feet, or about 0.25 cubic yards. That is 7 bags of 1 cubic foot, or 9 bags of 0.75 cubic feet.

Worked example: top-dressing three raised beds

Three 4-by-8-foot raised beds total 96 square feet. A yearly 1-inch top-dressing needs 96 ÷ 12 = 8 cubic feet, or 4 bags of 2 cubic feet.

Worked example: a lawn after aeration

A 1,000-square-foot lawn top-dressed at a quarter inch needs 1,000 × 0.25 ÷ 12 ≈ 20.8 cubic feet, or about 0.77 cubic yards. At this size, bulk delivery is usually cheaper than about 21 bags.

Bags, weight and bulk compost

Bagged compost is often sold by weight as well as volume, for example in 40-pound bags that hold around three-quarters of a cubic foot. Moisture content changes how much a bag weighs, so always count by the cubic feet printed on the label rather than by the pound figure. Common bag sizes are 0.75, 1, 1.5 and 2 cubic feet.

Bulk compost is sold by the cubic yard. Depending on moisture and ingredients, a cubic yard of finished compost often weighs somewhere around 1,000 to 1,600 pounds. That is lighter than topsoil, but still more than many small trucks should carry in one load. To compare bulk and bags fairly, divide the bulk price per yard by 27 and add delivery, then compare that with the bag price divided by the bag size in cubic feet.

Many cities and counties sell or give away compost made from yard waste. It is usually inexpensive and perfectly good for beds and lawns. Ask whether it has been screened, because unscreened compost can contain sticks and debris.

Types of compost

Compost varies a lot depending on what it was made from. All good compost is dark, crumbly and smells earthy, not sour or like ammonia.

• Yard-waste or leaf compost is made from leaves, grass and plant trimmings. It is balanced and gentle, and a good default for most gardens.
• Manure compost from cattle, horses or poultry is richer in nutrients. It is useful for heavy feeders, but it can be high in salts and phosphorus, so use it at the lower end of the rates above.
• Mushroom compost is the spent growing medium from mushroom farms. It is often alkaline and salty, so it suits vegetable beds and lawns better than acid-loving plants such as blueberries and azaleas.
• Leaf mold is decomposed leaves alone. It is low in nutrients but excellent for soil structure and water holding.
• Vermicompost (worm castings) is rich and finely textured. It is usually used in small amounts, such as in potting mixes or planting holes, rather than spread across whole beds.

Compost versus soil

Compost and topsoil do different jobs. Topsoil provides the mineral body of a bed: the sand, silt and clay that hold roots and store nutrients. Compost provides organic matter, which feeds soil life and improves structure. A bed made of pure compost tends to shrink dramatically as it breaks down and can be overly rich. Most gardens do best with a soil base improved by compost, rather than compost alone.

If you are filling new raised beds, the raised bed soil calculator splits the total into topsoil, compost and an aerating ingredient. For filling or leveling ground, use the soil calculator.

When to add compost

Compost can go on at almost any time the ground is not frozen, but two windows make the most sense for vegetable gardens.

• Fall: spread compost after cleanup, so it has the winter to settle and begin breaking down. This is a good time for fresher or coarser compost.
• Spring: add finished compost a couple of weeks before planting and mix it into the top few inches.

For perennial beds and shrubs, a thin layer in spring under the mulch is enough. For lawns, the best time is right after core aeration in spring or early fall, when the grass is actively growing.

Making your own compost

A home compost pile is the cheapest source of compost, but it shrinks a lot. A heap that starts at one cubic yard of leaves, trimmings and kitchen scraps often ends up as half that volume of finished compost, or less. If you plan to make all your own compost, start with roughly twice the volume you need.

Finished home compost should be dark and crumbly, with few recognizable scraps left. If it still smells sour or is full of whole leaves, give it more time. Partly finished compost can still be used as a surface mulch, but it is better not to dig it into beds just before planting.

Can you add too much compost?

Yes. Compost is beneficial in sensible amounts, but heavy yearly applications, especially of manure-based compost, can build up phosphorus and salts over time. Very high organic matter also makes soil hold so much water that it stays soggy in wet years. Signs of overdoing it include lush leafy growth with poor fruiting, salt crust on dry soil, and soil test results showing very high phosphorus. Sticking to about an inch a year on established beds avoids most problems.

Common mistakes

• Counting bags by weight. A 40-pound bag is not a fixed volume. Use the cubic feet on the label.
• Using compost as the whole bed. It shrinks and can be too rich. Blend it with soil.
• Burying fresh compost deep. Unfinished material decomposes in the root zone and can harm seedlings. Mix finished compost into the top layer only.
• Applying manure compost every year at high rates. Rotate with plant-based compost and test the soil every few years.

Compost for raised beds and containers

Raised beds use compost in two ways. When a bed is first filled, compost is usually about a third of the mix, blended with topsoil and an aerating ingredient. The raised bed soil calculator splits the fill into those parts. After the first season, the bed needs a yearly top-up of 1 to 2 inches, because the soil settles and organic matter is used up. That top-up is exactly what the existing bed option in this calculator works out.

Containers are different. Garden compost on its own is too heavy and dense for pots, and it can hold too much water. For containers, compost generally makes up no more than about a quarter to a third of the mix, combined with a potting mix based on peat or coir, bark and perlite. Many gardeners mix one part compost with two or three parts bagged potting mix, which adds nutrients and biology without making the pot soggy.

How to spread compost

1. Clear the bed. Remove weeds and old crop residue, which can carry pests and diseases into the next season.
2. Spread evenly. Tip small piles across the bed and rake them out to the target depth. A garden rake held upside down helps level the surface.
3. Mix in for new beds. Work the compost into the top 6 to 8 inches with a fork or spade. In established beds, leave it on top and let worms and roots do the work, which disturbs soil life less.
4. Water it in. A gentle watering settles the compost and helps it start to integrate with the soil.
5. Mulch if needed. Compost on the surface can dry out and grow weeds from seeds it contains. A thin mulch layer on top prevents both.

For lawns, spread compost with a shovel in small flicks, or use a peat spreader, then rake or brush it so it falls between the grass blades and into aeration holes. The grass should still be clearly visible afterward.

Frequently asked questions