When purchasing compost for a landscape or garden project, buyers often encounter a measurement mismatch. Bulk compost, typically sold by local suppliers, is measured in cubic yards. Conversely, compost bought from a garden center comes in individual bags measured by cubic feet. Accurately converting between these two volume units is necessary for proper budgeting and ordering the correct amount of material.
Understanding Volume: Cubic Yards and Cubic Feet
Volume in gardening and construction is primarily measured using the cubic yard and the cubic foot. A cubic yard represents the volume of a cube where each side measures three feet in length. This unit is the standard for large-scale material delivery, such as for mulch, topsoil, or compost. The fundamental relationship between these two units is mathematically precise: One cubic yard is exactly equal to 27 cubic feet. This conversion factor of 27 allows for the accurate translation of bulk volume into the number of individual bags required.
Standard Compost Bag Sizes
Compost is packaged in a few common sizes that determine how many bags are needed to equal 27 cubic feet. The most frequent volume-based bag sizes found at retail outlets are 1.0 cubic foot, 1.5 cubic feet, and 2.0 cubic feet. Some products may also be sold in smaller volumes, such as 0.75 cubic feet, particularly for specialty amendments.
A few brands market their compost by weight, such as a 40-pound bag, instead of volume. When sold by weight, an exact cubic foot measurement is not possible because the density and moisture content of the compost vary widely. For accurate conversion, rely only on bags clearly labeled with cubic foot volume.
Calculating Bags Per Cubic Yard
Calculating the number of bags required to fill a cubic yard involves a straightforward division: the total volume of 27 cubic feet is divided by the volume of a single bag.
- For a 1.0 cubic foot bag, 27 bags are required (27 / 1.0 = 27 bags).
- If a bag holds 1.5 cubic feet of compost, 18 bags are necessary (27 / 1.5 = 18 bags).
- Choosing a 2.0 cubic foot bag means 13.5 bags are needed.
- The 0.75 cubic foot bag requires the largest quantity, totaling 36 bags (27 / 0.75 = 36 bags).
Since compost cannot be purchased in partial quantities, any calculation resulting in a decimal must be rounded up to the next whole number. For instance, the 13.5 bags calculation requires purchasing 14 bags to ensure sufficient material. Rounding up ensures the entire volume of a cubic yard is covered.
Factors Influencing Compost Needs
Before converting to the number of bags, the total cubic yard volume needed for the project must first be determined. This calculation depends on the size of the application area and the desired depth of the compost layer. The first step involves measuring the area in square feet—the length multiplied by the width of the garden bed or lawn.
The application depth is then determined by the project type, as different uses require varying amounts of material. For top dressing a lawn, a depth of 0.25 to 0.5 inches is typically recommended, translating to a much smaller volume requirement. Projects like filling a raised garden bed or amending poor soil for planting often require a much thicker layer, sometimes 6 to 12 inches deep, which drastically increases the total cubic yard requirement.
Once applied, compost will naturally settle and compact over time, often reducing the initial volume by a small percentage. It can be wise to account for this settling by adding a slight buffer to the required depth calculation. Accurately measuring the application area and selecting the appropriate depth is the only way to correctly establish the total cubic yards needed, which can then be converted into the final number of bags.