Materials like topsoil, mulch, and compost are sold in two different volume units: bulk purchases measured by the cubic yard, and retail pre-packaged options measured in cubic feet. Understanding the relationship between these units is necessary to avoid over-purchasing expensive bagged material or underestimating the volume needed for a large area. This guide provides a straightforward method for accurately converting a project’s volume requirement into the number of bags needed.
Defining the Measurements
The term “yard” in landscaping refers specifically to a cubic yard, which represents a three-dimensional volume. A cubic yard is defined as a cube measuring three feet long, three feet wide, and three feet high, equating to exactly 27 cubic feet.
Pre-packaged topsoil is sold in smaller, fixed quantities measured in cubic feet (cu ft). The most common retail sizes for topsoil bags are 1.0 cubic foot, 1.5 cubic feet, and sometimes 0.75 cubic feet.
Determining Your Required Volume
The initial step is calculating the total volume of material required for the specific area. This calculation requires three measurements: the length, the width (both measured in feet), and the desired depth of the topsoil layer.
The intended depth is usually measured in inches, so it must be converted into a fraction of a foot before calculating the volume. For example, a depth of six inches converts to 0.5 feet, while four inches converts to approximately 0.33 feet. Multiplying these three dimensions—length, width, and depth—yields the total volume needed in cubic feet.
For a practical example, consider an area 10 feet long and 10 feet wide where a six-inch layer of new topsoil is desired. The calculation is 10 feet multiplied by 10 feet, multiplied by the converted depth of 0.5 feet, resulting in a total volume requirement of 50 cubic feet of topsoil.
Calculating Bags Per Cubic Yard
Once the total required volume in cubic feet is established, convert that volume into the necessary number of bags based on the specific retail size available. The conversion formula divides the total cubic feet needed by the volume of a single bag. For instance, if 50 cubic feet are needed and bags are 1.5 cubic feet each, dividing 50 by 1.5 yields approximately 33.33 bags.
Knowing how many bags are in one cubic yard provides a simple baseline for scaling the purchase. Since one cubic yard contains 27 cubic feet, a standard 1.0 cubic foot bag requires exactly 27 bags to equal one yard of bulk material. If the topsoil is sold in 1.5 cubic foot bags, 18 bags are required to cover the same volume (27 divided by 1.5).
Larger bags, such as those containing 2.0 cubic feet, reduce the total bag count significantly. Dividing 27 by 2.0 indicates that 13.5 bags are technically needed to equal a cubic yard. It is prudent to round up to the next whole number when calculating material needs to ensure sufficiency, meaning 14 bags would be the correct purchase.
Accounting for Real-World Adjustments
While the calculated bag count provides a theoretical minimum, topsoil volume is subject to settling and compaction after application. When soil is poured, its particles are loosely arranged, creating air pockets that contribute to the initial volume. Watering and natural gravity cause the particles to rearrange and fit more closely together, reducing the overall depth of the layer.
This compaction effect means that the final depth will be less than the initial application depth unless an additional buffer is included in the purchase. It is advisable to add a 10 to 15 percent buffer to the calculated bag count to account for this natural settling. For example, if the calculation required 30 bags, applying a 10% buffer would mean purchasing an additional three bags.
The degree of settling depends on the composition of the material. Finer, loam-heavy topsoil tends to compact more significantly than soil that contains a higher percentage of coarse organic matter or mulch. Including this buffer ensures that the desired depth is maintained even after the soil has settled following the first few watering cycles.