Topsoil is required for landscaping projects, such as amending existing ground or filling garden beds. This material is typically sold in bags labeled by volume, measured in cubic feet, or by weight, measured in pounds. While bag sizes vary, volumes commonly found in stores range from 0.75 to 2.0 cubic feet. Understanding how these measurements relate to the physical space you need to fill is the first step toward accurately calculating your project’s needs.
Common Topsoil Bag Sizes
While topsoil bags are not standardized, certain volumes are customary across the industry. The smallest and most frequent size for general use is the 0.75 cubic foot bag, which is convenient for small patching jobs or minor soil amendments. This size is widely available at most garden centers and hardware stores. For larger projects, manufacturers commonly offer bags containing 1.0, 1.5, or 2.0 cubic feet of material. Buyers should always check the volume printed on the packaging, as selecting a larger bag size can reduce the total number of bags needed for substantial filling tasks.
Understanding Volume vs. Weight Labeling
The most accurate way to measure topsoil for a project is by volume, specifically in cubic feet, because this measurement represents the fixed amount of space the soil will occupy. However, many bags are prominently labeled with a weight, such as 40 pounds, which can be misleading for calculation purposes. A 40 lb bag of topsoil is generally equivalent to 0.75 cubic feet, but this relationship is not always consistent.
The weight of a soil bag is affected by its moisture content, which can fluctuate based on weather and storage conditions. A bag of soil stored outside in the rain will weigh significantly more than a bag stored indoors, even though the actual amount of dry soil material is the same. The cubic footage, by contrast, remains a constant measure of the physical volume of the soil, regardless of how much water it holds. For this reason, always rely on the cubic foot measurement for calculations, as weight is a variable and unreliable metric for determining coverage.
Step-by-Step Guide to Calculating Bag Needs
Accurately determining the number of topsoil bags needed involves four simple steps that convert your project’s dimensions into a total volume. First, measure the length and width of the area you plan to cover, such as a garden bed or a patch of lawn, recording these measurements in feet. Next, determine the required depth of the soil layer in inches, which is often 3 to 6 inches for top-dressing or up to 12 inches for a new raised bed.
Before calculating the volume, convert the desired depth from inches to feet by dividing the inch measurement by 12. For example, a 6-inch depth converts to 0.5 feet (6 ÷ 12 = 0.5 ft). The third step is to calculate the total volume required by multiplying the length, width, and converted depth: Length × Width × Depth = Total Cubic Feet.
Example Calculation
Consider a raised bed that is 4 feet wide by 8 feet long, requiring a soil depth of 6 inches. The calculation is 4 ft × 8 ft × 0.5 ft, which equals 16 total cubic feet of soil needed.
The final step is to divide this total volume by the cubic footage of the specific bag you plan to purchase. If you are buying 0.75 cubic foot bags, dividing 16 cubic feet by 0.75 results in 21.33 bags. Since you cannot purchase a fraction of a bag, you must round up to 22 bags of topsoil for this specific project.
Real-World Variables Affecting Soil Volume
The calculated volume represents the theoretical minimum needed, but you should anticipate needing a slightly greater quantity in practice. Soil settles naturally over time due to gravity and the introduction of water, causing the material to compact. This settling reduces the effective volume and level of the soil after installation.
When filling a raised bed or container, the loose soil poured from the bag contains many air pockets. Once watered, the fine particles shift and collapse into these spaces. To account for this natural settling and compaction, it is advised to purchase 10 to 15% more topsoil than your calculation suggests. This small over-order ensures you have enough material for a final top-off after the initial settling period.