When undertaking landscaping or gardening projects, determining the correct volume of soil is the first practical step. Cubic footage (CF) provides a standardized, three-dimensional measurement for the space your soil will occupy, such as a raised bed, container, or in-ground plot. Accurately calculating this volume prevents ordering too much material, which wastes money, or too little, which halts project completion.
Calculating Volume for Rectangular and Square Spaces
The volume calculation for simple rectangular or square areas, such as standard raised garden beds, relies on a straightforward geometric principle. The fundamental equation is length multiplied by width multiplied by depth. All three measurements must first be expressed in feet.
If the measured depth is given in inches, that figure must be divided by 12 to convert it into feet before multiplication occurs. For instance, 6 inches is 0.5 feet, and 18 inches is 1.5 feet. Ensuring uniformity in the units is necessary for arriving at a correct cubic foot total.
Consider a typical raised bed measuring 8 feet long and 4 feet wide, with a desired soil depth of 18 inches. First, the depth is converted to 1.5 feet. The calculation then proceeds as 8 feet x 4 feet x 1.5 feet.
Multiplying these dimensions yields a total volume of 48 cubic feet. This figure represents the precise amount of loose soil needed to fill the container to the desired level. This process applies to any plot with four right-angled sides.
Calculating Volume for Circular and Irregular Areas
Not all planting spaces are square or rectangular, requiring different mathematical approaches. For circular containers, like large pots or tree rings, the area of the circle must be determined before multiplying by the depth. The volume formula is \(\pi\) (approximately 3.14) multiplied by the radius squared, which is then multiplied by the depth in feet.
The radius is half the diameter, so measure across the center and divide that figure by two. If a circular planter has a 3-foot diameter and a 2-foot depth, the radius is 1.5 feet. The calculation becomes 3.14 x (1.5 x 1.5) x 2, resulting in a volume of 14.13 cubic feet.
Calculating volume for oddly shaped garden plots, such as those following a curving path, requires dividing the area into multiple standard geometric shapes. These shapes might include smaller rectangles, squares, or triangles, which are simpler to measure individually. Once the cubic footage for each shape is calculated, the separate volumes are added together to find the overall total.
This method allows the gardener to estimate the total soil requirement for a complex space with reasonable accuracy. Breaking the area into a series of small, measurable rectangles and summing their volumes provides a close approximation of the total cubic footage required.
Converting Cubic Feet to Purchase Units
Once the final cubic footage (CF) is determined, the next step is converting this number into the units used by suppliers for purchasing. Bulk soil is most frequently sold by the cubic yard (CY), which is a much larger unit of measure. There are exactly 27 cubic feet of material contained within one cubic yard.
To convert the calculated CF requirement into cubic yards, the total cubic footage must be divided by 27. For example, a project requiring 108 cubic feet of soil would necessitate ordering exactly 4 cubic yards of material (108 divided by 27). This conversion is necessary for large-scale projects where soil is delivered loose by truck.
For smaller projects, where soil is purchased in bags from a garden center, the conversion involves checking the volume printed on the bag packaging. Common bag sizes often contain 1.0, 1.5, or 2.0 cubic feet of soil. To find the number of bags needed, the total calculated cubic footage is divided by the volume of a single bag.
If the calculated requirement is 15 cubic feet, and the chosen bag size is 1.5 cubic feet, the total number of bags to purchase would be 10. This direct mathematical division ensures the appropriate quantity is bought without unnecessary excess.
Accounting for Soil Settling and Waste
The calculated volume represents a static, ideal amount, but real-world conditions necessitate ordering a slightly larger quantity than the mathematical total. Soil materials naturally settle and compact over time, especially after the first watering, reducing the initial volume by a noticeable amount. It is a standard practice to adjust the final order quantity upward to compensate for this anticipated loss.
Experts commonly suggest adding an extra 5% to 10% to the calculated cubic footage to account for this inevitable settling and compaction. This extra allowance ensures the beds remain full and at the optimal level after a few weeks of use. This buffer also covers minor losses due to spillage during the transfer from the delivery area to the planting location.
Furthermore, the required depth itself may need adjustment based on the planned planting material. Root vegetables, such as carrots or potatoes, require a minimum of 12 to 18 inches of loose soil to grow properly. Conversely, shallow-rooted annual flowers or ground covers may thrive in as little as 6 to 8 inches of depth.
Before placing the final order, these practical considerations should be integrated into the initial calculations, either by increasing the target depth by a few inches or by applying the percentage buffer to the final CF total. This strategic over-ordering ensures project success and eliminates the need for a costly, small follow-up delivery.