When planning any outdoor project that involves adding soil, such as building a raised garden bed or leveling a section of your lawn, accurately estimating the required volume is the first step. Proper calculation prevents the costly problems of over-ordering material or facing delays due to insufficient supply. This guide provides the practical methods necessary to measure your project space and determine the precise amount of dirt you need, ensuring your project runs smoothly and efficiently.
Calculating Volume for Rectangular Spaces
The simplest calculation applies to areas with a uniform rectangular or square shape, such as a raised bed or a patio space being graded. Begin by measuring the length and the width of the area in feet. You must also decide on the final depth of the soil layer, measured in feet, before performing any calculations.
To find the raw volume, multiply these three dimensions: Length × Width × Depth. This calculation yields the volume in cubic feet, the standard unit for three-dimensional space. For example, a raised bed 10 feet long by 5 feet wide, requiring a 6-inch (0.5 feet) depth of soil, results in a raw volume of 25 cubic feet.
It is crucial that all three measurements are in the same unit for an accurate outcome. If you measure the depth in inches, convert it to feet by dividing the inch measurement by twelve before multiplying. This consistency prevents calculation errors and serves as the basis for all subsequent adjustments.
Formulas for Circular and Irregular Areas
Projects involving curved areas, such as filling a circular planter or surrounding a tree base, require a different geometric approach. For any circular area, first determine the radius (the distance from the center point to the edge). The volume of a cylinder is calculated by multiplying Pi (approximately 3.14159) by the radius squared, and then multiplying that result by the required depth.
Irregularly shaped projects, such as flower beds with curved edges, cannot be measured with a single formula. The most effective method involves breaking the complex shape into several smaller, manageable geometric figures. These smaller sections can be treated as rectangles, squares, or triangles.
Calculate the volume for each of these smaller figures individually using the appropriate formula. Once all individual volumes have been determined, add them together to find the total raw volume needed for the entire space.
Adjusting for Settling and Compaction
The raw volume calculated represents the final, settled amount of soil required, but it does not account for real-world changes during installation. Soil, especially loose topsoil, contains air pockets that compress and settle over time or through watering. Therefore, you must always order more material than your initial geometric calculation suggests.
To compensate for this compaction effect, increase the calculated volume by a specific percentage. For most landscaping projects, adding an extra 10% to 20% to the raw volume is standard practice to ensure the final depth is achieved. Very loose materials, like shredded bark mulch or lightweight potting mixes, require an adjustment toward the higher end of the range.
The type of dirt affects the necessary adjustment; dense fill dirt compacts less than light topsoil. After applying the percentage increase, you arrive at the adjusted volume, which is the amount you should actually purchase.
Converting Volume to Purchase Quantities
The final step is converting your adjusted volume, currently in cubic feet, into the quantity used by suppliers. Bulk landscaping materials are most commonly sold by the cubic yard. To convert, use the standard factor: there are exactly 27 cubic feet in one cubic yard.
Divide your total adjusted volume in cubic feet by 27 to determine the required volume in cubic yards. For example, if your adjusted volume is 135 cubic feet, dividing by 27 results in 5 cubic yards of material needed for the project. This is the figure you will use when ordering from a bulk supplier.
If the project is smaller, you may purchase soil in bags. Standard bags typically hold 1 or 2 cubic feet of material. To estimate the number of bags, divide your total adjusted cubic feet volume by the volume of a single bag. This helps you decide the most cost-effective option.