A raised garden bed is a contained planting area built above the surrounding landscape. These structures offer significant advantages by improving drainage, warming the soil earlier in the spring, and allowing the creation of an ideal growing medium free from native soil deficiencies. Calculating the correct volume of soil is the necessary first step to prevent costly over-ordering or frustrating under-filling. Knowing precisely how much material to purchase ensures the project stays on budget while providing the plants with the required space for root development.
Determining Volume Based on Bed Dimensions
The process of calculating the required soil volume begins with the basic geometric formula for a rectangular prism: Length multiplied by Width multiplied by Depth equals Volume (L x W x D = V). Measure all dimensions using the same unit, typically feet, to determine the volume in cubic feet. For example, if a bed is 4 feet wide, 8 feet long, and 12 inches deep, convert the depth to feet (one foot). Multiplying 4 x 8 x 1 yields a total volume of 32 cubic feet of material.
Since soil is often purchased in cubic yards or standard bags, the cubic feet volume may need conversion. To convert cubic feet to cubic yards, divide the total cubic feet by 27, as there are 27 cubic feet in one cubic yard. For the 32 cubic feet example, this equates to approximately 1.19 cubic yards of soil. If purchasing bagged soil (e.g., 1.5 cubic foot bags), dividing the total cubic feet (32) by the bag size indicates that roughly 21.3 bags are required.
Essential Components of a Quality Raised Bed Mix
Once the total volume is determined, the next consideration is the composition of the growing medium, which should be a specialized blend rather than standard garden soil. A high-quality mix is engineered to provide superior aeration, water retention, and nutrient availability. The ideal raised bed mix typically consists of three primary components: a bulk filler, a nutrient-rich amendment, and a drainage agent, all combined by volume.
A common and effective blend uses topsoil, compost, and an aeration material. Topsoil provides the necessary structure and density for root support. Compost supplies organic matter that feeds the soil’s microbial life and slowly releases nutrients to the plants. A widely recommended ratio for a nutrient-rich blend often involves a 50% topsoil and 50% compost mix, or a refined blend of 50% topsoil, 30% compost, and 20% organic matter like aged bark or leaves.
The third component, a drainage or aeration material such as coarse vermiculite or perlite, is incorporated to prevent compaction and improve the movement of water and oxygen. Vermiculite retains moisture while perlite assists with porosity and drainage, creating the loose, crumbly texture that healthy plant roots require. Using this blend ensures the soil remains friable over time, supporting robust root growth.
Accounting for Soil Settling and Depth Requirements
A significant factor to incorporate into the initial volume calculation is the inevitable settling of the soil mix after installation. Because raised bed mixes are rich in porous materials like compost and organic matter, they will compress and decompose over time, leading to a loss of volume. Compost-heavy blends can settle by approximately 10% to 20% after the initial watering and decomposition. To compensate for this compaction, increase the calculated volume by at least 10% to ensure the bed remains full after the first few waterings.
The required depth of the soil is another consideration that impacts the volume calculation and should be based on the specific plants being grown. While a 12-inch depth is a suitable standard for most common vegetables, shallower-rooted crops like herbs and lettuce can thrive in a minimum of six inches. Deep-rooted plants such as carrots, tomatoes, and larger brassicas require a minimum depth of 12 to 18 inches for proper root extension. Selecting the appropriate depth ensures that the calculated volume provides enough vertical space for the root systems to develop fully.