Setting up a raised garden bed is an excellent way to control soil quality and drainage for successful planting. Before starting, gardeners must accurately determine the volume of soil required to fill the structure. This volume is measured in cubic feet, a standard unit necessary for purchasing the correct amount of material. Calculating the exact cubic footage prevents multiple trips to the store or costly over-ordering of soil. For a standard 4-foot by 8-foot raised bed, understanding the relationship between the bed’s fixed dimensions and the variable depth is the first step.
Understanding the Volume Calculation
The total amount of soil needed is a measure of volume, which defines the three-dimensional space inside the raised bed. Volume calculation for a rectangular structure is straightforward, using the fundamental geometric formula: Length multiplied by Width multiplied by Height. To ensure the final result is in cubic feet, all three measurements must first be converted into feet. The base dimensions of a 4-foot by 8-foot bed provide a constant surface area of 32 square feet.
The height, or desired depth of the soil, is the only variable in the calculation that the gardener controls. Since garden bed depth is often measured in inches, it must be divided by 12 to convert it into the proper feet measurement for the formula.
Cubic Feet Required for Common Depths
The required cubic feet changes significantly based on the root depth requirements of the planned crops. For a 4×8 raised bed, the 32 square feet base area simplifies the calculation, making the cubic footage exactly 32 times the depth in feet. Shallow-rooted crops like lettuce, spinach, and herbs can thrive in soil depths as low as 6 inches.
A 6-inch depth converts to 0.5 feet, requiring 16 cubic feet of soil for the entire bed. A more common depth for mixed vegetable gardening, accommodating plants like peppers and broccoli, is 8 inches, or approximately 0.67 feet. This depth requires about 21.33 cubic feet of soil to fill the bed.
Deeper-rooted vegetables, such as carrots, tomatoes, or potatoes, perform best in a minimum of 10 to 12 inches of soil. A 10-inch depth (0.83 feet) demands approximately 26.67 cubic feet of material. A standard 12-inch depth, equivalent to one full foot, simplifies the math, requiring exactly 32 cubic feet of soil. Gardeners constructing very deep beds, perhaps 18 inches high (1.5 feet), will need a volume of 48 cubic feet. Selecting the appropriate depth based on plant needs ensures the roots have sufficient room for growth.
Translating Volume into Purchase Units
Once the necessary cubic footage is determined, the next step is translating that number into the units used by soil suppliers. Soil is typically sold in bags or in bulk measured by the cubic yard. Bagged soil is generally available in common sizes of 1.0, 1.5, or 2.0 cubic feet, which is convenient for smaller projects or for topping off existing beds. To find the number of bags needed, the total cubic feet required is divided by the size of the bag being purchased.
For example, a 12-inch deep bed requiring 32 cubic feet would necessitate 16 bags of 2-cubic-foot soil. For larger volumes, purchasing in bulk by the cubic yard is generally more economical and efficient. One cubic yard is equal to 27 cubic feet.
To convert the required cubic feet into cubic yards, the total cubic footage is divided by 27. The 4×8 bed filled to a 12-inch depth requires 32 cubic feet of soil, which converts to approximately 1.18 cubic yards. Bulk purchasing often involves delivery by a landscaping company, which is the preferred method for any volume exceeding 1.5 cubic yards.