The cubic foot (CF or cu ft) is the fundamental unit of volume used when measuring and purchasing soil for landscaping and gardening projects. This measurement represents the space inside a perfect cube that is one foot long, one foot wide, and one foot deep. Understanding this three-dimensional measurement is the first step in accurately estimating the material needed for any project. Calculating the total volume required prevents the common problem of over-ordering or running out of soil.
Defining the Volume: The 3D Measurement
Determining the total volume of soil needed involves a straightforward calculation of the space you intend to fill. The formula for cubic feet is simple: multiply the area’s Length by its Width by the desired Depth. All three measurements must be in feet to yield a result in cubic feet.
For projects like raised garden beds or planting holes, depth is typically measured in inches. To correctly integrate the depth into the formula, you must first convert the inches into a decimal fraction of a foot. Since there are 12 inches in a foot, you divide the depth in inches by 12.
For example, a depth of four inches converts to approximately 0.33 feet (4 inches รท 12 = 0.33). If your garden bed is six feet long and four feet wide, and you need a depth of eight inches (0.67 feet), the calculation becomes 6 feet x 4 feet x 0.67 feet, equaling 16.08 cubic feet of soil.
Real-World Coverage and Common Conversions
Translating the cubic foot volume into practical coverage depends entirely on the depth you plan to spread the soil. A single cubic foot of material will cover a space differently based on whether you are top-dressing a lawn or filling a deep container. The volume is a constant, but the surface area it covers is inversely related to the layer’s thickness.
For instance, one cubic foot of soil is sufficient to cover an area of 12 square feet when spread at a shallow depth of just one inch. If you increase the soil layer to a depth of three inches, the same volume will cover a smaller area of four square feet. When filling a deep space, such as a large patio planter that is one foot deep, one cubic foot of soil will cover exactly one square foot of the planter’s base.
Soil is often sold in bags labeled by volume to make purchasing easier. While bulk soil is measured in cubic yards (equivalent to 27 cubic feet), bagged products are typically sold in smaller increments. Common bag sizes include 1.5 cubic feet or 2 cubic feet. A standard 40-pound bag of topsoil usually contains about 0.75 cubic feet of material.
To help visualize this volume, one cubic foot of soil is roughly equivalent to 7.5 gallons. A standard five-gallon bucket, when filled, holds about two-thirds of a cubic foot of soil. Understanding these consumer conversions helps in quickly estimating how many bags or buckets are needed.
Why Soil Weight is Never Standard
While the volume of a cubic foot of soil is mathematically fixed, its weight is highly variable and depends on several environmental and compositional factors. Because of this variability, soil is almost always sold by volume (cubic feet or cubic yards) rather than by weight.
Factors Affecting Weight
Moisture content is the single largest factor influencing the weight of a cubic foot of soil. Since water is heavy, a cubic foot of soil saturated with water can easily weigh over 100 pounds. In contrast, the same volume of completely dry soil may weigh as little as 70 to 80 pounds.
The composition of the soil also plays a significant role in its mass. Soil with a high mineral content, such as dense clay or sand, is inherently heavier than soil rich in organic matter like peat moss or compost. Finally, the degree of compaction affects the density, as loosely packed soil contains more air pockets and weighs less than tightly compressed material.