Purchasing soil requires understanding the difference between volume and weight. Soil is primarily sold by volume, measured in cubic feet, because this metric accurately reflects the space the material will fill. Knowing the cubic feet in a bag of soil is essential for calculating project needs accurately. This precise measurement prevents running out of material mid-task and ensures budget efficiency by avoiding the purchase of excess material.
Defining Cubic Measurements for Soil
The standard unit for measuring soil volume in retail is the cubic foot (cu ft), which represents the space inside a cube measuring one foot in length, one foot in width, and one foot in height. Volume is the reliable standard for soil because it accounts for the three-dimensional space a material occupies, unlike weight, which can fluctuate. The weight of soil can change dramatically based on moisture content and the materials it contains, but the labeled volume remains constant.
For projects requiring large quantities of material, soil is often sold in bulk measured in cubic yards (cu yd). One cubic yard is a significantly larger volume, equivalent to 27 cubic feet. This conversion is useful when transitioning from bagged products to bulk delivery for extensive garden beds or large-scale landscaping features.
Common Retail Bag Volumes
Bagged soil, potting mix, and other growing media are consistently labeled with their volume in cubic feet, providing a clear metric for purchase. The most frequently encountered bag sizes in garden centers are 1 cubic foot, 1.5 cubic feet, 2 cubic feet, and 3 cubic feet. Smaller bags may contain volumes like 0.75 cubic feet, particularly for denser products like topsoil.
It is important to rely on the cubic foot measurement rather than the bag’s listed weight, as weight is highly misleading for volume. For instance, a 40-pound bag of dense topsoil may only contain about 0.75 cubic feet of material. In contrast, a 40-pound bag of lightweight mulch or peat moss can easily contain 2 or 3 cubic feet.
Determining Project Coverage
Calculating the total volume of soil needed for a project, such as a raised bed, involves a simple three-step formula. Measure the length, width, and desired depth of the area to be filled, ensuring all measurements are in feet.
The calculation then becomes: Length (ft) x Width (ft) x Desired Depth (ft) = Total Cubic Feet Needed. Since the desired depth is often measured in inches, it is necessary to convert this measurement to feet by dividing the number of inches by 12. For example, a depth of six inches converts to 0.5 feet.
If a gardener is filling a raised bed that is 10 feet long by 5 feet wide, and they want a soil depth of 0.5 feet (6 inches), the calculation is 10 ft x 5 ft x 0.5 ft, resulting in a total need of 25 cubic feet of soil. This total cubic footage is then divided by the volume of the bag being purchased. For example, if the bags are 2 cubic feet each, the gardener will need 12.5 bags, which should be rounded up to 13 bags to ensure sufficient material.
Why Soil Volume Varies
While the labeled cubic footage on a bag is accurate at the time of packaging, the actual volume of the material once spread can appear to differ due to several physical factors. Moisture content is a primary cause for this variation, as soil is typically measured and packaged at a certain moisture level. If the soil in the bag dries out, it may seem to occupy less space when poured out.
Compaction is another variable that affects the final volume of soil in a garden bed. Different types of media, such as lightweight potting mix versus heavy topsoil, will compact differently after they are spread and watered. Over time, the material settles and its bulk density increases, reducing the overall depth of the soil layer.