Soil products are often measured and packaged by volume, such as cubic feet or quarts, which creates confusion for consumers intending to cover a flat area measured in square feet. Understanding the difference between volume-based and area-based measurements, and knowing the simple conversion formulas, is the most effective way to accurately determine how many bags of soil your gardening project requires. This practical knowledge prevents buying too much or too little material for your needs.
Decoding Soil Bag Measurements
Soil and soil-like growing media are typically sold by volume, with the most common unit being the cubic foot (ft³). Large bags of topsoil or garden soil frequently list their contents in cubic feet, often in sizes like 1.0, 1.5, or 2.0 ft³. Potting mixes, used for containers, are often measured in dry quarts, especially for smaller bags.
A significant point of confusion arises because some bags are labeled as “compressed,” meaning the material inside will expand once opened and loosened. Weight in pounds is sometimes listed, but this is a poor indicator of usable volume since the moisture content of the soil can vary dramatically, changing the weight without affecting the actual volume.
Calculating Coverage for Wide Areas
When spreading soil thinly over a garden bed or lawn, the calculation must convert the bag’s three-dimensional volume into a two-dimensional area based on a specific depth. The fundamental relationship is that the total volume of soil equals the area covered multiplied by the depth of the layer. To find the square footage a bag covers, you divide the cubic feet of the bag by the desired depth, which must also be in feet.
This calculation requires converting the desired depth from inches into feet. Since there are 12 inches in a foot, a 2-inch layer is calculated as 2 divided by 12, or approximately 0.167 feet. Using this conversion, a 1.5 cubic foot bag would cover roughly 9 square feet at a 2-inch depth (1.5 ft³ / 0.167 ft ≈ 8.98 ft²).
Determining Volume for Containers and Raised Beds
Filling containers and raised beds requires measuring the three-dimensional volume of the space to be filled, which is a simpler calculation than estimating coverage. For rectangular raised beds, the volume is found by multiplying the length, width, and depth of the bed in feet (Length × Width × Depth = Volume in cubic feet). This result directly matches the cubic feet measurement on the soil bag.
Calculating Volume for Cylindrical Pots
For cylindrical pots, the volume calculation is based on the formula for a cylinder: Pi multiplied by the Radius squared multiplied by the Height. The height and radius (half of the diameter) must be measured in the same unit, like inches, which results in a volume in cubic inches. To compare this result to a bag measured in cubic feet, you must divide the total cubic inches by 1,728, as there are 1,728 cubic inches in one cubic foot. If the bag is measured in quarts, there are approximately 30 dry quarts in a single cubic foot.
How Soil Density Affects Actual Yield
The calculated volume provides a theoretical requirement, but the actual performance of the soil often depends on its composition and density. Lightweight potting mixes, which are often “soilless” and contain high amounts of organic materials like peat moss or coir, are engineered to remain loose and airy. This lower density means the volume measurement on the bag more closely reflects the usable volume, as the material resists compaction.
In contrast, topsoil, which is often heavier and contains more mineral content like clay and silt, is naturally denser. This higher density means the material packs down more easily, and the usable volume may be slightly less than the labeled cubic footage once settled. Topsoil is generally used for filling large areas or amending native garden beds, while the lighter, fluffier potting mix is preferred for containers where drainage and aeration are paramount.