Home gardeners often face the challenge of reconciling two different units of measurement: the cubic volume used for potting soil bags and the liquid volume used for containers. Potting soil is sold in bags labeled with cubic units, such as cubic feet or liters. Conversely, garden pots, especially those purchased at nurseries, are typically measured by their capacity in US liquid gallons. This difference creates confusion when matching the volume of soil purchased to the volume of the pot. Understanding the precise mathematical relationship between these two units is the first step in accurately budgeting for a gardening project.
The Standard Conversion Rate
The foundation for all calculations is the direct mathematical relationship between the two volume units. One cubic foot, the standard unit for potting mix bags, is precisely equal to 7.48 US liquid gallons. This conversion factor allows for straightforward calculation of the theoretical gallon capacity of any bag labeled in cubic feet. Common bag sizes translate directly into predictable gallon volumes: a 1.0 cubic foot bag contains 7.48 gallons, a 1.5 cubic foot bag holds 11.22 gallons, and a 2.0 cubic foot bag provides 14.96 gallons.
Understanding Bag Volume Measurements
The volume printed on a bag of potting soil represents a theoretical measure that is often an estimate of the “loose fill.” The true usable volume of the material can differ from the stated cubic measurement due to a physical phenomenon called compression. Potting mix is a blend of materials like peat moss, coir, and perlite, which contain a high degree of air space. During the packaging and transportation process, external pressures can reduce this air space, causing the mixture to settle and compact, meaning a bag labeled 1.5 cubic feet may not actually fill a container mathematically requiring that volume. Some manufacturers may label bags as “compressed,” which indicates the volume of the material before it was pressed down, requiring the user to fluff or break it up to achieve the full volume.
Calculating Soil Needs for Containers
Accurately determining the volume of a pot in gallons is necessary to know how much soil to buy. For containers with a uniform shape, such as cylindrical or square pots, the volume is calculated using their interior physical dimensions (height, length, and width) in inches to find the cubic volume. This cubic volume is then converted to US liquid gallons by dividing the total cubic inches by the standard conversion factor of 231. For example, a rectangular pot measuring 12 by 12 by 10 inches has a volume of 1,440 cubic inches, which converts to approximately 6.23 gallons. Because of the volume variability in bagged soil and the settling that occurs after watering, it is recommended to round up the purchase to the next bag size to ensure adequate material.