Calculating the volume of soil needed for containers can be confusing because pots are typically measured in liquid gallons while soil is sold by dry volume in cubic feet. Understanding the direct conversion between these two units is the first step toward accurate purchasing. This calculation ensures gardeners buy the precise amount of material, preventing waste and unnecessary trips to the store. The following information simplifies this process by providing the exact conversion for a common five-gallon container and offering practical purchasing advice for all your gardening projects.
The Direct Volume Conversion
A standard five-gallon container, when referring to US liquid volume, corresponds to a precise dry volume measurement. This theoretical volume is exactly 0.6684 cubic feet of space. Therefore, a five-gallon pot requires slightly more than two-thirds of a cubic foot of soil to be completely filled. In practical terms, you can round this figure to 0.67 cubic feet for purchasing and planning purposes. The actual amount of soil placed into the pot will be slightly less than this theoretical volume, accounting for necessary headspace below the rim and any pre-existing drainage layer materials.
Relating Pot Size to Bag Size
The calculated volume of 0.67 cubic feet must be related to the way soil is commercially packaged for consumers. Most gardening centers sell potting mix and other substrates in bags labeled with volumes like 1.5 cubic feet or 2.0 cubic feet. Smaller bags are often measured in quarts or liters, which can further complicate the purchasing decision.
A common retail bag size of 1.5 cubic feet offers enough soil to fill two full five-gallon pots. Two pots require 1.34 cubic feet of soil, leaving a small surplus of 0.16 cubic feet from the 1.5 cubic foot bag. Buying in larger cubic foot bags is generally more economical and efficient than purchasing smaller bags measured in quarts.
Understanding the difference between dry volume (cubic feet) and liquid volume (gallons or quarts) is important when comparing products. For gardeners needing to fill many containers, calculating the total cubic feet required allows for accurate bulk purchasing and minimizes the number of individual bags needed.
General Volume Calculation Methods
To determine the soil volume for any size container, you can use a geometric formula. The volume of a cylinder, which closely resembles a round pot, is calculated by multiplying Pi by the radius squared, and then by the height. Here, ‘r’ is the radius of the pot’s interior and ‘h’ is the height of the soil column.
Measuring the pot’s interior dimensions in inches provides the volume in cubic inches. Once you have the cubic inch total, you must convert it to cubic feet to match the commercial bag labeling. This conversion is done by dividing the total cubic inches by 1,728, since one cubic foot contains 1,728 cubic inches.
For irregularly shaped containers or for those who prefer to avoid manual calculations, numerous online volume calculators are available. These tools allow you to input the container’s dimensions and the desired depth to receive an immediate volume output in cubic feet. Regardless of the method chosen, accurately measuring the container’s interior dimensions ensures you buy the correct amount of material for your planting project.