Potting soil is sold by dry volume, not weight. Distributors generally use the dry quart or cubic inch for small quantities and the dry cubic foot for larger quantities. It can be helpful to convert from one measuring unit to another when planning a potting soil purchase. The U.S. dry volume system starts with the dry quart, equal to 67.2 cubic inches in volume.
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Find the Volume
It takes 25.7 dry quarts or 1,728 cubic inches to equal 1 cubic foot of potting soil. To know how many flower pots a quart or cubic foot of potting soil will fill, you need to know the volume of each pot. Most flower pots are cylinders. The formula for the volume of a cylinder is V = 3.14 times radius squared times height.
Filling Flower Pots
For example, to find the volume of a flower pot 6 inches in diameter and 6 inches high, divide the diameter by 2 to get a radius of 3. Multiply the radius of 3 by itself to get 9, multiply 9 by 3.14 to get 28.26, and multiply 28.26 times the 6-inch height to get 169.56 cubic inches per pot. Divide 1,728 by 169.56 to get 10 pots per cubic foot of soil. Put another way, each of these 10 pots will take 2.5 dry quarts of soil, calculated by dividing 169.56 cubic inches per pot by 67.2 cubic inches per dry quart.