The cylinder is one of geometry's most basic forms -- essentially a series of circles stacked on top of each other. While geometric circles are two-dimensional (and thus have no "depth"), cylinder size in the physical world is calculated by assuming that each circle is one "unit" high. A measurement in fluid ounces is easy to calculate, requiring only one extra step beyond calculating volume in cubic inches.
Measure the cylinder diameter in inches, then divide by two to derive the cylinder's radius. Multiply that radius by pi (3.141), and then multiply that figure by itself to get the volume of a circle 1 inch high.
Multiply the volume of that circle by the cylinder's height in inches to get the cylinder's displacement in cubic inches.
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Multiply the displacement in cubic inches by 0.554 to convert it to fluid ounces.
As an example, we'll calculate the ounce displacement of a cylinder measuring 3 inches across and 6 inches high. We'll first divide the diameter (3) by two (equals 1.5), multiply by 3.141 (equals 4.7115), and multiply that by itself (equals 22.19).
Next, we'll multiply 22.19 by the cylinder's height (6) to derive the displacement in cubic inches (133.14). That displacement in cubic inches multiplied by 0.554 gives us a displacement in fluid ounces of 73.75.