The vertical liquid volume inside a cylindrical container can be readily calculated with a tape measure and this guide. The volume of liquid that occupies the container, such as water in a glass, is related to the height of the water in the glass. Also relevant is the radius of the cylindrical container. The radius is equal to half the diameter of the glass. The top of the glass is a circle, so the diameter is the straight line distance that measures the width of the circle and passes through the circle's centre.
Measure the height of the liquid in the container in inches. As an example, say this height is 10 inches.
Measure the diameter of the container. Assume a diameter of 3 inches for the example calculation.
Calculate the radius of the container by diving the diameter by 2. This yields a radius of 1.5 inches (3 inches divided by 2).
Calculate the volume of a cylinder in cubic inches using the formula:
V = pi x r^2 x h,
where V is the volume, r is the radius, and h is the height. Completing the example you have:
V = (3.1415) x (1.5 in)^2 x (10 in)
The volume of the liquid is 70.7 cubic inches.
Cubic inches may be converted to gallons by dividing by 231, since 231 cubic inches equals 1 gallon.