Spheres such as pool balls are measured by their circumference and diameter. Squares and sided objects are measured according to width and length, but round three-dimensional objects are measured by calculating the distance around and through the object. Calculate the circumference of a pool ball with a string and a measuring tape, and the diameter with a ruler or measuring tape.
Hold the end of a piece of yarn or string against any part of the pool ball. Wrap the yarn or string around the centre of the ball until it meets the end of the yarn/string you are holding against the ball's surface.
Cut the piece of yarn or string exactly where the yarn/string meets the yarn or string end being held against the ball's surface. According to Science is Fun website, measuring circumference of a ball is easiest when using a piece of string. Any string or yarn will work.
Place the string or yarn onto a flat surface such as a table. Align the yarn or string with a ruler or measuring tape. Measure the length of the piece of yarn or string. This measurement is the circumference of your pool ball.
Hold a measuring tape or ruler above the pool ball. The diameter of a sphere or orb is the measurement of a "line" running through the centre of the widest part of the shape.
Touch the measuring tape or ruler against the ball's surface, but don't follow the contours of the ball. The measuring device should be rigid above the ball.
Measure the width of the widest part of the ball, which is normally the ball's centre. This is the diameter of the pool ball.
According to World Pool-Billiards Association, regulation pool balls should measure 2-1/4 inches in diameter and weigh about 170gr. Pool balls can be cleaned with soap and water. A set of balls consist of 15 colour striped or solid balls with numbers, and one solid white ball known as a cue ball.