Table tennis balls are 40mm in diameter and weigh 2.7 grams. Top quality table tennis balls are graded by a 3 star system which differentiates ball quality levels by seam quality, roundness and consistency.
Tournament grade table tennis balls have smooth seams and consistent wall thickness.
Competition table tennis balls have true flight to allow you to play your best.
Competition table tennis balls give you consistent bounce allowing you to develop a consistent stroke.
3 stars are the highest quality table tennis balls as used in table tennis tournaments.
2 stars are the next quality level of table tennis balls good for home use and and robots (ping pong robots usually require 2 or 3 star table tennis balls for best operation).
1 star is the lowest quality in star graded table tennis balls and are ok for practice.
There also are larger (55mm diameter) balls for a slower game for novices. These larger table tennis balls will not work in ping pong robots and are not acceptable for table tennis tournaments.
The international rules specify that the game is played with a light 2.7 gram, 40 mm diameter ball. The rules say that the ball shall bounce up 24–26 cm when dropped from a height of 30.5 cm on to a standard steel block thereby having a coefficient of restitution of 0.89 to 0.92. The 40 mm ball was introduced after the 2000 Olympic Games. However, this created some controversy as the Chinese National Team argued that this was merely to give non-Chinese players a better chance of winning since the new type of balls has a slower speed, while at that time most Chinese players were playing with fast attack and smashes. A 40 mm table tennis ball is slower and spins less than the original 38 mm (1.5 inch) one. The ball is made of a high-bouncing air-filled celluloid or similar plastics material, colored white or orange, with amattefinish. The choice of ball color is made according to the table color and its surroundings. For example, a white ball is easier to see on a green or blue table than it is on a gray table. Stars on the ball indicate the quality of the ball. Three stars indicate that it is of the highest quality, and is used in official competition.