An official tennis ball is between 2.575 inches and 2.7 inches in diameter as defined by the International Tennis Federation. Tennis balls also must conform to other criteria if they are to be used in regulation play.
Tennis balls are covered in felt and have an internal pressure of 12 pounds per square inch. Tennis balls used in regulation play are optic yellow in color because the bright hue makes the balls highly visible both on the court and on television. The U.S. Open uses 70,000 tennis balls per year between practice and competitive game play. This equals 700 square yards of felt and more than 3,900 pounds of rubber.