Most officially sized tennis balls have a circumference of about 8.258 inches. This is because modern tennis balls are manufactured to have a width of 2.630 inches.
Though tennis balls tend to be produced at a width of 2.630 inches, the International Rules of Tennis and the United States Tennis Association allow some leeway in ball size, with balls as small as 2.575 inches and as large as 2.700 inches being accepted. Therefore, the actual circumference of a ball in play could be anywhere from 8.086 inches to 8.478 inches. Tennis balls intended for use by young children and beginners are larger than official balls.