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From wikianswers>>>The circumference of a basketball varies according to the governing organization's rules. According to the FIBA, it is 78 cm (30.7"). In the NBA, it is 29 1/2" to 29 3/4" around the channels and 29 5/8" to 29 7/8" across the channels. In the NCAA the maximum Circumference is 30".
The NBA also recommends these sizes for different athletes
Size 3: circumference 22 to 22.5 inches, weight 10-11 ounces, recommended for small children (age 5-8). It has a diameter of 7 to 7.16 inches and a radius of 3.5 to 3.58 inches
Size 5: circumference 27.25 to 27.75 inches, weight 16.6-18 ounces recommended for boys and girls under 12 years old (junior league). It has a diameter of 8.67 to 8.83 inches and a radius of 4.34 to 4.42 inches.
Size 6: circumference 28.5 to 29 inches, weight 20 ounces, recommended for boys and 12-14 and girls and women age 12 and up. This is the official game ball for women's high school, university and professional basketball. It has a diameter of 9.07 to 9.23 inches and a radius of 4.54 to 4.62 inches.
Size 7: circumference 29.5 to 30 inches, weight 22 ounces, recommended for men and boys age 15 and up. This is the official game ball for men's high school, university and professional basketball. It has a diameter of 9.39 to 9.55 inches and a radius of 4.7 to 4.77 inches.

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