About 22.86 cm. The inch of pizza refers to inches, named after its diameter, 1 inch = 2.54 centimeters, 9 inches is 9*2.54 = 22.86 centimeters, so a 9-inch pizza is about 22.86 centimeters. Common pizza sizes and corresponding sizes are: 6-inch pizza 15.24 cm, 7-inch pizza 17.78 cm, 8-inch pizza 20.32 cm, 9-inch pizza 22.86 cm, 10-inch pizza 25.4 cm, 11-inch pizza 27.94 cm, 12-inch pizza 30.48 cm, 13-inch pizza is 33.02 cm, 14-inch pizza is 35.56 cm, and 15-inch pizza is 38.1 cm.