9 millimeter is the caliber.
IMPROVING ANSWER: A "9mm" bullet is actually .355 or .356 of an inch in diameter. So the caliber is about .36, but nobody calls it that because it would be confusing. Everybody calls it a 9mm and if somebody asks you what "caliber" that pistol is, you can answer "It's a 9mm" without having to add or explain anything.
further improve: 9mm is the correct answer. you can find the caliber by measuring the diameter of the bullet. translate to millimeters or inches and that's the caliber. 9mm is a 9mm caliber not "around .36"
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