another rule of thumb
Drop 2 from the nominal width of the lumber, then double it for the acceptable span on 16" centers:
2x6: (6-2) x 2 = 8
2x8: (8-2) x 2 = 12
2x10: (10-2) x 2 = 16
you can also google "span tables" and get other acceptable solutions with other "on center" layouts
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