Outside Dia. of a Standard External Gear
(Number of Teeth + 2) / Diametral Pitch
Example: 21tooth 6DP Gear
(21 + 2) / 6 = 3.833
Outside Dia. of an External Involute Spline
(Number of Teeth + 1) / Diametral Pitch
Example: 24 Tooth 16/32DP Spline
(24 + 1) / 16 = 1.563