 The Diametral Pitch (DP) is the number of teeth per inch of diameter of the pitch circle.The units of DP are inverse inches (1/in).DP = Diametral Pitch 

PD = Pitch Circle Diameter in inches 

CP = Circular Pitch in inches

n = Number of Teeth 
DP = n / PD
The Diametral Pitch (DP) is equal to π divided by the Circular Pitch (CP).