 MM = Metric Module

PD = Pitch Circle Diameter in mm

n = Number of Teeth
MM = PD / n
English system (inch measure) gears are more commonly specified with the Diametral Pitch (DP) which 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

n = Number of Teeth
DP = n / PD
When converting between module and DP there is an inverse relationship and normally a conversion between the two units of measure (inches and millimeter).