The diameter pitch (DP), as a unit of gear tooth size, is widely used in countries such as the United States and the United Kingdom where length units are in inches.
The diameter pitch is determined by dividing the number of teeth by the pitch diameter (d, in inches).
DP=z/d (inches)
That is to say, the diameter pitch is a unit that represents the number of teeth within a diameter of 1 inch. The more teeth in a 1-inch diameter, the smaller each tooth, so the larger the diameter pitch, the smaller the size of the teeth. On the contrary, when the modulus (m) is used as a unit of tooth size, the larger the modulus value, the larger the tooth size.
The conversion between modulus and radial pitch is shown below.
M=25.4/DP
Internationally, according to ISO regulations, although the unit for representing the size of gear teeth is established as a module based on the metric system, there are still many gears with radial pitch specifications used in the United States.