Metric gears are defined by the module system, as follows:
The length in mm of the pitch circle diameter per tooth.
MOD = PCD / N
Number of Teeth
The number of teeth on the gear.
N = PCD / MOD
Pitch Circle Diameter
The diameter of the pitch circle.
PCD = N x MOD
The outside diameter of the gear.
OD = (N + 2) x MOD
The distance between the axes of two gears in mesh.
C = PCD(g) + PCD(p)
The distance between adjacent teeth measured along the arc at the pitch
CP = x MOD
Circular Tooth Thickness
The width of a tooth measured along the arc at the pitch circle
CTT = CP / 2
The height of the tooth above the pitch circle diameter.
A = MOD
The depth of the tooth below the pitch circle diameter.
D = H - A
The total depth of the space between adjacent teeth.
Finer than 1.25 MOD:
H = 2.4 x MOD
1.25 MOD and coarser:
H = 2.25 x MOD