The center distance is defined as the smallest distance between the axes running through the centers of pinion and wheel. The symbol a is often used for the center distance. In general, the center distance is calculated from the sum of the working pitch radii:

a = (d_w1 + d_w2) / 2 .

In the special cases of pairing gears without profile shift (i.e., X-Zero gears) or gears for which the sum of their respective profile shifts equals zero, the working pitch circles are equal to the reference circles (= manufacturing pitch circles). The corresponding reference center distance a_d is:

a_d = (d₁ + d₂) / 2 .

The relation between the center distance of a general profile shifted gearing and the reference center distance is:

a = a_d cos(α_t) / cos(α_wt) ,

wherein α_t denotes the transverse pressure angle and α_wt is the transverse working pressure angle.