Gear Center Distance Calculator
Find the center distance between two external spur gears from module and tooth counts — the shaft spacing you need before you can lay out a gearbox.
Enter your values
Module and tooth counts for both gears.
How this calculator works
For two external spur gears meshing on standard (non-profile-shifted) center distance, the spacing between their shaft centers depends only on module and the total tooth count:
Where m is module in mm, and z₁, z₂ are the tooth counts of the pinion and gear. This is the same as adding the two pitch-circle radii, since pitch diameter d = m·z for each gear.
Internal gear pairs
When a pinion meshes inside a ring (internal) gear, the pinion sits inside the larger gear rather than beside it, so the center distance is the difference of the two pitch radii instead of the sum:
Here z₂ is the internal ring gear's tooth count, which must be larger than the pinion's tooth count z₁ for a valid mesh.
Frequently asked questions
What if my gears use profile shift?
Profile-shifted gears can run at a center distance slightly different from the standard value calculated here. This calculator gives the standard (non-shifted) center distance, which is the right starting point for most designs.
Does module have to match on both gears?
Yes — two gears can only mesh correctly if they share the same module (or the same diametral pitch, in imperial systems). Mismatched modules will not mesh properly regardless of tooth count.