Spur Gear Design Guide
How module, tooth count, and pressure angle combine to define a spur gear, and how to choose them for a real mechanism.
A standard involute spur gear is fully defined by three numbers: module (tooth size), tooth count (z), and pressure angle (flank steepness, almost always 20° on modern gears, including every gear in this catalog). From those three, every other dimension follows directly:
- Pitch diameter
= m × z - Base diameter
= m × z × cos(pressure angle) - Outside (tip) diameter
= m × (z + 2) - Root diameter
≈ m × (z − 2.5)
Every gear page on this site shows these calculated directly from the same formulas the interactive generator uses, so what you see on a gear's page is exactly what you'll get in the download.
Choosing tooth count
For a given module, more teeth means a larger, smoother-running gear with a higher pitch-line velocity for the same rotational speed; fewer teeth means a more compact gear but, below about 17–18 teeth at a 20° pressure angle, the involute flank starts to undercut at the root, weakening the tooth base. The catalog starts at 10 teeth so you can see this regime — compare a 10-tooth gear against a 20-tooth gear at the same module to see the difference in root shape.
Choosing the ratio
For a meshing pair, the ratio is simply teethdriven / teethdriver, and center distance is m × (z1 + z2) / 2. See the Gear Ratio Guide for worked examples.
Where to go next
Once you've picked module and tooth count, browse the matching page in the gear catalog for exact dimensions and downloads, or open the generator to build a custom pair with a specific face width, bore, backlash, or helix angle that isn't in the fixed catalog.