How this calculator works
Diametral pitch counts teeth per unit of pitch diameter, whereas module is pitch diameter per tooth. The engine converts reciprocal inches to reciprocal metres before calculation. To avoid over-specifying the same geometry, module mode converts a known pitch; diameter and tooth modes use pitch. The pitch circle is a reference geometry and usually lies below the tooth tips. Matching pitch alone does not ensure compatible pressure angles or tooth forms.
Formula
P = N/D; module = D/N = 1/P. For module in mm and pitch in teeth/in: module = 25.4/P.
Worked example
A 40-tooth gear with a 100 mm pitch diameter has pitch 400 m⁻¹, or 10.16 in⁻¹, and module 2.5 mm. Twenty teeth at pitch 1000 m⁻¹ give a 20 mm pitch diameter.
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Frequently asked questions
Can I use the outside diameter?
No. Use pitch-circle diameter. Outside diameter depends on addendum and profile shift.
Is a higher diametral pitch a larger tooth?
It means smaller teeth for a fixed diameter; module increases with tooth size.
What does reciprocal length mean here?
It is a count per unit diameter, such as teeth per inch. The value scales inversely with the chosen length unit.