Comparison: Diametral Pitch vs. Pitch Diameter

Date: May 30, 2024

Both pitch diameter and diametral pitch are essential factors in gear measurements, but they represent different parts of a gear’s geometry. Diametral pitch uses pitch diameter as a relative measurement to quantify spacing and size of gear teeth.

In simple terms, pitch diameter is the diameter at which the gear teeth are fully engaged at their pitch circle.

Diametral pitch is the measure of the size and shape of gear’s teeth, defined as the number of teeth per inch of the pitch diameter. Diametral pitch is found by dividing the number of teeth in a gear by the pitch diameter.

What is Pitch Diameter?

An estimate for pitch diameter can be made by establishing the diameter of the gear at the center of the gear tooth.

illustration of gear pitch diameter measurement

A gear's pitch diameter is also the diameter of its pitch circle

What does it measure?

Pitch diameter is used to determine the size and spacing of a gear’s teeth, and to calculate other gear parameters such as:

  • Contact ratios
  • Tooth dimensions
  • Gear ratios

Pitch diameter is a diameter measurement of the pitch circle, which is an imaginary cylinder passing through the tooth at the pitch point between the addendum circle and the dedendum circle.

illustration of two gears with pitch circles meeting where the gear teeth mesh

A gear's operating pitch circle tangentially meets the pitch circle of a meshing gear

What is Diametral Pitch?

Diametral pitch expresses the size of the gear teeth, typically as the number of teeth per inch. The pitch diameter is an essential measurement for calculating the diametral pitch.

What does it measure?

Diametral pitch measures the size of a gear’s teeth and their spacing, which is then used to determine whether two gears will mesh correctly and work together. Additionally, it can be a helpful factor in determining a gear tooth’s strength and load-bearing capacity.

Diametral pitch calculation

D = N / P

Where:

D is Diametral pitch

N is the number of teeth

P is the pitch diameter

gear diametral pitch illustration showing number of teeth divided by pitch diameter to calculate diametral pitch

Diametral pitch helps determine the right size and fit for gears to mesh correctly

WM Berg manufactures high-precision gearing including custom precision gears. Contact our engineering team with questions or to request technical support with your application.

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