Metric module and diametral pitch describe tooth size in different unit systems. A module value is a length in millimeters; diametral pitch is the number of teeth per inch of pitch diameter. The exact conversion is DP = 25.4 ÷ m, where m is module in millimeters. This table calculates the equivalent diametral pitch for selected common module values. It helps translate a specification or compare catalogs, but it does not make two gears compatible by itself.

Gear Module to Diametral Pitch Conversion

For example, module 2 corresponds to 12.7 teeth per inch because 25.4 divided by 2 is 12.7. Module 2.5 corresponds to 10.16 DP. Values may be decimal even when a catalog lists only selected preferred diametral pitches. Keep the exact calculated result during comparison; do not round to a nearby catalog size until the gear system and design requirements are checked.

Understanding the units

Module equals pitch diameter divided by tooth count, with both dimensions expressed in millimeters. Diametral pitch equals tooth count divided by pitch diameter in inches. Combining the definitions gives DP = 25.4/m. The conversion uses the exact millimeter-to-inch factor and is reciprocal: m = 25.4/DP. A larger module means larger teeth; a larger diametral pitch means smaller teeth. This inverse relationship is a useful way to spot a misplaced decimal.

The reference values in the table are calculations from selected module sizes. They are not a complete list of available stock gears, preferred series, or manufacturing tolerances. The source provides the module and diametral-pitch concepts; the numeric equivalents follow directly from the unit relationship. If a catalog offers a listed gear with the nearest rounded DP, treat it as a candidate for review, not as a confirmed replacement.

Keep module and DP labeled with their units so the conversion remains traceable on drawings and inspection records.

Where the conversion helps

Suppose a metric drawing specifies a module 2.5 gear and an imported component is labeled 10 DP. The exact conversion gives 10.16 DP, which indicates that the sizes are close but not identical. Before treating them as a pair, compare the actual pitch diameters and tooth counts, pressure angle, tooth profile, helix angle where applicable, and backlash. A small tooth-size mismatch can prevent proper meshing even when the nominal numbers appear similar.

The relation also helps when translating a catalog request, checking a legacy drawing, or identifying whether a gear is likely to use a metric or inch tooth-size system. It does not replace inspection measurements. If pitch diameter is unknown, tooth count alone cannot identify module or DP; you need another dimensional measurement or the original specification. When the original drawing is incomplete, record the measured dimension, instrument uncertainty and tooth-count method so another engineer can reproduce the identification.

Compatibility limits

A module-to-DP conversion is dimensional bookkeeping, not a design approval. Gears with equivalent tooth size may still differ in pressure angle, tooth form, addendum modification, accuracy grade, material, heat treatment, backlash or center distance. Spur and helical gears also use normal and transverse module distinctions. Verify which module definition the source uses before applying this chart to a helical gear.

Do not choose a replacement solely because the calculated DP rounds to a familiar catalog number. Use the manufacturer’s drawing or gear inspection data, and evaluate tooth contact, center distance, load and lubrication for the application. Strength and life calculations require additional geometry and operating data.

Source

The definitions of module and diametral pitch are described in KHK Gears’ technical reference on gear module. Conversion values here are calculated with 25.4 mm per inch; no gear performance rating is implied.