Gear Module and Diametral Pitch Reference Table
Standard gear modules matter because cutters, hobs and inspection gages are stocked in preferred sizes. This reference lists the ISO 54 module series with the equivalent diametral pitch, circular pitch and tooth proportions, so you can convert between metric and inch practice.
| Module m (mm) | ISO 54 series | Size class | Equivalent DP (teeth/in) | Circular pitch (mm) | Addendum (mm) | Dedendum (mm) | Whole depth (mm) |
|---|---|---|---|---|---|---|---|
0.5 | I (preferred) | Fine | 50.8 | 1.5708 | 0.5 | 0.625 | 1.125 |
0.6 | I (preferred) | Fine | 42.3333 | 1.885 | 0.6 | 0.75 | 1.35 |
0.8 | I (preferred) | Fine | 31.75 | 2.5133 | 0.8 | 1 | 1.8 |
1 | I (preferred) | Small | 25.4 | 3.1416 | 1 | 1.25 | 2.25 |
1.125 | II (second choice) | Small | 22.5778 | 3.5343 | 1.125 | 1.4063 | 2.5313 |
1.25 | I (preferred) | Small | 20.32 | 3.927 | 1.25 | 1.5625 | 2.8125 |
1.375 | II (second choice) | Small | 18.4727 | 4.3197 | 1.375 | 1.7188 | 3.0938 |
1.5 | I (preferred) | Small | 16.9333 | 4.7124 | 1.5 | 1.875 | 3.375 |
1.75 | II (second choice) | Small | 14.5143 | 5.4978 | 1.75 | 2.1875 | 3.9375 |
2 | I (preferred) | Small | 12.7 | 6.2832 | 2 | 2.5 | 4.5 |
2.25 | II (second choice) | Small | 11.2889 | 7.0686 | 2.25 | 2.8125 | 5.0625 |
2.5 | I (preferred) | Small | 10.16 | 7.854 | 2.5 | 3.125 | 5.625 |
2.75 | II (second choice) | Small | 9.2364 | 8.6394 | 2.75 | 3.4375 | 6.1875 |
3 | I (preferred) | Medium | 8.4667 | 9.4248 | 3 | 3.75 | 6.75 |
3.5 | II (second choice) | Medium | 7.2571 | 10.9956 | 3.5 | 4.375 | 7.875 |
4 | I (preferred) | Medium | 6.35 | 12.5664 | 4 | 5 | 9 |
4.5 | II (second choice) | Medium | 5.6444 | 14.1372 | 4.5 | 5.625 | 10.125 |
5 | I (preferred) | Medium | 5.08 | 15.708 | 5 | 6.25 | 11.25 |
5.5 | II (second choice) | Medium | 4.6182 | 17.2788 | 5.5 | 6.875 | 12.375 |
6 | I (preferred) | Medium | 4.2333 | 18.8496 | 6 | 7.5 | 13.5 |
7 | II (second choice) | Medium | 3.6286 | 21.9911 | 7 | 8.75 | 15.75 |
8 | I (preferred) | Medium | 3.175 | 25.1327 | 8 | 10 | 18 |
9 | II (second choice) | Medium | 2.8222 | 28.2743 | 9 | 11.25 | 20.25 |
10 | I (preferred) | Large | 2.54 | 31.4159 | 10 | 12.5 | 22.5 |
11 | II (second choice) | Large | 2.3091 | 34.5575 | 11 | 13.75 | 24.75 |
12 | I (preferred) | Large | 2.1167 | 37.6991 | 12 | 15 | 27 |
14 | II (second choice) | Large | 1.8143 | 43.9823 | 14 | 17.5 | 31.5 |
16 | I (preferred) | Large | 1.5875 | 50.2655 | 16 | 20 | 36 |
18 | II (second choice) | Large | 1.4111 | 56.5487 | 18 | 22.5 | 40.5 |
20 | I (preferred) | Large | 1.27 | 62.8319 | 20 | 25 | 45 |
25 | I (preferred) | Heavy | 1.016 | 78.5398 | 25 | 31.25 | 56.25 |
32 | I (preferred) | Heavy | 0.7938 | 100.531 | 32 | 40 | 72 |
40 | I (preferred) | Heavy | 0.635 | 125.6637 | 40 | 50 | 90 |
50 | I (preferred) | Heavy | 0.508 | 157.0796 | 50 | 62.5 | 112.5 |
Module series follow ISO 54 and DIN 780 practice: series I should be used first, series II only when necessary. Derived columns assume a standard full-depth tooth (addendum 1 m, dedendum 1.25 m, whole depth 2.25 m). Equivalent DP is 25.4 / m and is rarely a standard DP value, so a metric module gear cannot be meshed with an inch DP gear unless the match is exact. Common inch diametral pitches include 4, 5, 6, 8, 10, 12, 16, 20 and 24. Size classes are an informal grouping for orientation only.
Structured gear attributes in ERP
Optional: a short review of how module, pressure angle and quality grade could become searchable item attributes.
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How to use the table
Search for a module or filter by series or size class. Read across to find the equivalent diametral pitch (teeth per inch of pitch diameter), circular pitch and the tooth proportions. Use series I values wherever possible: hobs, cutters and master gears are most available and cheapest in those sizes. Series II modules are valid but often mean special tooling and longer lead times from a gear shop.
Module versus diametral pitch
Module m is pitch diameter divided by number of teeth in millimeters, while diametral pitch P is teeth per inch of pitch diameter. They are reciprocals scaled by 25.4: m = 25.4 / P and P = 25.4 / m. Because inch DP values are whole numbers, their metric modules are not round: 8 DP is 3.175 mm. A module 3 gear is close to 8.47 DP, so it will not mesh correctly with an 8 DP gear.
Using the derived proportions
For a full-depth tooth, addendum equals the module, dedendum is 1.25 times the module, and whole depth is 2.25 times the module. Circular pitch is pi times the module and equals the arc distance between corresponding points on adjacent teeth. These figures set the blank size, cutter depth and the clearance needed in housings. For stub or long-addendum designs the proportions change, and the tables from the cutter maker should be used.
Common mistakes
The usual mistake is assuming module and DP can be mixed because they look close. Another is choosing a series II module for convenience and then discovering a four week tooling lead time. Remember that pressure angle must also match, and that helical gears are specified by normal module, not transverse module, which is a frequent source of wrong hobs on purchase orders.
Frequently Asked Questions
How do I convert module to diametral pitch?
Divide 25.4 by the module to get diametral pitch in teeth per inch. A module of 2 is 12.7 DP, and a module of 3 is about 8.47 DP. To go the other way, divide 25.4 by the DP value.
Which gear modules are standard?
ISO 54 lists series I as 1, 1.25, 1.5, 2, 2.5, 3, 4, 5, 6, 8, 10, 12, 16, 20, 25, 32, 40 and 50 plus finer values below 1. Series II fills the gaps and should be avoided when possible.
Can a module gear mesh with a diametral pitch gear?
Only if their tooth sizes are identical. For example 8 DP equals module 3.175, which does not match module 3. Mixed pairs run with poor contact and noise, so specify gears in one system for a mating set.
What does normal module mean for helical gears?
Normal module is measured perpendicular to the tooth, while transverse module is measured in the plane of rotation. Standard hobs cut a normal module, and transverse module equals normal module divided by the cosine of the helix angle.
If gear specifications are scattered across drawings and quotes, Netray can help structure them as searchable attributes in your item master.
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