Diametral Pitch (DP)

What is diametral pitch (DP)?

The diametral pitch (DP) is often used in the United States and the United Kingdom, where the inch is the unit of measure, as a unit to express the size of gear teeth. Gears that use diametral pitch (DP) as a unit tooth size are called imperial gears or inch gears.

The diametral pitch (DP) is obtained by dividing the number of teeth (z) by the pitch circle diameter (d, in inches).

DP = z / d (inch)

In other words, the diametral pitch can be expressed as the number of teeth per 1" inch.
More teeth per inch mean a smaller tooth profile, so in the case of diametral pitch, the larger the DP value, the smaller the size of the teeth.
Inversely, for metric gears, the larger the module number means the larger the tooth profile.

The conversion formula for the module (m) from the diametral pitch (DP) is as follows:

m = 25.4 / DP

Internationally, the ISO designates the use of the module based on the metric system as the unit of measurement for gear tooth size.
In the United States, however, inch (DP) standards are the most common gearing used.

Below is a table comparing the diametral pitch (DP), module (m), and circular pitch (CP).

Module m	Circular Pitch CP	Diametral Pitch DP
0.39688	1.24682	64
0.5	1.57080	50.8
0.52917	1.66243	48
0.6	1.88496	42.33333
0.79375	2.49364	32
0.79577	2.5	31.91858
0.8	2.51327	31.75
1	3.14159	25.4
1.05833	3.32485	24
1.25	3.92699	20.32
1.27000	3.98982	20
1.5	4.71239	16.93333
1.59155	5	15.95929
1.58750	4.98728	16
2	6.28319	12.70
2.11667	6.64970	12
2.5	7.85398	10.16
2.54000	7.97965	10
3	9.42478	8.46667
3.17500	9.97456	8
3.18310	10	7.97965
4	12.56637	6.35
4.23333	13.29941	6
4.77465	15	5.31976
5	15.70796	5.08
5.08000	15.95929	5
6	18.84956	4.23333
6.35000	19.94911	4
6.36620	20	3.98982
8	25.13274	3.175
8.46667	26.59882	3
10	31.41593	2.54