Comparison · Gear basics

Module vs. diametral pitch (DP): conversion table and how to read a gear drawing

Metric drawings describe tooth size by module; American drawings use diametral pitch (DP). They describe the same thing in opposite directions and different units, and the conversion is a single line — yet "can a 12 DP gear be cut with a 2-module hob?" has the answer no. This guide gives the conversion table and, while it is at it, the other parameters you must read correctly on a gear drawing.

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Comparison · Published 2026-09-12 · Wolfer Transmission

Module and DP: one multiplication apart

Module m = pitch diameter d ÷ number of teeth z, in millimetres; a larger module means a larger tooth. Diametral pitch DP = teeth z ÷ pitch diameter d in inches, in 1/in; a larger DP means a smaller tooth. The two are reciprocals scaled by 25.4: m = 25.4 ÷ DP and DP = 25.4 ÷ m. Circular pitch p = π·m (mm) = π ÷ DP (in). So an 8 DP gear is module 3.175, not 3, and a 10 DP gear is module 2.54, not 2.5.

That is the answer to the hob question: a tooth cut with a 2.5-module hob differs from a 10 DP tooth by 1.6 %, so pitch and tooth thickness are both wrong and it meshes badly with the original DP mate. DP gears need DP hobs, or tooling made to the converted non-standard module. Wolfer routinely covers module 0.5–12 (about DP 50–2) and machines DP gears to drawing — just state whether the drawing is in module or DP when you ask for a quote.

The other items on the drawing

Pressure angle: 20° is the modern default; 14.5° is common on older American gears and 25° on some heavy-duty ones. Gears with different pressure angles cannot mesh, so check it whenever you replace one half of a pair.

Profile shift coefficient x: the cutter is moved radially by x·m from its standard position. Spur gears with fewer than 17 teeth at 20° are given positive shift to avoid undercut; shifting both gears also produces non-standard centre distances and balances the strength of pinion and wheel. The x value sets the tip diameter and tooth thickness, so it cannot be left off the drawing.

Tooth-thickness allowance and backlash: the accuracy class governs profile and pitch errors only; backlash comes from the tooth-thickness allowance, called out with a system such as DIN 3967 — Wolfer's milled spur gears are made to DIN 3967 8d25 by default and the ground 40Cr series to JIS grade 2. Together with helix angle and hand (helical gears), face width, bore tolerance and keyway, material and heat-treatment hardness, and the accuracy class (ISO 1328 / DIN / AGMA / JIS), these make a complete gear drawing; whatever is missing, the quotation has to assume.

Diametral pitch (DP) to module conversion (m = 25.4 ÷ DP)
DP (1/in)	Module m (mm)	Circular pitch p (mm)	Common module m (mm)	Equivalent DP (1/in)
48	0.529	1.662	0.5	50.8
32	0.794	2.494	1	25.4
24	1.058	3.325	1.5	16.93
20	1.27	3.990	2	12.7
16	1.588	4.987	2.5	10.16
12	2.117	6.650	3	8.467
10	2.54	7.980	4	6.35
8	3.175	9.975	5	5.08
6	4.233	13.30	6	4.233
5	5.08	15.96	8	3.175
4	6.35	19.95	10	2.54
3	8.467	26.60	12	2.117
2	12.7	39.90	—	—

The left three columns convert from DP, the right two from common modules; Wolfer's routine range is module 0.5–12, and DP gears are cut to drawing with matching tooling.

Working the module back out of an old gear

For a standard spur gear without profile shift the tip diameter is da = m·(z + 2), so m ≈ da ÷ (z + 2): count the teeth and measure the tip diameter with a caliper. A result near 2.117, 2.54, 3.175, 4.233 or 6.35 almost certainly means a DP gear (12, 10, 8, 6 or 4 DP); a result near a whole or half number means a module gear. Helical gears need the normal module and the helix angle, and profile-shifted gears drift away from the formula — when in doubt, send a sample and Wolfer measures the tooth form on the gear measuring center before fixing the parameters.

FAQ

Common questions on this topic

Can you make DP (diametral pitch) gears?

Yes. DP gears are cut with the matching DP hob or with tooling made to the converted module, to the tooth form on the drawing; the routine range is about DP 50–2 (module 0.5–12). State whether the drawing is DP or module, and give the pressure angle, when you ask for a quote.

Is module 2.5 the same as 10 DP?

No. 10 DP converts to module 2.54, 1.6 % away from 2.5, so pitch and tooth thickness differ and the two will not mesh. Likewise 8 DP (3.175) is not module 3 and 12 DP (2.117) is not module 2.

How do I measure the module of an existing gear?

For a standard spur gear, m ≈ tip diameter ÷ (teeth + 2); a result near 2.117, 2.54 or 3.175 indicates a DP gear. The formula is not reliable for helical or profile-shifted gears — send a sample and Wolfer measures it on the gear measuring center to fix module, pressure angle and profile shift.

Should I choose a 20° or 14.5° pressure angle?

New designs: 20°, which gives a thicker root, higher load capacity and undercut only below a smaller tooth count. Use 14.5° only when replacing a mate in an old American gear set; gears of different pressure angles cannot mesh.

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