Working with both inch and metric gears can lead to mistakes if unit assumptions go unchecked. This post clears up confusion between DP (diametral pitch) and module, explains their unit systems, and helps you avoid errors when switching or comparing gears.
DP is almost always specified in inches, and module in millimeters.
These systems are not directly interchangeable — converting between them without adjusting other specs can introduce backlash, misfit, or ordering issues.
Read on to see how they relate to circular pitch, when conversions are safe, and how to avoid CAD or supplier mix-ups.
Table of Contents
What’s the difference between DP, module, and circular pitch?
Diametral pitch (DP), module, and circular pitch all describe gear tooth spacing—but they’re not interchangeable.
DP belongs to the imperial system and is defined as the number of teeth per inch of pitch diameter. Module is the metric equivalent, calculated by dividing pitch diameter in millimeters by the number of teeth. Circular pitch refers to the physical arc length between teeth and can be expressed in either unit system, but is rarely used in sourcing.
To convert between them, use:
DP = 25.4 / module
Circular Pitch = π / DP = π × module
For example, a 2.0 module gear equates to ~12.7 DP. But despite this mathematical link, DP and module live in different ecosystems—with DP common in U.S. legacy prints and local catalogs, and module standard in global ISO-based design.
Using circular pitch for inspection or conversion makes sense, but specifying gears this way is uncommon. Vendors typically quote in DP or module—not CP—so misunderstanding which unit system a catalog or drawing uses can easily cause quoting errors or mismatched parts.
Design Takeaway: Always match your gear specs to the dominant unit system used in your CAD model, vendor list, or regional standard. Don’t treat DP and module as cosmetic differences—they impact quoting, meshing, and downstream fit. Circular pitch can bridge the two, but it’s not a replacement for clear spec alignment.
Is diametral pitch (DP) always measured in inches?
Yes — diametral pitch (DP) is strictly defined in inches.
By definition, DP is the number of gear teeth per inch of pitch diameter. For example, a 16 DP gear has 16 teeth for every 1 inch of pitch circle diameter. This system is standardized under AGMA 910 and used exclusively in inch-based mechanical design. There is no metric variant of DP — mixing units leads to invalid specs.
DP is most often found on legacy drawings, U.S. gear catalogs, or assemblies with inch-based fasteners. Errors commonly occur when a DP gear is modeled in a metric CAD template or quoted by a vendor working in module — leading to tooth count mismatches or backlash.
While it’s possible to convert DP to module using circular pitch, doing so should only happen when transitioning an entire design ecosystem, not a single spec.
Design Takeaway:
Use DP only when your CAD, BOM, and vendor stack are entirely inch-based. If you’re collaborating internationally or modernizing toward ISO specs, convert DP to module early to avoid unit conflicts and downstream fitment issues.
Is module always defined in millimeters?
Yes — module is a metric-only gear specification defined in millimeters.
It represents the ratio of pitch diameter (mm) to the number of teeth. For example, a module 2.0 gear with 10 teeth has a 20 mm pitch diameter. This definition is governed by ISO 54:1996 and ISO 701, which standardize gear tooth proportions in metric systems globally.
Module is the default for most modern CAD environments (e.g., SolidWorks in mm mode) and supplier catalogs across Europe and Asia. It is not interchangeable with DP, just as you wouldn’t describe an M6 fastener using inch threads.
While circular pitch can be used to bridge systems mathematically, vendors rarely list specs this way — and relying on it introduces sourcing risks unless every part in the drive system has been validated. Mixing module gears into inch-based assemblies often leads to alignment errors or inconsistent fits — even when diameters appear similar on paper.
Design Takeaway:
Use module only when your design, modeling, and supplier base are fully metric. Avoid partial conversions or mixing specs across BOMs. Module-based gears are not compatible with DP components unless circular pitch is verified — which is rarely practical during quoting or production.
How does circular pitch relate to module and DP?
Circular pitch (CP) is the physical spacing between gear teeth — and the common ground between DP and module.
It’s defined as the arc length between adjacent teeth along the pitch circle. CP is what gives gears their actual meshing distance, regardless of whether you’re using inch or metric specs. It can be calculated in both systems:
- CP = π / DP (for inch-based designs)
- CP = π × module (for metric-based designs)
This means circular pitch is the geometric bridge between DP and module, even though they belong to different unit systems. If two gears — one DP-based and one module-based — have the same circular pitch and pressure angle, they may mesh. But that’s a big “if.”
You’ll often see circular pitch listed in CAD software exports, gear inspection reports, or standards tables — but almost never in vendor catalogs or BOMs. That’s because CP is not a specification system — it’s a derived value. Specifying gears by CP alone usually leads to confusion during quoting or design reviews.
Design Takeaway:
Use circular pitch as a conversion or verification tool — not a primary design spec. It’s useful when comparing imported drawings or checking mating parts, but avoid using it as a substitute for DP or module in production specs, CAD models, or supplier documentation.
Can I convert between DP and module directly?
Yes — DP and module are mathematically linked by a simple formula:
DP = 25.4 / module
Module = 25.4 / DP
That’s because 25.4 mm = 1 inch. A module 2.0 gear converts to 12.7 DP. This allows you to convert values for comparison, especially when reviewing mixed BOMs or supplier specs. But conversion alone doesn’t guarantee compatibility.
Each system assumes its own pressure angle conventions, tooth profile standards, and gear family availability. For instance, many DP gears use 14.5° or 20°, while module gears often assume 20° by default. Converting specs without confirming these details can result in mismatched teeth, excess backlash, or even unmachinable features.
We’ve seen real-world cases where a designer converted a 2.0 module gear to 12.7 DP and ordered it from a U.S. vendor. The gear arrived — but with a mismatched pressure angle. It meshed, but ran loud and wore quickly. The conversion was correct, but the assumption wasn’t.
Also note: while some converted values fall nicely into standard DP or module sizes, others may not be stocked — making sourcing inefficient or impossible.
Design Takeaway:
Use conversion formulas for comparison — not for spec substitution. If you must cross systems, confirm not just pitch, but pressure angle, gear availability, and mating geometry. Conversion is just step one — validation is what prevents failure.
Will converting between systems affect accuracy or backlash?
Yes — converting between DP and module without full geometric validation can absolutely affect backlash, center distance, and meshing accuracy.
Even if the circular pitch matches, pressure angle, tooth shape, or rounding tolerances may differ — especially between inch-based legacy prints and modern ISO gear libraries.
For example, a module 2.0 gear converts to roughly 12.7 DP. That suggests compatibility — but if one gear has a 14.5° pressure angle and the other 20°, the tooth flanks won’t align. This leads to measurable backlash, reduced torque transfer, increased wear, or gear noise under load.
In multi-stage assemblies, the impact is worse. Small mismatches at each stage create tolerance stacking, disrupting center distances, alignment, or preload. This is especially risky in applications requiring quiet operation, sealed housings, or precise mechanical timing.
Even if you’re only converting for a one-to-one mesh, assumptions can be expensive. Misalignment may not be obvious until final assembly — and by then, rework or reordering delays are inevitable.
Design Takeaway:
DP ↔ module conversions should never happen in isolation. Always validate pressure angle, circular pitch, and total gear geometry — including tolerances and material choices — before releasing to sourcing. Matching the math doesn’t mean matching the function.
When should I choose module gears for my design?
Module gears are the right choice when your entire design environment operates in metric units.
Module is standardized under ISO 54 and ISO 701, and widely used in Europe, Asia, and most modern CAD systems. If your fasteners, shafts, and housing specs are all in millimeters, module aligns naturally with your workflow.
Common module sizes—like 1.0, 1.5, 2.0, 3.0—are easy to source globally, and larger module values offer stronger, more robust teeth. That makes them ideal for high-load systems, heavy-duty drive assemblies, and industrial applications where durability is prioritized over compactness.
Global sourcing teams, metric-based QA systems, and CNC manufacturers working under ISO constraints also expect module-based specifications. Most modern CAD libraries assume module by default, and sticking with it keeps design intent consistent across tools and teams.
⚠️ Switching to module mid-project after inch-based tooling is defined often leads to mismatched shaft spacing, tooth interference, or failed assemblies.
Design Takeaway:
Use module gears when designing new systems in metric CAD, or when your supplier base includes international vendors. Starting with module from day one ensures compatibility, reduces revision rounds, and eliminates conversion-based fitment risks.
When does DP make more sense than module?
Diametral pitch (DP) gears are the better fit when your entire system is inch-based or grounded in legacy U.S. engineering standards.
DP measures the number of teeth per inch of pitch diameter and is codified under AGMA 910. It’s prevalent in U.S. machine shops, aerospace and defense tooling, and service manuals for legacy systems.
If your BOM includes inch threads, shafts sized in fractions, or housing layouts based on imperial dimensions, DP provides natural alignment. You’ll also find gear vendors in the U.S. more likely to stock 16, 20, 24, and 32 DP parts—with matched ratios and standard bore sizes readily available.
DP also avoids the need for retrofitting or CAD rework when updating older assemblies. If the original machine was built around inch units, sticking with DP preserves center distances, avoids interference, and simplifies maintenance sourcing.
⚠️ Redrawing a DP-based design in module without adjusting pitch diameter or spacing can lead to center misalignment and gear mesh failures—even if conversion math looks correct.
Design Takeaway:
Choose DP when you’re maintaining, retrofitting, or extending U.S.-based systems. It ensures legacy compatibility, avoids CAD errors, and keeps inch-unit tolerance chains intact through CNC manufacturing and inspection.
Should I standardize gear specs across my design?
Yes — standardizing all gears to either DP or module within a single design is critical for alignment, sourcing clarity, and downstream manufacturability.
Mixing both systems introduces risk at every stage: from CAD modeling and BOM generation to vendor communication and tolerance stacking. What looks interchangeable numerically may not perform the same mechanically.
For example, module 5 converts to roughly 5.08 DP. A tiny mismatch—but in multi-stage gear trains, that difference alters tooth alignment, center distance, and contact ratio. The result: increased backlash, premature wear, or rejection at QC.
Vendors may also misread drawings if units aren’t clearly separated. We’ve seen cases where one spec line used module and another used DP—leading to mismatched teeth that couldn’t mesh during assembly.
⚠️ Even minor system-mixing often introduces invisible errors that only show up during testing or inspection.
Design Takeaway:
Pick one system—DP or module—and apply it consistently across CAD, BOMs, and supplier files. Standardizing ensures smoother CNC quoting, eliminates ambiguity, and helps maintain mechanical integrity throughout gear-driven assemblies.
Can mixing gear specs cause confusion in CAD or drawing files?
Yes — mixing module and DP units within a CAD model or drawing set is a common source of gear failures, quoting errors, and machining delays.
Most CAD software allows you to model either system, but unit mismatches often go unnoticed until it’s too late. For example, one designer models a gear in module, while a teammate adds a mating gear in DP — assuming the numbers are “close enough.” They rarely are.
Center distances, pitch diameters, and tooth counts may appear valid in CAD but fail during inspection or assembly. Even worse, many exported step files and DXFs don’t include unit metadata. We’ve received drawings where the pitch circle looked right — but the units were off, and the gears didn’t mesh.
These problems compound in quoting. CNC shops may misread your intent or apply the wrong standard. That’s why we routinely check gear specs for hidden DP/module mismatches, especially in assemblies sourced internationally.
💡 Tip: Use filename suffixes like “-mod2.0” or drawing title blocks to flag pitch systems. That small habit avoids confusion during collaboration.
Design Takeaway:
Avoid mixing pitch systems in your design files. Stick to one unit type and label it clearly across drawings, filenames, and BOMs to prevent translation errors and scrap downstream.
Do gear vendors sometimes list specs in both systems incorrectly?
Yes — gear suppliers often list module and DP interchangeably, and the conversions aren’t always accurate or complete.
This happens in dual-language catalogs, e-commerce listings, or spec sheets where imperial and metric data are shown side-by-side. Sometimes, pressure angles are assumed equal when they’re not. Or face widths are listed in inches, while pitch specs use module — creating inconsistencies during quoting or inspection.
We’ve seen listings where a gear marked “module 2.0” was actually built to 12.7 DP, with mismatched pressure angles and an imperial bore. On paper, the gear fit. In practice, it generated uneven wear and required rework.
Some vendors round off values or omit tooth counts entirely. That’s why we always cross-check circular pitch, tooth count, and pressure angle when validating supplier data — especially for gears sourced from international distributors.
💡 Tip: Don’t rely solely on catalog headers. Always confirm tooth count and center distance in CAD to verify what the listing actually means.
Design Takeaway:
If a gear is listed in both module and DP, don’t assume it’s interchangeable. Verify pressure angle, bore specs, and circular pitch before placing an order — or work with a CNC partner that reviews critical geometry before production.
Conclusion
Understanding the differences between DP, module, and circular pitch helps you avoid costly gear mismatches. Whether you’re designing new assemblies or converting legacy specs, we help validate geometry before production. Contact us to explore manufacturing solutions tailored to your gear-driven product requirements.
Frequently Asked Questions
Not reliably. Without knowing the pitch system (DP or module) and pressure angle, center distance and tooth count alone aren’t enough to define gear geometry. You may end up with mismatched tooth profiles or incorrect mesh ratios.
Use the pitch diameter and tooth count:
- If pitch diameter ÷ tooth count = a clean mm value, it’s likely module.
- If the result fits cleanly in inches, it may be DP.
You can also compute circular pitch and compare against π / DP or π × module to verify.
Only if your model includes clearly defined specs: pressure angle, pitch system, and tolerances. Relying on a 3D file alone is risky—especially if it was converted from another unit system. Always provide a spec sheet or drawing alongside the model.
You can—but it introduces risk. Changing pitch systems mid-design affects shaft spacing, gear size, and mesh compatibility. It often requires a full redesign of the mating components. Choose early and lock it in before prototyping.
Yes. Gears must match racks or worm gears not just in pitch, but also pressure angle and handedness (for worms). Mixing pitch systems across components usually results in incorrect motion or excessive wear.
That’s where custom machining helps. If your pitch choice isn’t available in standard catalogs, we can produce gears based on your specs—while helping you assess if minor adjustments (like switching to a nearby standard size) can reduce cost without compromising function.
