How To Measure Bearings & Identify Them By Code.

Whether you're specifying a bearing for a new design or replacing a worn one, the process is the same: measure the bearing and housing carefully, confirm any bearing codes if they’re visible and verify the details before ordering. This applies whether you are working on a production line, a workshop project or a personal build.

This guide explains how to take bearing measurements accurately and how to decode bearing numbers and suffixes. It covers both metric and imperial conventions, the most common bearing types and the tools you’ll need to make the job straightforward.

Contents:

• What Measurements Define a Bearing?

• Required Tools.

• The Order Bearing Dimensions are Written In.

• How to Measure Bearings Step by Step.

• Metric vs Imperial Bearings

• How to Identify a Bearing by Its Number.

What Measurements Define a Bearing?

Every rolling-element bearing is defined by three principal dimensions;

• Inner Diameter (ID); Also sometimes called the bore. This is the diameter of the central hole that fits over a shaft.

• Outer Diameter (OD); The external diameter of the outer ring. This is the dimension that fits inside a housing or bore.

• Width (W); Also referred to as height or thickness, depending on the bearing type. It is the distance measured across the bearing from one face to the other.

Together, these three values uniquely identify most standard bearing sizes.
They are almost always written in the order ID × OD × W and quoted in millimetres for metric bearings or inches for imperial types.

Required Tools To Measure Bearings.

A basic tooling setup is all that most users need to measure bearings. The table below summarises the tools, which measurement aspect they suit best and when each one matters.

 

 

For most replacement tasks, a digital caliper is sufficient. It handles all three measurements and gives readings accurate enough to identify a standard bearing size.

If greater precision is required, for example, when assessing wear on a used bearing or verifying conformance to a specific tolerance class, a micrometer is the better choice. Accu stock both calipers and micrometers suited to this kind of work.

Having the correct tools on hand, with a solid understanding of how they work for the respective areas of a bearing, is essential for accurate measurement and will prevent you from mis-ordering or specifying the wrong bearing for the project.

For a broader overview of measurement tools and their applications, see our guide to metrology

The Order Bearing Dimensions Are Written In.

Before picking up a caliper, it is worth understanding how bearing dimensions are written. The standard convention is ID × OD × Width; this order is used almost universally across catalogues, datasheets and supplier listings. Knowing this upfront does two things: it gives your measurements a clear structure to follow, bore first, outer ring second, face-to-face width last and it means the numbers you record will already be in the format that manufacturers, suppliers and search tools expect.

This convention applies to both metric and imperial bearings, though imperial dimensions are written in inches or fractions of an inch, depending on the project and bearing measurement resolution.

When searching for a replacement or specifying a new part, always confirm that the source you are reading follows this order. The vast majority of manufacturers and suppliers do, but a small number of specialist catalogues list OD first. If in doubt, cross-reference with the bearing's printed code, which we will cover later in this article, or speak with our team to ensure you order the correct bearing for your project.

How to Measure Bearings Step by Step.

Before measuring, clean the bearing thoroughly. Residual grease, dust or corrosion can add fractions of a millimetre to a reading, enough to push you towards the wrong size. Inversely, wear can also lead to an undersized measurement when specifying a replacement bearing. Use a lint-free cloth and, if needed, a light solvent such as IPA to remove contamination. Allow the bearing to dry completely before taking measurements.

If the bearing still has a readable code printed on it, note that first. The code is typically the fastest route to identification, which we will cover in a later section. Measurement should then serve as confirmation rather than your only reference, helping ensure you select the correct component the first time.

Be sure to identify the unit system your bearing is specified in prior to measurement and ensure your caliper or micrometre utilises the same system. For digital tools, this is not a problem as they typically feature both Imperial and Metric units, but for analogue tools, this becomes more critical.

 

How to Measure the Inner Diameter (ID).

In this example of measuring the inner diameter of the bearing, we will be utilising a caliper. As a reminder, the inner diameter is the bore of the bearing that sits on a shaft

Firstly, zero the caliper before starting. Close the inside jaws fully and confirm the display/scale reads 0.00 mm. If it does not, recalibrate or use the zero-set function.

 

 

 

 

 

 

 

 

 

 

Position the inside jaws inside the bore and open them slowly, so they sit against the inner bearing ring. 

Keep the caliper perpendicular to the bore axis and ensure the jaws are centred across the full diameter, not offset to one side. An off-centre reading will measure a chord rather than the true diameter, giving a figure that is too small.

 

 

 

 

 

 

 

Finally, read the measurement, then rotate the bearing roughly 90° to measure again across a different working surface. If the bore is worn, readings may vary at different angular positions. A difference of more than 0.01 mm on a standard-tolerance bearing suggests wear or deformation. On wider bearings, repeat at the front and rear faces of the bore; a difference between these readings can indicate taper wear along the width.

 

Engineering Tips: On miniature bearings, the bore can be small enough that caliper jaws struggle to seat properly. In these cases, measuring the shaft the bearing fits on may give a more reliable figure. On sealed or shielded bearings, the bore remains fully accessible from either side, so seals do not affect this measurement.

 

 

 

 

How to Measure the Outer Diameter (OD).

Continue to use your caliper for the next step, where we will measure the OD of the bearing.
As a reminder, the outer diameter is the external measurement of the outer ring of the bearing.

Zero the caliper before starting, just as with the bore measurement. Close the outside jaws fully and confirm the display/scale reads 0.00 mm.

Place the bearing flat on a clean surface and close the outside jaws of the caliper around the outer ring, measuring across the widest point. Make sure you are using the larger, lower jaws; the smaller inside jaws are for internal measurements only.

 

 

 

 

 

 

Ensure the jaws are perpendicular to the bearing face and seated squarely against the material of the outer ring, not resting on any flanges, seals or any rubber lip that extends beyond the ring edge. Any axial-tilt will produce an artificially high reading and measuring over a seal can add several tenths of a millimetre.

Read the measurement, then rotate the bearing 90° and measure again. A difference between the two readings indicates ovality, which is common in bearings that have operated under heavy radial load or in housings with uneven clamping. As with the bore, a discrepancy greater than 0.01 mm on a standard-tolerance bearing points to wear.

Engineering Tips: If the bearing is still pressed into its housing, you may not be able to measure the OD directly. In that case, measuring the housing bore will give you the nominal OD, though you should account for any interference fit allowance when cross-referencing against standard sizes.

 

 

How to Measure Bearing Width.

Still using your caliper from the previous steps, next we will measure the width of the bearing. The width is the simplest of the three bearing measurements, but should still be approached with care.

Zero the caliper before starting. Close the outside jaws fully and confirm the display/scale reads 0.00 mm, following the same routine as the bore and OD measurements.

Stand the bearing on one face on a flat, clean surface, or hold it between finger and thumb. Close the outside jaws of the caliper across both faces of the bearing, ensuring the jaws sit flat against the full face on each side simultaneously.

Check the jaw position carefully. If the bearing has a snap ring groove, an extended inner ring or a seal lip that sits proud of the face, position the caliper on the main body faces of the outer ring. Measuring across a protruding seal or snap ring will add to the reading and lead you to the wrong measurement.

 

 

 

 

Read the measurement, then rotate the bearing and repeat at a different point around the circumference. Width should be consistent; any variation suggests uneven wear or damage. On wider bearings, also confirm the jaws are parallel and not canted across the faces, as even slight misalignment will overstate the true width.

Engineering Tips: Some bearing types, such as tapered roller bearings, have separable components. Always measure the assembled width (inner ring, rollers and outer ring together) unless you specifically need the individual component dimensions. For flanged bearings, measure the width of the main body only; the flange is not part of the catalogued width dimension.

How To Measure Bearings With a Flange.

Flanged bearings differ from their standard counterparts in both the method of measurement and the way the dimensions are recorded and shown. The flange itself is a protruding lip on the outer bearing ring that acts as a built-in locating feature, removing the need for a separate housing shoulder to position the bearing axially.

When measuring a flanged bearing, take the three standard dimensions (ID, OD and width) from the main body of the bearing as covered in the previous steps of this guide and ignore the flange entirely to begin with. For clarity, we have broken these steps down again here and included where the flange factors in and why it should be ignored for initial measurements.

 

 

Firstly, measure your bearings' inner diameter following the same bore measurement process as a standard bearing covered earlier in this article. The flange does not affect the bore, so it can be ignored for this step.

Then measure the outer diameter across the outer bearing ring and behind the flange as pictured, not across the flange itself. The flange diameter is always larger than the body OD, so measuring across it will give you the wrong figure for catalogue matching.

 

 

 

 

 

 

 

 

 

 

Measure the bearing width across the main body faces, excluding any flange thickness that extends beyond the flat area of the flange, such as lips or notches. These three figures are what you will use to search bearing catalogues and match the part to a standard designation as with a normal bearing.

 

 

 

 

 

 

 

 

 

Finally, measure the flange separately. Use the outside jaws of your calliper to take the flange outer diameter across its widest point, then measure the flange thickness from its face to where it meets the main body of the outer bearing ring. These dimensions are not part of the standard product code and can vary between manufacturers, so always confirm them against the supplier's datasheet or product page before ordering.

Engineering Tips: The flange has its own dimensions, typically considering its outer diameter and its thickness, but these are not encoded in the standard bearing product code. A designation like F6202-2Z tells you the bearing type, series, bore and shielding, but nothing about the flange geometry.

The flange dimensions sit in the product-specific technical data instead: the datasheet, product page or engineering drawing for that individual part.

This is standard practice across bearing suppliers, not specific to any one manufacturer or catalogue. For clarity on this and to ensure you get the right part, feel free to contact Accu’s customer service team.

Metric vs Imperial Bearings

All Bearings use one of two measurement systems: metric (millimetres) or imperial (inches). Which you encounter depends on where the equipment was manufactured and when. European, Japanese and most modern international machinery utilise the metric system. Older American-made equipment, legacy agricultural machinery and mid-century automotive applications are more likely to use imperial.

The physical measuring process is identical regardless of the unit system. What changes is the reference system you use to match the measurement to a part number. The only thing you need to be sure of when taking your measurements is that the tool you are using has the same measurement system as the bearing.

How to Tell Whether a Bearing Is Metric or Imperial.

Engineering Tips: 
If your measurement produces a value like 25.40 mm (exactly 1 inch) or 19.05 mm (3/4 inch), you are almost certainly dealing with an imperial bearing. If the bore reads 25 mm or 20 mm with no fractional tail, it is metric.

Where ambiguity remains, measure all three dimensions and compare against both. Most suppliers, including Accu, list bearings by their metric dimensions, so converting your measurement to millimetres and searching by ID × OD × W is usually the most efficient approach.

How to Identify a Bearing by Its Number

If a bearing has a legible code printed or engraved on its outer ring or shield, that code is the single most reliable way to identify it. The basic designation (the number itself) is internationally standardised and encodes the bearing type, dimensional series and bore size. It is consistent across manufacturers, meaning a 6202 from SKF, NSK, FAG or NTN describes the same bearing geometry.

Bearing code suffixes, which appear after the basic number, describe additional features such as sealing, shielding, clearance and cage type. Unlike the basic designation, suffixes are not fully standardised. Different manufacturers use different codes to describe the same feature. This is not an error or inconsistency; it is simply how the industry has developed. Understanding what suffixes describe matters more than memorising every manufacturer's specific code.

How Bearing Codes Work.

A typical bearing code reads left to right and breaks into four segments. Not every segment is always present; the type prefix, for example, is omitted on standard deep groove ball bearings because they are the default. The table below explains each segment, what it tells you and how to interpret it.

 

 

Engineering Tips: if you see the code 6202-ZZ on a bearing, reading left to right gives you no type prefix, which denotes it as a deep groove ball bearing, dimension series 62 (light series), bore code 02 (a fixed designation corresponding to a 15 mm bore) and the suffix ZZ (two metal shields). From six characters and a suffix, you already know the bearing type, its dimensional family and its bore diameter before reaching for a caliper.

Bore Code Conventions for Metric Bearings.

The bore code is the segment most people need to decode quickly and is the aspect of identifying bearings which is least immediately obvious.
The previous table explained where the bore code sits within a bearing designation; this table shows how to convert that code into an actual bore diameter in millimetres. Keep it to hand when cross-referencing a printed code against your caliper or micrometer readings.

The bore code conversion depends on the size range being considered and follows three distinct rules.

Rule 1) For miniature bearings with a bore below 10 mm, the bore diameter is written directly as the last digit of the full bearing designation. The preceding digits identify the type and series. For example, 608 is a series 60 bearing with an 8 mm bore; 623 is a series 62 bearing with a 3 mm bore. There is no calculation involved; the final digit is the bore in millimetres.

Rule 2) For bore codes 00 to 03, the bore sizes are fixed designations that do not follow any mathematical rule. These four codes map to 10, 12, 15 and 17 mm, respectively and are worth committing to memory.

Rule 3) For bore codes 04 and above, the rule is straightforward: multiply the bore code by 5 to get the bore diameter in millimetres.

Keep these three rules in mind as we proceed through our worked example in the next section and refer to the table below where required.

Engineering Tips: For bores above 480 mm, the diameter is often written directly after a slash (e.g. /500 = 500 mm bore). These larger sizes are uncommon in most precision engineering applications, but the convention is worth knowing if you encounter one.

Metric Worked Example: 6202-2Z

To see how all of this fits together, take a bearing stamped 6202-2Z and read the code left to right using the rules covered above.

The first digit after the prefix, 6, is the type indicator. A leading 6 identifies this as a deep groove ball bearing. The second digit, 2, is the diameter series, telling you this is a light-series bearing, meaning its outer diameter and width are moderate relative to the bore size. Together, 62 gives you the dimensional series.

The final two digits of the basic number, 02, are the bore code. This is one of the four fixed designations covered in the bore code table above, so the multiplication rule does not apply here. A bore code of 02 corresponds to a 15 mm bore diameter.

The suffix 2Z tells you the bearing is fitted with two non-contact metal shields, one on each side. You may also see this written as ZZ as shown in our example here; both notations mean the same thing. If additional suffixes were present (C3, for example), they would follow in sequence.

From the code alone, you now know this is a 15 mm bore, light-series deep groove ball bearing with double metal shields. Cross-referencing against a bearing catalogue confirms the full dimensions: 15 × 35 × 11 mm (ID × OD × Width).

This is worth practising on a few bearings. Once the pattern is familiar, you can decode most standard designations in seconds and confirm whether the code matches your caliper readings before ordering. Where a suffix is unfamiliar or does not appear in the table later in this guide, consult the manufacturer's catalogue or contact the supplier directly; some suffixes are proprietary to individual manufacturers.

Imperial Bearing Codes.

The metric ISO system covered above is the most widely used bearing designation standard, but it is not the only one. Imperial bearings, dimensioned in inches, follow their own code conventions. If you are working on older American-made equipment, legacy agricultural machinery or certain automotive applications, you are likely to encounter these. In the sections below, we will cover Imperial Bearing specific codes and dimension series.

Imperial R-Series Ball Bearings.

The most common imperial ball bearing designation uses an R prefix followed by a number that represents the bore diameter in sixteenths of an inch. The rule is straightforward: divide the number after R by 16 to get the bore in inches. 

Unlike the metric system, with Imperial bearings there are no fixed designations or special rules to memorise.
The conversion is the same at every size: the number after R divided by 16 gives the bore in inches.

Suffixes on R-series bearings work the same way as on metric bearings. An R8-2RS is a 1/2 inch bore imperial ball bearing with two rubber contact seals. An R6-ZZ is a 3/8 inch bore bearing with two metal shields. The suffix tables and cross-reference later in this section apply equally to imperial and metric designations.

Imperial Worked Example: R8-ZZ

Now lets see how the imperial system works in practice, take a bearing stamped R8-ZZ and read the code left to right using the rules covered above.

The prefix R identifies this as an imperial ball bearing. This immediately tells you the bore will be expressed in sixteenths of an inch. Where a metric bearing starts with a type digit (6 for deep groove), an imperial ball bearing starts with R.

The number 8 is the bore code. Applying the sixteenths rule, 8/16 = 1/2 inch, which converts to 12.700 mm. Unlike the metric system, there are no fixed designations to remember here; the same division-by-16 rule applies at every size, making it simple to follow.

After the hyphen, the suffix ZZ tells you the bearing has two non-contact metal shields, one on each side. This works exactly as it does on a metric bearing; the suffix system is shared across both measurement systems. If additional suffixes were present (C3, for example), they would follow in sequence, just as they would on a metric designation.

From the code alone, you now know this is a 1/2 inch bore imperial ball bearing with double metal shields. Cross-referencing against an imperial bearing catalogue confirms the full dimensions: 1/2 × 1 1/8 × 5/16 inches (ID × OD × Width). Most suppliers list imperial bearings with both inch and millimetre dimensions, so you can cross-reference using whichever unit system your caliper is set to.

Common Bearing Suffixes and What They Mean.

Suffixes appear after the basic bearing number and describe how the bearing is built, not just its size. Two bearings with the same base number but different suffixes can behave very differently in service, so understanding what each suffix category describes is essential when specifying or replacing a bearing.

There is an important distinction to make here. The basic designation of a bearing code is internationally standardised; a 6202 is a 6202 regardless of who made it.
Suffixes, however, are not fully standardised in the same way. Some codes are universally recognised across all manufacturers (Z, ZZ, C3). Others vary significantly; what SKF calls 2RS1, NSK calls DDU, FAG calls 2RSR and NTN calls LLU. All four describe the same feature: two rubber contact seals.

The tables below focus on what each suffix category means functionally, with the most commonly encountered codes and universal codes supplied. A cross-reference table at the end of this section maps the most common features across the major manufacturers.

Sealing and Shielding.

Internal Clearance.

Internal clearance codes are one of the few genuinely universal suffix categories. C0, C2, C3 and C4 mean the same thing regardless of manufacturer or other bearing specifications.

Bearing Cage Type Codes.

Cage suffixes vary between manufacturers. The table below shows the most commonly encountered generic codes; the cross-reference table at the end of this section maps these across major brands.

 

Bearing Retention Feature Codes.

Engineering Tips: Multiple suffixes can appear on the same bearing and they are read independently. For example, 6202-2Z C3 denotes a double-shielded deep groove ball bearing with C3 (greater than normal) internal clearance. The sealing suffix tells you how the bearing is protected; the clearance suffix tells you how it is fitted.

Bearing Suffix Cross-Reference by Manufacturer.

As outlined at the start of this section, the basic bearing code designation is universal, but seal, shield and cage suffixes are not. The table below maps the most commonly encountered features across major bearing manufacturers. If you are replacing a bearing from one brand with another, this table will help you identify the equivalent suffix.

This table covers the most common features for deep groove ball bearings. Other bearing types (cylindrical roller, angular contact, spherical roller) have additional manufacturer-specific suffixes that go beyond the scope of this guide. When in doubt, consult the manufacturer's catalogue directly or contact the supplier.

If a suffix does not appear in any of the tables above, it is likely proprietary to a specific manufacturer or relates to a specialist feature such as heat stabilisation, precision class or a non-standard grease fill. The manufacturer's technical documentation will give the definitive meaning.

Common Bearing Types and Where They Are Used.

Not every bearing looks or behaves the same way and knowing which type you are dealing with matters when it comes to measurement. A deep groove ball bearing and a tapered roller bearing may share similar outer dimensions, but they are not interchangeable; the rolling elements, raceway geometry and load capabilities are fundamentally different.

Understanding the basic categories helps you identify what you are measuring, search for the right replacement or specification and avoid ordering a bearing that fits dimensionally but fails functionally.

 

For most precision assemblies and mechanical designs, deep groove ball bearings and thin-section bearings cover the majority of requirements. The measurement process and code structure covered in this guide apply to all of the types listed above, regardless of rolling element or raceway geometry.

Bearing Measurement Mistakes to Avoid.

Even straightforward measurements can go wrong. With bearings, a small error can lead to ordering the wrong part entirely. Most of these mistakes are easy to make and just as easy to prevent once you know what to watch for.
The table below covers the most common errors, explains why they happen and gives you a practical way to avoid each one.

 

 

Wrapping Up.

Identifying and measuring a bearing is a practical skill, not a complex one. With a clean bearing, a caliper and an understanding of the ID × OD × W convention, most standard bearings can be identified in minutes.

Where a printed code is available, use it. It is faster and more reliable than measurement alone. Where it is not, careful measurement and a reference table will get you to the right answer.

Accu’s bearing range covers deep groove ball bearings, thin-section ball bearings, slide bearings and plate bearings, all with full dimensional specifications and technical data. If you need calipers or micrometers to carry out the measurement, those are available too.

Further Reading:

• Coloured Electrical Tape: An Engineer’s Guide - Discover The In-and-outs of coloured electrical tape with Accu.
• How to Use Vernier Calipers Like An Engineer - Discover how to get the most our of your metrology equipment with Accu.
• Threaded Inserts and Fasteners for Plastics and 3D Printing - Disover threaded fastener solutions for 3D printing with Accu.

FAQs

Q: How do I measure a bearing size?

A: Measure the inner diameter (bore), outer diameter and width using a caliper. Clean the bearing first and measure the metal rings directly, not over any seals or shields. Record all three values in the order ID × OD × W, then match them to a standard bearing dimension table or catalogue.

Q: What do the numbers on a bearing mean?

A: The printed number encodes the bearing type, dimensional series and bore size. The last two digits of the base number typically represent the bore code: for codes 04 and above, multiply by 5 to get the bore diameter in millimetres. Codes 00 to 03 map to fixed bore sizes (10, 12, 15 and 17 mm respectively). Suffixes after the base number describe features such as seals, shields and clearance class.

Q: What does 2RS mean on a bearing?

A: 2RS means the bearing has two rubber contact seals, one on each side. This provides good protection against dust and moisture ingress and retains the grease inside the bearing. The trade-off is slightly higher friction compared to a shielded (ZZ) variant.

Q: What does ZZ mean on a bearing?

A: ZZ (sometimes written as 2Z) means the bearing has two metal shields, one on each side. Shields are non-contact, meaning they do not rub against the inner ring. They keep out larger contaminants while generating less friction than rubber seals.

Q: What does C3 mean on a bearing?

A: C3 indicates greater than normal internal radial clearance. This is commonly specified for applications involving higher operating temperatures or interference fits, where thermal expansion or press-fit compression would otherwise reduce the internal clearance to an unacceptable level.

Q: How do I know if a bearing is metric or imperial?

A: Measure the bore. If it reads a clean whole number in millimetres (e.g. 10, 17, 25, 30 mm), it is almost certainly metric. If it converts neatly to an inch fraction (e.g. 6.35 mm = 1/4″, 12.7 mm = 1/2″, 25.4 mm = 1″), it is likely imperial. Check OD and width for the same pattern to confirm.

Q: What tool do I need to measure a bearing?

A: A digital caliper is sufficient for most bearing identification tasks. For tighter tolerances or wear assessment, an outside micrometer (for OD) or inside micrometer (for ID) improves precision. See Accu’s guides on how to use vernier calipers and how to use a micrometer for detailed usage instructions.

Q: What if the markings on my bearing are worn off?

A: Measure all three dimensions (ID, OD and width) carefully and search bearing catalogues or supplier websites by those dimensions. Most standard bearings can be identified from their measurements alone. If the dimensions fall between standard sizes, consider whether the bearing may be imperial or worn beyond nominal.