Although a bearing designation may provide important information about its series and bore size, the complete dimensional requirements of a bearing are normally defined by several measurements. The most important are the bore diameter (d), outside diameter (D), bearing width (B or T), and corner or chamfer dimensions (r).

These dimensions determine whether a bearing can fit correctly onto a shaft, inside a housing, and within the available axial installation space.

For standardized rolling bearings, dimensional relationships are not arbitrary. International dimensional standards and established bearing series allow bearings with equivalent boundary dimensions to be specified consistently across manufacturers and applications.

However, bearing size charts should not be treated simply as lists of numbers. Two bearings with the same bore diameter can have substantially different outside diameters, widths, load capacities, internal designs, and intended applications.

This guide explains how to read bearing size charts and dimension tables, understand standard bearing dimensions, interpret bearing series, and use dimensional data correctly during bearing identification and selection.

These are commonly called the boundary dimensions of the bearing.

For example, a deep groove ball bearing specified as:

d = 25 mm
D = 52 mm
B = 15 mm

requires approximately a 25 mm shaft seat, a housing bore corresponding to its 52 mm outside diameter, and sufficient axial space for its 15 mm width.

The actual shaft and housing dimensions cannot simply be manufactured to exactly these nominal numbers. Appropriate fits and tolerances must also be specified according to load conditions, operating requirements, accuracy, and mounting arrangement.

This distinction is important:

Bearing dimensions define the nominal envelope of the bearing; fits and tolerances define how that bearing interfaces with the shaft and housing.

A properly selected bearing can:

• Reduce friction between moving components
• Support radial and/or axial loads
• Maintain shaft alignment
• Control rotational accuracy
• Reduce wear
• Allow higher operating speeds
• Improve machine efficiency
• Help maintain predictable service life

Consider an electric motor. The bearings support the rotor shaft while maintaining the small and relatively precise air gap between the rotor and stator. Excessive bearing clearance, wear, or misalignment can therefore affect more than bearing life—it can influence the performance of the entire motor.

The same principle applies to pumps, gearboxes, conveyors, machine tools, fans, agricultural equipment, and industrial machinery.

Read the Complete Guide → How Bearings Work

It determines the basic radial envelope required inside the bearing housing.

Importantly:

Bearings with the same bore diameter do not necessarily have the same outside diameter.

Consider several common deep groove ball bearing series with a 25 mm bore:

All three fit nominally onto a 25 mm shaft, but their radial and axial envelopes differ considerably.

This illustrates why selecting a bearing solely by bore diameter is insufficient.

Different diameter series allow engineers to balance factors such as:

• available installation space
• radial load capacity
• stiffness
• bearing mass
• operating speed
• housing dimensions
• and application requirements

A compact machine may favor a lighter dimensional series, while an application requiring greater load capacity may require a larger cross-section.

For many radial ball and roller bearings, the symbol B represents bearing width.

For tapered roller bearings, dimensional tables often use several different symbols because the bearing consists of separable inner-ring and outer-ring components.

Typical tapered roller bearing dimensions can include:

d — bore diameter
D — outside diameter
T — total bearing width
B — cone width
C — cup width

Therefore, engineers should never assume that every dimension table uses B in exactly the same way.

The bearing type and the table’s dimensional definitions must always be checked.

This becomes particularly important when replacing tapered roller bearings or designing axial shoulders and spacers.

These normally include:

Bore diameter (d)
Outside diameter (D)
Width or overall width (B/T)
Corner dimensions (r)

Standardized boundary dimensions provide a major engineering advantage: interchangeability.

A machine designer does not need to create completely different shaft and housing geometry every time a bearing manufacturer changes.

Where bearings conform to the same applicable dimensional standard and specification, equivalent bearing types may share standardized boundary dimensions.

However, same dimensions do not automatically mean identical performance.

Bearings sharing d × D × B dimensions may still differ in:

• internal geometry,
• cage design,
• material,
• internal clearance,
• accuracy class,
• seal configuration,
• lubrication,
• load rating,
• speed capability,
• and service life.

Dimension tables should therefore be considered an identification and dimensional-selection tool, not a complete engineering selection method.

Standardized bearing systems organize many bearing dimensions according to combinations of:

Diameter Series + Width Series

These relationships allow different bearing cross-sections to be created around the same nominal bore.

Conceptually, the progression can be understood as:

Smaller cross-section → Medium cross-section → Larger cross-section

As outside diameter and width increase, the bearing generally has more internal space available for larger rolling elements and more robust internal geometry.

This often permits increased load capacity, although actual ratings must always be taken from the manufacturer’s engineering data.

This is why:

6005 ≠ 6205 ≠ 6305

even though all three have a nominal 25 mm bore.

The series designation helps identify the dimensional family to which the bearing belongs.

A simplified example of commonly encountered 62-series dimensions is shown below:

These tables are particularly useful when:

• identifying an existing bearing,
• checking available installation space,
• comparing adjacent sizes,
• reviewing shaft diameters,
• or narrowing down candidate bearing series.

But the table should not be used alone to determine whether a bearing is suitable for a particular load or speed.

For a nominal 20 mm bore:

The bore remains constant while the outside diameter and width increase.

This creates different cross-sectional envelopes.

60 Series

Generally provides a relatively compact radial envelope where installation space is limited.

62 Series

Offers a widely used balance between dimensions, load capacity, speed capability, and general industrial availability.

63 Series

Provides a larger cross-section and is commonly considered when greater load capacity or robustness is required.

The correct choice cannot be made from dimensions alone. Load, speed, lubrication, operating temperature, desired service life, and surrounding component design must also be evaluated.

Different bearing families require different dimensional parameters.

Deep Groove Ball Bearings

Usually:

d × D × B

Angular Contact Ball Bearings

Usually:

d × D × B

but contact angle, arrangement and preload can be equally important.

Cylindrical Roller Bearings

Typically:

d × D × B

plus internal design and flange configuration.

Spherical Roller Bearings

Typically:

d × D × B

with additional attention to internal clearance and tapered/cylindrical bore configuration.

Tapered Roller Bearings

Often require:

d × D × T

together with B and C dimensions for the cone and cup.

Thrust Bearings

Dimensional notation can differ substantially because the bearing primarily supports axial loading.

This is why a universal “bearing size chart” must always identify the bearing type before presenting dimensions.

Metric bearings normally specify dimensions in:

millimeters (mm)

while inch-series bearings may specify dimensions in:

inches

Tapered roller bearings are one area where both systems are commonly encountered.

When replacing an existing bearing, engineers should not convert approximate inch measurements into a nearby metric bearing and assume equivalence.

For example, two bearings may appear dimensionally close while differing enough to create improper fits or assembly problems.

Always verify:

Bore × Outside Diameter × Width

using the original dimensional specification.

A basic identification procedure is:

Step 1 — Measure the bore diameter (d)
Measure the inner-ring bore.

Step 2 — Measure the outside diameter (D)
Measure across the largest outside diameter of the outer ring.

Step 3 — Measure the width (B/T)
Measure the total axial width.

Step 4 — Identify bearing type
Determine whether the bearing is a ball bearing, tapered roller bearing, cylindrical roller bearing, needle bearing, thrust bearing, etc.

Step 5 — Compare the dimensions with the appropriate dimension table.

For general workshop identification, a vernier caliper may provide sufficient information to narrow the possibilities.

For precision inspection and engineering verification, suitable calibrated measuring equipment and proper measurement procedures should be used.

1. Start with the shaft diameter

Determine the required nominal bore d.

2. Identify suitable bearing types

Determine whether the application requires a deep groove ball bearing, angular contact bearing, roller bearing, thrust bearing, or another design.

3. Compare dimension series

Compare available D and B/T dimensions for the same bore.

4. Check available housing space

Confirm that the outside diameter can be accommodated.

5. Check axial installation space

Confirm the bearing width and shoulder arrangement.

6. Evaluate performance requirements

Check load ratings, speed, service life, accuracy and operating conditions.

7. Determine fits and clearance

Select appropriate shaft/housing fits and bearing internal clearance.

8. Verify the complete bearing designation

Finally confirm seals, shields, cage, clearance, precision and any other suffixes.

This method prevents a common purchasing error: identifying a bearing only by d × D × B while ignoring its engineering configuration.

This is one of the most important engineering principles on this page.

After identifying candidate bearings from dimensional tables, engineers should evaluate:

Load
Radial, axial and combined loads.

Speed
Operating and limiting speed requirements.

Life
Required fatigue life.

Clearance
Internal clearance before and after mounting.

Accuracy
Required dimensional and rotational precision.

Lubrication
Grease or oil requirements.

Environment
Temperature, contamination, moisture and corrosion.

Fits
Shaft and housing tolerance requirements.

Sealing
Open, shielded or sealed bearing configurations.

Therefore, the correct engineering workflow is:

Available Space → Bearing Type → d/D/B → Candidate Series → Load & Life → Speed → Clearance → Fits → Lubrication → Final Bearing Selection

A dimension table helps solve the first part of the problem—not the entire problem.

The three most important dimensions for many radial bearings are:

d = Bore Diameter
D = Outside Diameter
B = Bearing Width

For tapered roller bearings, T is commonly used for overall width, while additional cone and cup dimensions may also be specified.

Bearings with the same bore can belong to different dimension series and therefore have different outside diameters and widths. For example, 6005, 6205 and 6305 all use a 25 mm bore but occupy different installation envelopes.

Standardized dimensions make bearing identification and interchangeability easier, but dimensional compatibility alone does not guarantee functional interchangeability.

After identifying a bearing from its size, engineers must still verify bearing type, load rating, speed, internal clearance, accuracy, lubrication, sealing, fits and operating environment.

In short:

Use bearing dimension tables to identify what fits. Use engineering calculations and operating requirements to determine what works.