How to Measure Bearing Size Accurately
Measuring a bearing correctly is essential when you need to identify a replacement bearing, cross-reference an old bearing with an unreadable part number, or compare equivalent bearings from different manufacturers.
For most standard bearings, the three main dimensions are expressed as:
ID × OD × Width
or
d × D × B
Example:
25 × 52 × 15 mm
This means:
-
ID (d) = 25 mm — Inside Diameter or Bore Diameter
-
OD (D) = 52 mm — Outside Diameter
-
Width (B) = 15 mm — Bearing Width
1. Tools Required for Measuring a Bearing
The most commonly used measuring tool is a:
Vernier Caliper
A Vernier Caliper is suitable for measuring:
-
Inside diameter
-
Outside diameter
-
Bearing width
-
Ring thickness
For general bearing identification, a digital or mechanical Vernier Caliper with a resolution of at least:
0.01 mm
is recommended.
For higher-precision inspection, you may also use a:
Micrometer
A micrometer is useful when checking bearing dimensions more precisely, especially for:
-
Outer diameter
-
Ring width
-
Worn bearing surfaces
-
Dimensional tolerance inspection
2. Measure the Inside Diameter — ID or Bore Diameter
The first dimension to measure is the bearing bore.
The standard symbol is:
d
For example:
Bearing 6205 = 25 × 52 × 15 mm
Therefore:
d = 25 mm
How to Measure
Use the inside measuring jaws of the Vernier Caliper.
Insert the jaws into the bore of the bearing and slowly open them until both jaws make contact with the inner surface of the Inner Ring.
Make sure the caliper passes through the centerline of the bearing.
Read the measurement.
Example:
25.00 mm
This value is the Inside Diameter or Bore Diameter.
Important Measuring Tips
Avoid measuring directly on the chamfered edge of the Inner Ring because this may produce an inaccurate reading.
Measure the bore at least 2–3 different positions.
Rotate the bearing approximately 90 degrees and repeat the measurement.
If the readings vary significantly, the Inner Ring may be worn, distorted, or out of round.
3. Measure the Outside Diameter — OD
The second important dimension is the:
Outside Diameter
The standard symbol is:
D
Example:
6205 = 25 × 52 × 15 mm
Therefore:
D = 52 mm
How to Measure
Use the external jaws of the Vernier Caliper.
Place the jaws around the outside surface of the Outer Ring.
Make sure the jaws contact opposite sides of the bearing and pass across the centerline.
Read the measurement.
Example:
52.00 mm
This value is the Outside Diameter or OD.
Recommended Method
Measure the OD in several directions, such as:
-
Vertical
-
Horizontal
-
Approximately 45 degrees
If the readings differ significantly, the Outer Ring may be worn, damaged, or deformed.
4. Measure the Bearing Width
The third major dimension is the bearing width.
The standard symbol is normally:
B
for the Inner Ring width.
For some bearing types, the symbol:
C
may be used for the Outer Ring width.
Example:
6205 = 25 × 52 × 15 mm
Therefore:
B = 15 mm
How to Measure
Use the external jaws of the Vernier Caliper.
Measure from one bearing face to the opposite face.
Keep the caliper perpendicular to the bearing face.
Read the measurement.
Example:
15.00 mm
This value is the Bearing Width.
5. Standard Bearing Dimension Format
Once the three main measurements are obtained, record them in the following format:
ID × OD × Width
Example:
25 × 52 × 15 mm
This dimension can then be used to identify a standard bearing.
For example:
25 × 52 × 15 mm = Bearing 6205
If the bearing has rubber seals on both sides, the full bearing designation may be:
6205-2RS
or
6205-2RSH
depending on the manufacturer.
6. Check the Bearing Number Before Measuring
Before measuring the bearing, always inspect the side surface of the Inner Ring or Outer Ring.
Most bearing manufacturers engrave or laser-mark the bearing number on the bearing.
Examples include:
SKF 6205-2RS1
NSK 6205DDU
NTN 6205LLU
KOYO 6205-2RS
Although the suffix codes may differ between manufacturers, a standard 6205 bearing generally has the basic dimensions:
25 × 52 × 15 mm
Suffixes normally provide additional information such as:
-
Seal type
-
Shield type
-
Internal clearance
-
Cage design
-
Lubrication
-
Internal design
If the original part number can still be read, use the bearing number as the primary identification method and use dimensional measurement as confirmation.
7. Identify Open, ZZ and 2RS Bearings
Bearing dimensions alone are not enough.
You should also check the bearing sealing arrangement.
Open Bearing
An open bearing has no shields or seals on either side.
The rolling elements and cage are visible.
Example:
6205
ZZ or 2Z
A bearing marked ZZ or 2Z has metal shields on both sides.
Examples:
6205ZZ
or
6205-2Z
These bearings are commonly used where protection from general dust is required while maintaining relatively low friction.
2RS
A 2RS bearing has rubber seals on both sides.
Example:
6205-2RS
It provides better protection