How to Measure a Bearing: Step-by-Step Guide to Finding Bearing Size
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How to Measure a Bearing: Step-by-Step Guide to Finding Bearing Size
Quick Answer: To measure a bearing, you need three main dimensions: the inner diameter (ID), outer diameter (OD), and width. Once you have those measurements, you can match the bearing to the correct replacement size.
If you need to replace a bearing but do not know the part number, measuring it correctly is the fastest way to find the right replacement.
This guide explains exactly how to measure a bearing step by step, what tools to use, and how to avoid common mistakes.
What Measurements Do You Need for a Bearing?
To identify a bearing size, you usually need these three measurements:
- Inner Diameter (ID): the hole in the center of the bearing
- Outer Diameter (OD): the full outside width of the bearing
- Width: the thickness of the bearing from side to side
These three numbers are enough to identify most standard ball bearings and many roller bearings.
Tools Needed to Measure a Bearing
The best tool for measuring a bearing is a digital caliper, because it gives a more accurate reading than a tape measure or ruler.
Helpful tools include:
- Digital caliper
- Vernier caliper
- Clean rag to wipe the bearing
- Notebook or phone to record measurements
If the bearing is dirty or greasy, wipe it clean first so you get an accurate measurement.
Step 1: Measure the Inner Diameter (ID)
The inner diameter is the size of the hole in the middle of the bearing. This is also called the bore size.
To measure it:
- Open the caliper slightly
- Place the inside measuring jaws into the center hole
- Expand them gently until they touch both sides
- Read the measurement
Example: a 6205 bearing has an inner diameter of 25 mm.
Step 2: Measure the Outer Diameter (OD)
The outer diameter is the full diameter across the outside of the bearing.
To measure it:
- Use the outside jaws of the caliper
- Place them across the widest outer edges
- Close the jaws until they gently touch both sides
- Read the measurement
Example: a 6205 bearing has an outer diameter of 52 mm.
Step 3: Measure the Width
The width is the thickness of the bearing from one side to the other.
To measure it:
- Stand the bearing on its side or hold it flat
- Use the outer jaws of the caliper across the side faces
- Measure straight across the full width
- Read the result
Example: a 6205 bearing has a width of 15 mm.
Example Bearing Measurements
Here are a few common examples:
- 6203 bearing: 17 mm x 40 mm x 12 mm
- 6205 bearing: 25 mm x 52 mm x 15 mm
- 608 bearing: 8 mm x 22 mm x 7 mm
If your measurements match one of these standard sizes, it becomes much easier to find the correct replacement.
How Bearing Sizes Are Written
Bearing dimensions are usually written in this order:
ID x OD x Width
For example:
- 6205: 25 x 52 x 15 mm
- 608: 8 x 22 x 7 mm
This format helps buyers quickly compare sizes and match a bearing to the correct product listing.
Common Mistakes When Measuring a Bearing
A few small mistakes can lead to ordering the wrong part.
- Measuring with a ruler instead of calipers
- Measuring a dirty bearing without cleaning it
- Confusing inner diameter with outer diameter
- Measuring at an angle instead of straight across
- Ignoring metric sizing
Most standard bearings are listed in millimeters, so metric measurements are usually the safest choice.
What If the Bearing Number Is Still Visible?
Before measuring, check the side of the bearing for a stamped part number. Many bearings have the number printed or engraved on the shield, seal, or outer ring.
If the number is still readable, you may be able to identify the bearing immediately without measuring.
How to Match the Bearing After Measuring
Once you have your dimensions, compare them to product listings or bearing charts. Make sure to also choose the correct bearing type, such as:
- Open for clean, protected environments
- ZZ for light protection and higher speed
- 2RS for better sealing against dirt and moisture
If you are replacing a common ball bearing, you can start by checking these collections:
Need Help Finding the Right Bearing?
If you have measurements but are not sure which bearing to order, contact us with the dimensions, application, and any visible part numbers.
If you need larger quantities for a repair shop, industrial use, or repeat orders, you can also request a quote here:
Final Thoughts
Measuring a bearing is simple once you know what to look for. Focus on the three key dimensions: inner diameter, outer diameter, and width.
With those measurements, you can identify most standard bearing sizes and find the right replacement faster.
Frequently Asked Questions
How do you measure a bearing size?
Measure the inner diameter, outer diameter, and width using a digital caliper. These three numbers identify the bearing size.
What is the order of bearing measurements?
Bearing sizes are usually listed as inner diameter x outer diameter x width.
Can I measure a bearing with a ruler?
You can, but a digital caliper is much more accurate and is the better option for finding the correct replacement size.
Are bearing sizes in inches or millimeters?
Most standard bearings are listed in millimeters, so metric measurements are usually the most useful.