When the bearing number is worn off or the original part is missing, measure the three key dimensions to identify the replacement. This guide shows how to measure a bearing size accurately with basic hand tools — no special equipment required.
Tools Needed
- Digital caliper (recommended) or micrometer
- Clean cloth to wipe the bearing
- A flat surface to rest the bearing on
A digital caliper is accurate to 0.01 mm and is sufficient for every bearing you are likely to identify. Micrometers give higher precision but are harder to position on a bearing race. For bearing identification, the caliper is the better tool.
Three Measurements
1. Bore (inner diameter, ID) — Measure inside the inner ring. This is the shaft size. Measure in two places and average.
The bore is the single most important measurement: it determines the shaft fit and usually identifies the bearing size by itself. Place the caliper jaws inside the inner ring, at the widest point (diametrically opposite), and measure in two positions roughly 90° apart. Take the average.
2. Outside diameter (OD) — Measure across the outer ring. This is the housing bore size.
Measure across the outer ring at its widest point. Again, take two measurements at 90° and average. The OD together with the bore is usually enough to pin down the exact bearing series.
3. Width (W) — Measure across the side faces. For tapered roller bearings, measure the overall width including both rings.
Width is measured across the side faces of the rings. For most ball bearings this is the inner ring face to outer ring face distance. For tapered roller bearings, measure the overall assembly width (cone face to cup face) — the dimension used in catalogue listings.
Common Bearing Dimension Patterns
Deep groove ball bearings follow standard ISO series patterns:
| Series | ID (mm) pattern |
|---|---|
| 6000 | Last 2 digits x 5 = bore (6005 = 25mm) |
| 6200 | Same rule (6205 = 25mm) |
| 6300 | Same rule (6305 = 25mm) |
For bore 20mm and above: multiply the last two digits by 5 to get the bore diameter. Example: 6208 = 08 x 5 = 40mm bore.
For bores below 20 mm the rule changes. The 6000, 6200, and 6300 series use non-linear bore codes for the smallest sizes: 6000 = 10 mm, 6201 = 12 mm, 6202 = 15 mm, 6203 = 17 mm. If your measured bore is 12 mm and the last two digits of the old number say “01”, the rule still checks out — but rely on the measured dimensions first.
How to Identify a Bearing by Dimensions
- Measure bore, OD, and width.
- Compare all three against the series tables on the bearing size chart.
- Match the bore to the series (6000, 6200, 6300, etc.) and confirm OD + width agree with one specific model.
- If bore matches one series but OD/width do not, the bearing is probably from a different series with the same bore — check adjacent series.
Two different series can share a bore (6205 and 6305 are both 25 mm bore) but they have different OD and width. That is why all three dimensions must be measured, not just the bore.
What About Closure and Clearance?
Dimensions alone identify the size, not the variant. Two bearings with identical dimensions can differ in:
- Closure — open, ZZ (metal shields), or 2RS (rubber seals). See ZZ vs 2RS.
- Internal clearance — C0 (normal), C3 (increased), C4. See C3 clearance explained.
- Cage material and precision grade.
If you are replacing a bearing and the old part number is unreadable, measure the dimensions AND note whether the old bearing had shields, seals, or neither. Send both to the quotation request.
Send the Measurements
When you send an RFQ to KIHOI with the three dimensions, we can identify the correct bearing series and offer the right replacement — even when the original part number is unreadable. Include the closure type (open/ZZ/2RS) if you can see it, and the application, and we will confirm the model, dimensions, and price before you order.