Guide
How to measure a bearing and find its size
Measure bore, outside diameter and width with calipers, allow for wear, tell metric from inch, and turn three numbers into a standard bearing designation.
When the marking on a bearing is worn off, painted over or simply unreadable, three measurements are usually enough to find the standard size: the bore (d), the outside diameter (D) and the width (B). This guide explains how to take those measurements reliably, how to deal with wear and inch sizes, and how to go from three numbers to a designation such as 6205.
What you need
- A pair of vernier or digital calipers that read to at least 0.05 mm. A steel rule is not accurate enough for bearings.
- A clean, lint-free cloth and some degreaser — dirt and old grease add tenths of a millimetre.
- A note of anything still legible on the rings or seals: part of a number, a brand, letters such as “2RS” or “C3”.
Measure the bearing off the shaft where possible. If it is still mounted, you can only measure the outside diameter and width reliably; the bore then equals the shaft diameter at the bearing seat, which you can measure once the bearing is removed.
Step 1: measure the bore (d)
Use the inside jaws of the calipers. Place them in the bore and open them until they touch the ring, then rock the calipers gently to find the largest reading — that is the true diameter, not a chord. Take readings in two directions at 90° and average them.
Standard metric bores are whole millimetres: 10, 12, 15, 17, 20, 25, 30 mm and so on. A reading of 24.98 mm is a 25 mm bore.
Step 2: measure the outside diameter (D)
Use the outside jaws across the outer ring, again checking two directions. Make sure you measure the ring itself and not a snap ring, a flange on a housing, or a seal lip that sticks out.
Step 3: measure the width (B)
Measure across both ring faces, parallel to the axis. For most ball bearings the inner and outer rings have the same width. If they differ, note both — that is a clue you may have a tapered roller bearing, a bearing with an extended inner ring, or an insert bearing for a housed unit.
For a thrust bearing, the equivalent dimension is the overall height of the assembled washers and ball set, usually written H.
Dealing with wear and damage
Worn or damaged bearings measure slightly off-standard:
| What you see | What it means | What to do |
|---|---|---|
| Bore reads 0.1–0.3 mm oversize | Ring has crept on the shaft or worn | Round to the nearest standard bore, then check the shaft |
| Outside diameter slightly undersize | Fretting or corrosion on the outer ring | Round to the nearest standard OD |
| Width varies around the ring | Damage or a deformed shield | Measure at several points and use the largest consistent value |
| Nothing rounds cleanly | Inch bearing or a special | See the next section |
A bearing that has spun on its shaft is a symptom, not just a measurement problem. Check the shaft and housing fits before fitting the replacement.
Metric or inch?
If your three readings land close to whole millimetres, you have a metric bearing. If they look odd in millimetres but clean in inches — for example a bore of 12.70 mm (½ inch) or 19.05 mm (¾ inch) — you are probably holding an inch-series bearing. Our guide to metric vs inch bearings explains how the two systems differ and why converting units does not turn one into the other.
From dimensions to a designation
Once you have d × D × B, look it up. On the MyBearings bearing search you can type the dimensions directly, for example 25x52 or 25x52x15, and you will see every standard bearing with that size.
Very often more than one bearing type shares the same envelope. A 25 × 52 × 15 mm bearing could be:
| Designation | Type |
|---|---|
| 6205 | Deep groove ball bearing |
| 1205 | Self-aligning ball bearing |
| 7205 | Angular contact ball bearing |
| NU205 | Cylindrical roller bearing |
So dimensions narrow the search, but the type comes from looking at the bearing: one row of balls in deep grooves is a deep groove ball bearing; two rows of balls with a spherical outer raceway is self-aligning; rollers instead of balls is a roller bearing; an offset outer ring shoulder suggests angular contact.
The size chart is useful when you want to see neighbouring sizes at a glance — for example to check whether 25 × 62 × 17 is the medium-series 6305 rather than the light-series 6205.
What measuring cannot tell you
Dimensions identify the basic size. They do not tell you:
- whether the bearing had shields or seals (ZZ, 2RS and the manufacturer equivalents) — see sealed vs shielded bearings;
- its internal clearance (CN, C3 and so on) — see C3 clearance explained;
- its cage material, precision class, lubricant or any special heat treatment.
Those details live in the suffix of the full designation. If any characters are still legible on the old bearing, write them down and decode them with the bearing number decoder or the suffix dictionary. If nothing is legible, match the application: the same machine’s manual, a twin bearing on the other end of the shaft, or the equipment maker’s spare parts list.
A quick checklist
- Clean the bearing.
- Measure d, D and B with calipers, two readings each.
- Round to standard millimetre values, or check for inch sizes.
- Identify the type by looking at the rolling elements and rings.
- Look up the size on the bearing search.
- Recover the suffix from the marking, the old packaging or the equipment documentation.
Apply it
Measured a whole shelf of unlabelled parts, or inherited a catalog full of dimension-only records? Turning dimensions into identified parts at scale is what part identification is about. Start with the bearing search for single parts, or talk to us about a batch.
Methodology article using synthetic examples only; it states no manufacturer specifications. Independent technical review: not yet assigned. Found an error? Tell us.
