How to read a bearing number
A metric rolling-bearing designation looks cryptic, but almost all of it follows one system defined in ISO 15 and ISO 355. Once you can read it, a string like 6204 or NU 2210 ECP tells you the type, the size series and the exact bore — before you ever open a catalogue.
The three parts of a designation
Nearly every standard metric bearing number breaks into three blocks:
- Type / series prefix — one or two characters (and sometimes letters) that say what kind of bearing it is and which dimension series (how heavy the section is).
- Bore code — the last two digits, which encode the bore diameter.
- Suffixes — letters and numbers after the base number for seals, clearance, cage, precision and internal design.
The bore code — the single most useful rule
For the vast majority of metric bearings, the last two digits of the base number give the bore, and the rule is almost absurdly simple once you know the four exceptions:
| Last two digits | Bore diameter |
|---|---|
| 00 | 10 mm |
| 01 | 12 mm |
| 02 | 15 mm |
| 03 | 17 mm |
| 04 and above | digits × 5 (04 → 20 mm, 05 → 25 mm, 20 → 100 mm) |
So a 6204 has a 20 mm bore (04 × 5). A 6220
has a 100 mm bore (20 × 5). A 6200 has a 10 mm bore. This one
rule covers deep groove ball, angular contact, self-aligning, cylindrical
roller, tapered roller and thrust ball bearings. The exceptions are bores below
10 mm and very large bores written as, e.g., 60/22 (22 mm bore).
The first digit(s) — the type
The leading character tells you the family:
| Prefix | Bearing type | Example |
|---|---|---|
| 6 | Deep groove ball | 6204 |
| 7 | Angular contact ball | 7204 |
| 1, 2 (…3) | Self-aligning ball | 1204, 2204 |
| N, NU, NJ, NUP, NF | Cylindrical roller | NU 205 |
| 3 (5-digit) | Tapered roller | 30204 |
| 2 (5-digit) | Spherical roller | 22212 |
| QJ | Four-point contact ball | QJ 206 |
| UC | Insert (mounted) ball | UC 204 |
| 51, 52 | Thrust ball | 51104 |
| NA, RNA, NK | Needle roller | RNA 4900 |
The second digit — the dimension series
In a four-digit ball-bearing number, the second digit is the diameter series — how large the outer diameter and how heavy the section is for a given bore. This is why bearings with the same bore can have very different outer diameters:
- 6800 — extra-thin section (10 × 19 × 5)
- 6900 — thin section (10 × 22 × 6)
- 6000 — light (10 × 26 × 8)
- 6200 — medium, the most common (10 × 30 × 9)
- 6300 — heavy (10 × 35 × 11)
All five have a 10 mm bore, but the outer diameter grows from 19 to 35 mm as the series number rises. When someone says "I need a 6204", the "62" already tells you it is a medium-series bearing before you look up 20 × 47 × 14 mm.
Worked examples
6204 → deep groove ball (6), medium series (2), bore 20 mm (04). Result: 20 × 47 × 14 mm.
NU 2210 ECP → cylindrical roller with ribs on the outer ring only (NU), wide 22 series, bore 50 mm (10). The E is a reinforced design, C a modified cage, P a polyamide cage.
30204 → tapered roller (30), normal-angle 302 series, bore 20 mm (04). Result: 20 × 47 × 15.25 mm.
22212 E → spherical roller (22), bore 60 mm (12), E = reinforced. Result: 60 × 110 × 28 mm.
UC 204 → insert ball bearing with a spherical outer surface, bore 20 mm — the bearing that goes inside a UCP204 pillow block.
The suffixes are where interchange goes wrong
The base number gets you the size. The suffixes decide whether two same-size bearings actually do the same job — seals, internal clearance, precision and cage all live there, and they are not standardised across brands. That is the subject of the rest of these guides, starting with what "interchangeable" really means.
Decoding reference only. Always confirm the exact dimensions and suffix meaning against the manufacturer's catalogue before ordering or substituting.