A metric bearing number is a compact code for its type and size. This tool reads the ISO 15 designation, decodes the type and dimension series, expands the bore code into millimetres, and matches the boundary dimensions from a table of common bearings.
How it works
The number is read digit by digit. The first digit (or letter) is the bearing type. The second is the dimension series — larger series numbers mean a heavier bearing for the same bore. The final two digits are the bore code:
if code >= 04: bore = code * 5 (mm)
00 -> 10 mm 01 -> 12 mm 02 -> 15 mm 03 -> 17 mmSo 6204 decodes as: type 6 (deep-groove ball), series 2 (light), bore code 04
→ 20 mm bore. The outside diameter and width come from the standard boundary
table because they depend on both the bore and the series.
Tips and notes
- The four low bore codes (00–03) break the times-5 rule — memorise 10, 12, 15, 17 mm.
- Same last two digits, different series digit, means same bore but different OD
and load rating (
6204vs6304). - Boundary dimensions are standardised, so any-brand replacements of the same number fit; suffixes like 2RS or C3 describe seals and clearance, not size.
- This tool covers the common 6000/6200/6300 deep-groove families; other types share the same numbering logic.
Understanding the dimension series
The second digit of a metric bearing number is one of the most practically important characters to understand, because it determines not just the size but the load capacity and the application a bearing is suited to. Within any given bore size, a higher series number means a heavier bearing with a larger outside diameter and wider race:
| Series | Character | Typical use |
|---|---|---|
| 6000 (series 0/1) | Extra-light | Instruments, precision spindles |
| 6200 (series 2) | Light | General machinery, motors, pumps |
| 6300 (series 3) | Medium | Heavier loads, gearboxes |
| 6400 (series 4) | Heavy | High-load applications, large equipment |
So a 6204 and a 6304 both fit a 20 mm shaft, but the 6304 has a larger OD, a wider inner ring, and a higher dynamic load rating. The trade-off is that it needs more housing space.
Suffixes: what comes after the base number
The base bearing number covers the type, series, and bore — everything needed for interchangeability. But real bearings carry suffixes that are equally important for selecting the right part:
- 2RS or 2Z — rubber seals (RS) or metal shields (Z) on both sides. Sealed bearings are pre-lubricated and low-maintenance. Open bearings (no suffix) take external lubrication and suit higher-speed applications.
- C3 — extra internal clearance, used where the bore will expand under heat or press-fit loading.
- P5, P6 — precision class. Standard is P0; P6 is closer tolerance for machine tools; P5 and P4 are for high-speed spindles and precision instruments.
- NR — snap-ring groove on the OD for axial positioning without a shoulder.
A full part number like 6204-2RS/C3 is therefore a single-row deep-groove ball bearing, light series, 20 mm bore, rubber-sealed both sides, and extra radial clearance — precise enough to order a replacement without ambiguity.