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How to Read Bearing Numbers (6205ZZ Explained)

6205ZZ means "a deep groove ball bearing with a 25 mm bore and metal shields on both sides." The 6 is the bearing type, the 2 is the dimension series, 05 is the bore code (05 × 5 = 25 mm), and the ZZ suffix means two shields. Read the type and size from the first four digits, and the details from the suffix.

A bearing number is a code, not a serial number

A bearing number, or designation, is not a serial number given to each product. It is a code that describes the type and size by agreed rules. For standard bearings that follow ISO dimension plans, the same basic number means the same bore, outside diameter and width, whoever made the bearing.

That is why drawings and parts lists say 6205 instead of "25 mm ball bearing": the designer, the buyer and the maintenance team all know exactly which part is meant.

But the same basic number does not mean everything is the same. Shields or seals, internal clearance, precision class, cage and grease are set by the suffixes. Miss a suffix, and you can fit a bearing of the right size but the wrong specification.

Reading it from the front

① 6 — bearing type

The first digit or letters show the bearing type. 6 is the most common type, the deep groove ball bearing. Here are common types, all with a 25 mm bore.

② 2 — dimension series

The second digit is the dimension series. Bearings with the same bore come in several outside diameters and widths. Put the 25 mm deep groove ball bearings side by side and the difference is easy to see.

(Dimensions in mm)

A larger section holds bigger balls and carries more load, but it takes more space. That is why two machines with the same shaft size may use a 6005 and a 6305.

③ 05 — bore code

• From 04 up, multiply the code by 5 to get the bore in mm. 04 → 20 mm, 05 → 25 mm, 08 → 40 mm, 10 → 50 mm.
• 00 to 03 follow their own rule, so they are worth memorizing. 00 → 10 mm, 01 → 12 mm, 02 → 15 mm, 03 → 17 mm.
• Below 10 mm, the last digit is the bore itself. The 608, common in skateboard wheels, has an 8 mm bore.
• Bores that are not multiples of 5, such as 22, 28 or 32 mm, and bores of 500 mm and above are written after a slash. For example, 62/22 has a 22 mm bore.

④ ZZ — suffixes

Suffixes after the basic number give the details. These are the ones you will see most often.

So the same bearing can be written 6205ZZC3 (NSK, NTN) or 6205-2Z/C3 (SKF). How to match numbers across brands will be covered in a separate article on bearing equivalents.

Step by step

1. Find the number. It is marked on the side of a ring or on the shield, together with the maker's name. You often have to wipe off the grease first.
2. Read the type and size from the first four digits. 6205 → deep groove ball bearing, 25 mm bore, 52 mm outside diameter, 15 mm width.
3. Check the details in the suffix. Look at the shields or seals (ZZ, rubber seals), the clearance (C3) and the precision (P6, P5). When you replace a bearing, match the suffix of the original as well.
4. If the number has worn off, measure it. Measure the bore, outside diameter and width with calipers and check the shield type; that is enough to find most bearings. Clearance and precision cannot be seen from outside, so check the machine's parts list.
5. Treat inch bearings separately. American inch tapered roller bearings carry separate numbers on the inner part (cone) and the outer ring (cup). Check both numbers and the dimensions.
6. Confirm with a dimension table. The numbering rules have exceptions. Before ordering or designing, check the maker's catalog.

Related articles

• Wind Turbine Bearings: The Five Places They Work
• Bearing Interchange: Finding Equivalents Across Brands (coming soon)
• Ball Bearings vs Roller Bearings: What Is the Difference? (coming soon)

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