Every bearing has three critical dimensions: the inner diameter (bore), the outer diameter, and the width. Measuring all three with a caliper or micrometer gives you everything you need to find a replacement. If the bearing still has a readable part number, you can also decode its size directly from those digits.
The Three Measurements You Need
A standard ball bearing is defined by three numbers, always listed in the same order: bore diameter, outside diameter, and width. A bearing sold as 6205, for example, has a 25 mm bore, a 52 mm outside diameter, and a 15 mm width. Every catalog and supplier uses this same trio, so once you have your three numbers, finding a match is straightforward.
Bore diameter (ID): The inside hole where the shaft passes through. Place your caliper jaws inside the bore and expand them until they contact the inner ring at its widest point. Take the reading at two or three positions around the bore to check for wear or out-of-round conditions.
Outside diameter (OD): The outer ring that sits inside the housing. Close your caliper jaws around the outside of the bearing at the widest point. Again, measure at a couple of positions to confirm consistency.
Width: The distance between the two flat faces of the bearing. Stand the bearing on a flat surface and measure from one face to the other. On sealed or shielded bearings, the seals sit flush with or slightly recessed from the outer ring faces, so measure the metal ring faces rather than a protruding seal.
Choosing the Right Measuring Tool
A digital caliper is accurate to 0.01 mm (0.001 inches), which is precise enough for most bearing replacements. Standard bearings are manufactured to tolerances that a good caliper can resolve, and you’re typically just trying to identify the correct size code rather than verify manufacturing precision.
A micrometer is ten times more accurate, reading down to 0.001 mm (0.0001 inches). You’d reach for one if you’re checking a shaft or housing bore for wear, verifying that a precision-ground component is still within spec, or working with high-tolerance bearings rated ABEC-5 or above. For simply identifying a bearing size to order a replacement, a caliper is the right tool.
Whichever tool you use, make sure the bearing is clean and dry before measuring. Dirt, old grease, or corrosion on the surfaces will throw off your readings. Wipe the bore, outer ring, and faces with a solvent or degreaser first.
Decoding a Bearing Part Number
If you can still read the number stamped or printed on the bearing’s outer ring, you can skip the calipers entirely. The standard numbering system used by most manufacturers follows a simple pattern.
The last two digits of the basic designation tell you the bore diameter. For bearings with a bore of 20 mm or larger, multiply those last two digits by 5 to get the bore in millimeters. A bearing stamped 6308, for instance: the last two digits are 08, so the bore is 08 × 5 = 40 mm.
Four small bore sizes break this rule and use fixed codes instead:
- 00 = 10 mm bore
- 01 = 12 mm bore
- 02 = 15 mm bore
- 03 = 17 mm bore
Bearings with bores smaller than 10 mm or 500 mm and above typically show the actual bore diameter in millimeters, separated by a slash. A designation like 618/8 means the bore is simply 8 mm. If the bore is a non-standard size (like an imperial conversion), you’ll see the full decimal value after the slash: 6202/15.875 indicates a bore of 15.875 mm, which is 5/8 inch.
The digits before the bore code identify the bearing type and dimension series, which together determine the outside diameter and width for that bore size. You won’t need to memorize those. Once you have the full part number, any bearing catalog or supplier search will return the complete dimensions.
Metric vs. Imperial Bearings
Most bearings worldwide follow metric ISO standards, but imperial (inch-sized) bearings are common in older American machinery, automotive applications, and aerospace. If your measurements come out to clean fractions of an inch rather than round millimeter values, you’re likely dealing with an imperial bearing.
Imperial bearing part numbers often encode dimensions in sixteenths or thirty-seconds of an inch. The last two digits of many imperial part numbers represent how many sixteenths of an inch the bore diameter is. If those digits are “04,” the bore is 4/16 inch, or 1/4 inch (6.35 mm). Width codes in some series use thirty-seconds of an inch: a width code of “12” means 12/32 inch (9.525 mm).
When your caliper reads something like 25.4 mm (exactly 1 inch) or 38.1 mm (1.5 inches), that’s a strong clue the bearing is inch-sized. Converting between systems is straightforward since one inch equals exactly 25.4 mm, but ordering the correct replacement means matching the original standard. An imperial bearing and a metric bearing with similar dimensions may differ by fractions of a millimeter, enough to cause fit problems.
Measuring Tapered Roller Bearings
Tapered roller bearings come in two separable pieces: the cone (inner ring with rollers) and the cup (outer ring). Because they come apart, you measure each piece individually.
For the cone, measure the bore diameter the same way you would a standard bearing. For the cup, measure the outside diameter. The width measurement is a bit different because the cone and cup overlap when assembled, so total assembly width depends on how far the cone is pressed into the cup. Catalog width for tapered bearings is measured with the components pushed fully together.
If you’re replacing a tapered bearing, the part number stamped on the cup or cone is the most reliable way to get an exact match. These bearings are manufactured as matched sets with specific internal geometry, so physical measurements alone can leave ambiguity that a part number eliminates.
What the Suffixes Mean
After the basic number, you’ll often see letters and numbers that describe the bearing’s features. These don’t change the physical dimensions, but they matter for ordering the right replacement.
- 2RS or 2RSH: Rubber contact seals on both sides, which keep grease in and contaminants out.
- 2Z or ZZ: Metal shields on both sides. These are non-contact, meaning they don’t touch the inner ring, so they create less friction than rubber seals but offer less protection from moisture and fine dust.
- C3: Greater-than-normal internal clearance between the balls and races. This is common in applications with heat or tight press fits that shrink the internal space.
- C2: Less internal clearance than normal, used where minimal play is needed.
If your old bearing is marked 6205-2RS/C3, you want a 25 mm bore deep groove ball bearing with rubber seals on both sides and C3 clearance. Matching the suffixes matters. Installing a bearing with normal clearance where C3 was specified can lead to premature failure if the application generates heat.
When Measurements Don’t Match a Standard Size
If your three measurements don’t line up with any standard bearing in a catalog, a few things could be going on. The bearing may be worn, especially on the bore, where a loose shaft fit can gradually enlarge the inner ring over time. Measure the shaft diameter as well and compare it to your bore reading. If the bore measures slightly larger than the shaft, wear is the likely cause, and you should order based on the shaft size.
You might also be dealing with a bearing from a proprietary series or a non-standard application. Agricultural equipment, some power tools, and certain European machinery occasionally use bearings with dimensions that don’t appear in the major catalogs. In these cases, searching by the original part number or the equipment manufacturer’s parts list is more productive than trying to cross-reference dimensions alone.