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Tutorial: How to Measure Bearings

Bearing sizes are often listed in a slightly different order or format depending on the source, which can make shopping for a replacement confusing. There is, however, a universal measuring convention used across the industry, and once you know it, sizing up any bearing becomes straightforward.

[[diagram:measuring-bearing]]

Tools You'll Need

Vernier calipers give you the most accurate reading of a bearing's dimensions. If you don't have a set on hand, a ruler will get you a rough estimate, though it won't be precise enough for a confident purchase.

Most calipers have two sets of jaws:

  • The outside (large) jaws measure the external or outer diameter, and are also used to measure width.
  • The inside (small) jaws measure the internal diameter, or bore.

The Order Bearing Sizes Are Measured In

It's easy to accidentally search for a 4x8x3mm bearing when what you actually need is a 3x8x4mm, simply because the numbers get transposed. Here's the order to measure in, using an 8x16x5mm bearing as the example:

1. Inner Diameter (ID) — measured first. This is the inner ring, sometimes called the shaft size or hole. In our example, that's 8mm. 2. Outer Diameter (OD) — measured second. In our example, that's 16mm. 3. Width (W) — measured third. In our example, that's 5mm.

Once you have all three measurements, you can search using the full size string — for example "8x16x5" — to find the matching bearing.

Imperial Bearings Follow the Same Order

Imperial-sized bearings are measured using the identical sequence. For example:

1/2" x 3/4" x 5/32" = (ID) x (OD) x (W)

This ID-OD-Width order is a universal convention, whether the bearing is destined for a fishing reel, a skateboard, an RC car, an engine, or anything else.

Measuring Linear Bearings

Linear bearings follow the same basic measuring method as standard radial bearings. The one thing that can trip people up is that the width is often the largest dimension on a linear bearing rather than the smallest, so keep that in mind when comparing the numbers to a standard bearing's proportions.

Need help figuring out which size you're looking at? Contact us with your measurements and we'll help confirm the correct bearing for your application.